Effect of Celestron Dew Ring on Star Shape

[@HeavenlyBackyardAstronomy]

Great to hear from you James. You and Chad from Patriot Astro have been MIA lately, but you're both back and all is well. Okay, about the heater ring. I just bought one for my C-11 so I was concerned when I saw your video title. However, my thinking is ... what could be the difference between the ring versus using dew straps? They both are generating heat from the outer edge of the corrector plate inward. Perhaps the only difference is the tube is not heating as much when using the ring. Other than that, I can't see what the concern would be. Also, GREAT mathematical discussion about the refraction of light through the glass. - Patrick -

[@Aero19612]

Thanks, Pat! Something about seemingly perpetual clouds that zaps my enthusiasm. Now, I'm in a 11-day clear sky period. Funny, I just thought of you the other day: finally getting around to testing out the Antlia ALP-T DualBand filter and trying it out on the Tulip. Making sure to frame the picture to get the Cygnus X-1 arc I first heard about in one of your videos. I'll be pointing the blame cannon at you when I get around to making a video about the ALP-T. Dew Ring: Not sure what people are seeing, but they're sure they are seeing something. I think Kurt (AstroQuest1) had a good observation. He saw star deformation and then backed off on the torque used to install the ring. It could be people are overtightening the ring and the heat just magnifies the effect with the ring too constrained to relieve the thermal expansion. The big difference with the Ring and the Strap is that heat from the strap has to go through the aluminum outer ring of the OTA before it gets to the glass. There's a lot of thermal mass in the outer alum ring to absorb heat. Plus, I don't think the strap is as effective of a heater as the ring is. I was having good results with my dew strap, but thought I'd test out the heated ring. It certainly does a trick on dew. I'll give it that. See ya on KZbin...

[@AnakChan]

I've always enjoyed to not only the way you approach your analysis but also your explanations. & LoL the conclusion wasn't what I expected! Well done!

[@Aero19612]

Haha. You wanted me to say the dew heater ring sucks!? Nah, it works well. As few good imaging nights as I’ve been getting lately, the last thing I want is to lose one to dew. Thanks for watching!

[@sergeyyatskevitch3617]

Great video, thank you. I greatly appreciate the clarity of your models and calculations. I might say that from what I have seen so far, your channel is the best in terms of actual understanding the phenomena you research. I have one question regarding your results. Is my eyes failing me, or the star picture comparison, indeed shows a bit more thermal noise on the RHS pic, where the dew ring was at 100%? I just installed a dew ring on my SCT 8 evo, and although I like how it deals with the dew, I am concerned that during the long nigh use, it may heat the tube, so I would get a stronger IR signal. Cheers!

[@astro-jessie]

Just came across this video and so thankful for all the information. Just had 2 clear nights and was seeing significant star deformation and spikes and didn't know what was causing it. I didn't see it before I installed my dew heater ring, so going to test it out. Glad I have a starting place to begin testing. Get so few clear nights, it's a shame to lose any. Again thanks so much!

[@Aero19612]

Thanks for watching. I thought I was on a path to solve this. Nope. Video out soon!

[@AstroQuest1]

Excellent research/experiment James. I always enjoy watching good science in action. Your images of the tests are indistinguishable so any differences in the two were most likely from some other cause as your calculations indicate. I did have some miss-shapen sars as well but I just lossened the Dew Ring and then retightened it but not as tight and that seemed to fix it. - Cheers Kurt

[@Aero19612]

Hey Kurt. You may have cracked it (not the corrector plate, haha)! Maybe the star shape issue people are referring to is actually caused by over-tightened screws and not the heat. Thanks for the great insight and comment!

[@NoahsAstrophotography]

Hi James, very interesting video, I'm glad I came across it! I have the Celestron 11" dew heater ring and had the exact same problem as you. Was able to diagnose the issue and I build a diy dew heater controller to lower the input power and shared my results in my latest yt video as well :) I never considered the glass could be warmer inside the tube with gradients. An interesting video, thanks!

[@Aero19612]

Very inventive, Noah! The stretched stars bother a lot of people. I have to say, I'm not that bothered about them as they aren't that noticeable at scale that I view an astrophoto. I understand that a lot of people like to zoom waaay in and inspect for aberrations. I do need to experiment more and see how low I can go on the power setting and still get dew protection. A 100% power level is definitely overkill. Thanks for watching and keep up the excellent work!

[@lesgatechair3907]

Hi Jay, Always amazing to see how scientific your analysis of problems is. Thanks!

[@Aero19612]

Hey Les. Hope your East Coast vacation went well. Thanks for watching these videos! Can’t believe how much this hobby pushes me into other technical areas. Always in over my head…I don’t know, maybe that’s one of the things I like about it.

[@lesgatechair3907]

@@Aero19612 Hope you are surviving the heat. Always look for Dalas area now to see how bad you have it compared to Tampa. But this week we are in Lake Tahoe w 23% humidity and 50 degF mornings. No dew strap needed! (but also no telescope). Saw the Milkey Way visually for the first time. Think I grew up believing that it was a cloud!

[@Aero19612]

Ahhh, the heat. Geez. It's depressing to walk outside after sundown and it's still > 100 F. Trying not to whine, but I'm pretty good at it. You certainly know how to plan your summers out. Wanna see me whine? Put me in Tampa in an off-the-charts humid summer. Also depressing to see a real night sky--don't have that problem in Dallas. I remember as a kid living in small town west Texas. Very clear Milky Way. Enjoy your trip and safe travels back!

[@elwaine1]

The heater ring can cause pinched optics. I use my C8 Edge for visual use only and so I do a quick check of collimation with each observing session. After purchasing a dew heater ring and the Celestron controller, I started to notice deterioration of the views after an hour or so. Rechecking the collimation showed clear signs of pinched optics. The diffraction rings took on an hexagonal shape instead of the concentric rings that were present at the start of the observation session. Turning off the power to the heater ring resulted in a return to perfectly concentric diffraction rings 10 minutes after the power to the ring was turned off. I’ll have to fiddle with the control settings to reduce the power to a level that prevents dew formation but doesn’t expand the corrector plate enough to mechanically stress the corrector.

[@Grolabolator]

Another goodie. Thank you. I have said Dew Heater Ring on my C11. Never had an issue with dew even with v. high relative humidity. Runs on auto power from the Pegasus Powerbox Advance Gen2. Set & forget :)

[@Aero19612]

I think the dew heater ring works very well for my C9.25. Good to hear that it handles the C11 with ease. Thanks for watching!

[@deep_space_dave]

Hey James, I think I figured out how the spikes are forming! If you look inside your SCT, the flange where the corrector plate sits on, there is a gap at the 5 o'clock and 11o'clock positions these gaps match perfectly with the spikes on my stars! Yes there is a little reflectivity there but the spike would have stayed when the dew heater was turned off. What I think is happening is because there isn't as much metal there compared to the rest of the flange, the glass is allowed to heat up more than the rest of the corrector because there is no metal to absorb the heat! Wow what a design oversight as most SCTs are made this way! Note: I didn't have this issue with my C6 both in HyperStar config and normal long FL. I think this mystery is solved! Now I am waiting for a new dew band to replace this as tired of having a slash on my stars 😕

[@Aero19612]

Excellent observation, Dave! I think you’re definitely on to something there. Thanks for reporting back. I’m sure others will find this post very useful. I’m not seeing an effect large enough for me to worry about it, but other folks sure seem to be having issues. I wonder if placing a thin shim (same width as the ring) between the ring and the glass would help if the shim had a gap to match the gap you refer to. Maybe that would prevent excess heat at those locations. I must say, I am surprised that the local hot spot doesn’t eventually average out. That’s a real good problem for someone with a computational heat transfer background! Good luck with the dew strap approach. I was having good luck with it before so you should be OK.

[@ldipenti]

Super interesting analysis! My personal experience with a C8 with a heater ring connected to an ASIAIR Pro is that I only saw some very visible spikes on super bright stars when having the heater at 50% power. Setting it down to 20% solved my problems. I read somewhere that the issue could be caused by hot air currents generated by the hotter zones on the ring.

[@Aero19612]

Yeah, it seems to only be noticeable with the brighter stars. If the 20% setting still prevents dew, then you’re all set. I don’t think the hot air currents hypothesis is reasonable - how could something as variable and random as air current cause spikes? See the comment from Deep_Space_Dave. He sees the spikes correlate with gaps in the corrector plate support ring inside the tube. Thanks for watching!

[@joshmccollumastrophotography]

I see like the tiniest amount of bloat, but that could be due to seeing and other facts. However new processing techniques on minimizing stars could probably effectively reduce it to an unnoticeable amount. I had my first go with it last night and it worked wonderfully. It was nearly 100% humidity with everything looking like it got rained on, and it was able to get rid of the dew.

[@johnmacdougall4545]

Hi James - As usual, nice video. I recently did an experiment addressing the Celestron Dew Ring observations and posted the results to Cloudy Nights - search for post number=160 in the topic called "Celestron Dew Heater ring for C11/RASA11 - comments?". I'm a pharmacologist as my day job and the dose/response (i.e. power/star distortion) relationship I saw was pretty clear. I posted images stepping down from 100% power to 20% power.

[@Aero19612]

Excellent experiment and approach, John! There’s definitely a heat-generated effect. Probably several variables involved. May be worse for longer focal lengths and over-sampled images. I’m using a camera with large pixels (ASI294MC at 1320 mm). I’m about to reconfigure for the native focal length of 2310 mm, so I’ll be sure to look for these effects. Thanks for watching, John!

[@oldfilmguy9413]

Great analysis, while still leaving open the possibility - that's just good science! Cheers!

[@Aero19612]

Haha. If you don’t want to see dew ring heater artifacts, just install something that produces larger artifacts! Like the Celestron focal reducer/corrector. Thanks for watching!

[@Rafastro]

And, like Paul Harvey would say, that is the rest of the story. Thank you I even consulted Damien peach on this issue. He uses see 14s for his planetary captures. He’s so temperature conscious inside the scope and its effects, he doesn’t even believe in dew straps. He’ll use a blow dryer when and if necessary. Of course, he can afford it since he only spent a short time outside, doing planetary . I feel that I will be safe installing this after your assessment. I’ll have you know that I did wait for this video before installing it . Thanks again …

[@Aero19612]

A follow-up. Check out Kurt’s (AstroQuest1 on YT) response here. Bottom line: don’t over tighten the screws when you install your ring. Quite likely that people are over-constraining the ring by over tightening the screws. The thermal stresses can be mitigated if the ring is allowed to expand a bit. But not too loose either!

[@808musicman808]

I was going crazy trying to figure out why my stars were getting spikes. This could possibly be the reason. I was using it during the winter time, but living in Hawaii , it doesn’t get that cold high 50’s at most. Since then, I haven’t been turning on the dew ring and spikes seem to be gone. I will test by turning heater on to max to see if spikes return and would confirm this. Thanks for the info!

[@Aero19612]

@@808musicman808 yes, a very frustrating “feature”. But on the good side, you live in Hawaii! Another to check is whether you installed the ring per Celestron directions about the clocking. Good luck!

[@davidleejenkins]

The Celestron Dew Ring definitely impacts my stars. I have tested by starting the night imaging with the dew ring turned off and my stars are perfectly round. Then, if I turn on the dew ring (100% power), and the stars become significantly misshaped. As a result of my test, I run my dew ring at 10% power and the stars aren’t perfectly round, but they are much better. I encourage you to try this test which only changes one variable. Clear skies.

[@operator911]

I absolutely see this too. See the spikes - and turn the heater off - ONE sub later - the spikes are gone...

[@brianwiley1009]

As you noted, I suspect thermal expansion radially (instead of through thickness) is the culprit here, causing warping/bending. I have experienced the distortion on my EdgeHD 925 and have adjusted my Pegasus PPBA heater output aggressiveness and the issue seems to be under control.

[@Aero19612]

Hey Brian. You might check out Kurt’s (AstroQuest1 on YT) comment here. He found that backing off on the installation screw torque resolved his star issues. Thanks for watching!

[@BruceMallett]

I am among those who have seen star distortion when using the heater ring and yet a dew strap around the front of the tube is not causing distortion. The distortion has been enough that it stood out in my images, even though I don't do the star-pixel-peeping thing. ;). Because of this I keep the Pegasus box's output very, very low to the heater ring and (as mentioned) added a dew strap around the front. I agree with other comments, I suspect the issue is not due to diffraction changing across the thickness of the glass so much as the change in temp. at the outside causing expansion that bows the glass radially, that will in turn cause distortion. One of the CN threads has a good sequence of photos taken at different power levels showing the effect quite clearly.

[@Aero19612]

Hi Bruce. One thing you might consider is Kurt's response (@AstroQuest1) here. He found star deformation as well, but then backed off on the clamping force of the installation screws and that seemed to solve his issue. It would be quite odd that I see no effect and others see so great an effect.

[@BruceMallett]

@@Aero19612 Thanks for that. I went out and checked the screws, they seemed not so tight already but I tried backing them off a bit. If we ever see a non-rainy day here again I'll test to see if there is any effect.

[@_Astrovert]

Love your videos, thanks for the time and effort you put into them!

[@Aero19612]

Welcome to the no-dew zone. Thanks! And thanks for watching!

[@williamhouse5081]

With my RASA 8 I don’t run the dew heater ring in the summer, even at low settings it builds to much heat in the dew shield which causes distortion and cause my camera to struggle maintaining temp. For summer use the camera generates enough heat and airflow to prevent dew build up. However it works great in the winter.

[@Aero19612]

Interesting. I had not considered the RASA/Hyperstar configuration. Thanks for the comment, William!

[@chrislee8886]

With my 8”SCT i can confirm (and have tested this) that i get a noticeable single star spike as the dew ring exceeds 20% and some weirder looking stars above 40%. I get no such spikes when using a dew band (the reason i changed to a dew ring was the ease with which to add my dew shield vs the band and the less than effective performance of the band during some heavy dew evenings. I have tweaked the torque of the dew ring but with little effect. I had wondered whether scopes that have cooling fans or vents might avoid this better than my “fully closed” system. I notice that if you defocus the stars by quite a bit you can see the thermal distortions appear at the point heat enters the ring and you can see these switch off when the power drops. Enough of this thermal disturbance might cause poor signal to reach the sensor (so not a glass distortion). Hence my query about scopes that are not thermally closed?

[@Aero19612]

Very interesting, Chris! Something sure is going on. I like your thought about open VS closed OTAs. Mine is closed. I was having good luck with the dew strap, so at least that's still a viable option.

[@PRASTRO]

Excellent information like always 👍🏻 Thanks!!!

[@Aero19612]

Glad you found it useful. Thanks for watching!

[@peters6500]

thanks again for that excellent presentation! ....maybe those who are experiencing distortion have a kind of external tubus seeing (like inside of Newton telescopes) due to the induced seeing in front of the corrector plate? maybe even more if an additional dew shield is in place?

[@Aero19612]

Good thought there, Peter. I think a local seeing disturbance would tend to produce an overall blooming effect rather than the “spike” effect that folks are seeing. There have been some good thoughts presented in the comments: 1. Gap in the corrector plate support ring I side the tube allows concentration of higher temp at the two gaps 2. Over-tightening the installation screws provides too much constraint and the corrector plate can’t expand with the heat. Thanks for watching, Peter!

[@ronlundgren4391]

Great analysis as always. Thank you.🍻

[@Aero19612]

Thanks, Ron!

[@IceNein763]

A lot of times people will say that something causes distortion without providing useful alternatives. Yes, it is true that a slab of glass with an uneven temperature will not be shaped exactly as it was designed to be, but what is the alternative to a dew ring? Dew. Dew has a worse impact on your image than an imperfect corrector plate. So unless someone has a useful solution, the appropriate action is to use a dew heater at the lowest setting that will guarantee no dew formation over your imaging session.

[@Aero19612]

Hi Clint, Dew can shut things down fast. There are a couple of things in your comment to respond to: 1. There is an alternative that works well. I had been using a dew strap wrapped around the end of the OTA with a shortened dew shield wrapped around that. It always worked (I have some vids on it), but I hate dew and the dew ring seemed like the nuclear option. 2. People are blaming the dew ring heat for the star distortions, but I’m not sure that’s the case. The comment from Kurt (AstroQuest1) shows that over tightening the screws can cause the deformed stars. The addition of thermal gradients will make things worse. Based on my video, I’m not seeing heat related issues, but I also didn’t over tighten the screws. Thanks for watching!

[@carlosespada6202]

Excellent repport! Thanks!

[@Aero19612]

Thanks for watching, Carlos!

[@GalacticConquerer]

Some comments: - Running both the ring and heating band/strip at full power and then comparing is not an “apples to apples” test, since the ring is much more efficient at heating the corrector. This is because the ring heats the glass directly, instead of the strips which heat indirectly through the lens cell. So comparing the band/strip at 100% vs. the ring at say, 50%, is probably a more fair test. - If you need to run the ring at 100% to prevent dew, then dew conditions are very very bad, and imaging will be poor anyway. I highly recommend using a Dew Heater Controller. - In bench testing I have done, there is no difference in star images whether a band/strip or ring is used, and corrector is at the same temperature. So for folks that are blaming the ring for poor images, it is either something else or you are running the ring with too much power.

[@Aero19612]

True. My test was only apples to apples in the sense of power usage. I suspect the ring can be used at a fraction of the current and still provide the dew protection. I have tried it at 50% and it seemed to work fine. There's more experimenting to be done on that front. Also, I'm not that bothered by the streaks that some people see in the brighter stars. I hate dew more than I hate streaks.

[@dumpydalekobservatory]

Interesting video I've got a RASA 11 with a dew ring I used a dew band previously & found no difference between the two, I do suffer with a bit of tilt on the imaging train but to be honest I'm not that bothered about it as the performance outweighs the downsides even though it has given me some grief in the past.

[@Aero19612]

This hobby is all about which grief bucket gets the treatment. My star shape grief bucket is pretty empty. My SCT focal reducer grief bucket overfloweth. Se la vie. Thanks for watching!

[@GregMcCall]

If the glass needs to be heated to a certain temperature to prevent dew, then that’s what you have to do if you want to prevent the condensation. I can’t see the difference between being heated by a dew strap or the celestron heater. What I can see is that the on glass heater could be over driven (so to speak) due to its better ability to heat the glass. I’m guessing one might be able to heat the glass too much with a 13.8v supply with good wiring (ie. little voltage drop) and just running it at 100% (no controller) in a warmer climate. I would argue that if you keep the glass just warm enough to prevent dew, you would have no discernible effect and your video shows that. I use the Celestron dew controller to keep the surface above the dew point because it utilises the thermistor in it’s feedback loop to control that temperature. I noticed that Prima Luci now has an adapter to allow their sensor input to utilise the celestron sensor as their system also utilises a feedback look in controlling dew. I think celestron contributes to the issue by saying in the manual that the dew heater can be connected to 12v which I think no dew heater should be connected as such.

[@Aero19612]

Hey Greg. There is a school of thought that heating the air close to the glass is good enough to prevent dew. I know there seems to be a lot of folks seeing deformed stars and blaming it on the nonuniform heating, but I'm not seeing that even at 100% power. Could be they have over tightened the screws which fights against the thermal expansion... Better to not crank down on those screws too hard. The dew strap isn't nearly as effective of a heater as the dew ring is. Plus, its heat has to get through the thick aluminum OTA ring before it can reach the glass. Thanks for watching!

[@aagifford]

How do you know it’s not the difference in CTE between the dew heater and the glass? If the are clamped together then as the dew heater warms up and expands while the glass is colder and remains overall smaller it will pull on the glass around each screw.

[@robertvanvugt3371]

Well, I would assume that the standard dew heater band will behave the same as the new ring heater in that it will heat unevenly as well with it more effective at the outer edges of the corrector. Maybe the screws are being re-installed too tight?

[@Aero19612]

I don't think so. First, the dew strap generates far less heat than the dew ring. Second, the dew strap is wrapped around the tube so there is more diffusion of the (smaller amount of heat) through the metal and then to the glass resulting in a more uniform and lower heat increase. Finally, I'm not convinced that heating the corrector plate is the only way to prevent dew (though that will work). In my case, the dew strap overhangs the end of the SCT tube by about half its width and probably does more to gently warm the air in front of the corrector plate than warm the corrector itself.

[@robertvanvugt3371]

So you think the corrector lens will be the same temperature at the outside edge as it will be in the center near the secondary. I seriously doubt that with either type heater with the larger aperture scopes likely more of a problem. It may well be the new ring heater gets hotter due to being directly in contact with the glass unlike an external band type and the higher heat is causing the issue or maybe overtightening the ring heater corrector screws but there is no way the corrector lens is the same temp from edge to center, also consider variations in the corrector/lens thickness effect on temperature.

[@Aero19612]

@@robertvanvugt3371 No. The corrector pate will always be warmer closer to the heat source. The dew ring gets hotter just based on its design (I have videos where I measure the heat of the dew strap and dew ring - the ring is very hot compared with the strap. Some have commented on the screws. I don't think that's the issue. The screws don't directly clamp the corrector plate they would if the screws went through the glass. The corrector lens heat-induced thickness would not produce the "diffraction" line artifacts.

[@robertvanvugt3371]

@@Aero19612 So you agree with my initial post that the corrector glass will have temperature variation as it will always be cooler toward the center farther from the heat source. We all know SCT type scopes require thermal equilibrium before use and without achieving this prior to use optical results are degraded thus the reason for CatCoolers, fans and cool down periods. This is a fact and readily visible within my own SCT which produces diffraction line artifacts on stars when it is not at temperature equilibrium so saying that temperature does not cause diffraction line artifacts is incorrect, it surely can and does at my end. As well I agree the screws themselves present a likely minimal effect on this but the torque on the screws can distort the glass much as they do on a mirror cell clips and the tighter they are installed the more heat will be transferred from the ring contact surface to the glass lens thru the direct contact of the ring heater to glass surface not thru the screw itself. Just as temperature changes effect focus over a nights shooting temperature effects glass shape/size with naturally larger temperature variations causing more severe optical degradation thus larger aperture scopes have more issue with this. I would not know what the temperature differential is between the two heaters as that is determined by many factors, heat element design, power settings, power supply, direct or indirect contact with lens etc... but I would assume power settings can make them equal. Since you say the ring gets much hotter than strap type heaters maybe many are not realizing they need to turn down the power?

[@PATTACAT72]

Like a telescope God.

[@GrundleStiltSkin]

i have an edge hd 8 with dew ring and imx 571. i get the flaring of stars when dew ring is turned up too high. I, like many others did not buy the regulated celestron dew controller. Turn down the power to the ring and stars will be fine.

[@vk3dgn]

I can see how a large change in refractive index across the plate might move the position of a star but not change its shape. The amount of glass the starlight passes through is so small, I don't think a change of shape is at all likely. The engineers at Celestron would be all over this sort of thing.

[@Aero19612]

Well, if the refractive effect were much larger, then I might expect a tear drop kind of artifact point toward or away from the center since the glass temperature varies along radial lines. But I don't see anything (or maybe its burried in the focal reducer artifacts, haha)... Thanks for watching!

[@vk3dgn]

@@Aero19612 I've got much bigger problems imaging than the effects of heat on refractive index. The best use for the reducer is getting the back focus to 105mm instead of 139.05. Thanks for the well thought out, and engineered, videos - I'm learning a lot.

[@deep_space_dave]

Update: I did a test last night where we had very heavy dew and I used my smaller pixel camera and guess what happen when I turned on the dew heater at 60%? I got JWST spikes on my stars! I turned it off and waited a while, and the stars were round again! This is very disappointing, looks like I will buying a dew band 😞 Thanks for bringing this to my attention! I am half way through the video but I felt it was important to share this before I forgot 🙂 Without posting a link, google search for starbright-xlt-optical-coatings and on the page that Celestron provides, you will find this bullet point "High-transmission water white glass is used instead of soda lime glass for the corrector lens. Water white glass transmits about 90.5% without anti-reflective coatings". Schott BK7 from the shallow rabbit hole went into researching this is in fact pretty close as it is a type of crown glass. From my RASA 8" using a camera with bigger pixels like a IMX571 I don't see any weird spikes in my stars but when I use a camera with smaller pixels like the new IMX715 camera made by QHY, I see odd spike and I now noticed that my stars are hexagonal like I imaged from the JWST! This makes me want to buy a dew strap to see if I can tell the difference. I thought it was guiding but I have a iOptron EC mount and still see this without dithering... This is a interesting discussion! Thank you for the video and clear skies!

[@Aero19612]

Very good info, Dave! Interesting note about the pixel size. My current setup uses the 4.65 um pixels in the ASI294MC. Of course, I don't expect full resolution with small pixels on an SCT - I assume they'd get eaten up by seeing. If you're still in the experimental mood, try backing off on the dew ring installation screw torque. Just enough clamping force to keep the corrector plate in place, but "loose" enough to permit thermal expansion....Maybe the 8 screws (I think there are 8 screws) could be responsible for the spikes(?).

[@deep_space_dave]

@@Aero19612 Actually I did that before I started testing so it must be some sort of uneven heating. Oh well I will be using my QHY600M until I get the dew strap. Can't live without dew control when we have humid summer nights 🙂 At least I finally proved to myself it is not tracking after all!

[@Aero19612]

Indeed. 90% of all problems is narrowing down which “thing” is causing the issue. Clear skies!

[@anata5127]

What is difference between dew ring and dew shield. For me, dew shield is absolutely essential; it shields scope from side lights coming from everywhere (my house, other houses, street lights, etc). I have not seen that dew shield do anything to stars.

[@Aero19612]

The dew shield is not likely to prevent dew for the most challenging nights (dew point = air temp) unless you have a heated dew shield (eg, AstroZap). The heated dew ring replaces the non-heated ring on your SCT that holds the corrector plate in place (check it out on the web). Some users claim that the relatively high heat applied to the corrector plate is causing stars to appear deformed. The dew ring will apply a lot more heat to the glass than a traditional dew strap, but I’m not convinced that’s causing the stars to be affected. Kurt (AstroQuest1) says that over tightening the installation screws will cause the deformed stars. I use my dew heater ring with my dew shield.

[@anata5127]

@@Aero19612 Ok. My one is a heater dew shield from AstroZap. I asked them to do it 8”-long. They did. It easily keeps away 99% humidity.

[@davidleejenkins]

The Celestron Dew Ring definitely impacts my stars. I have tested by starting the night imaging with the dew ring turned off and my stars are perfectly round. Then, if I turn on the dew ring (100% power), and the stars become significantly misshaped. As a result of my test, I run my dew ring at 10% power and the stars aren’t perfectly round, but they are much better. I encourage you to try this test which only changes one variable. Clear skies.

[@Aero19612]

You might try backing off on the installation screw torque. See Kurt's (AstroQuest1) response here. He saw some artifacts as well and re-installed the ring without clamping down so tight. A little bit of constraint will prevent the corrector from rotating but still allow thermal expansion to take place.

[@Steve_The_Ignorant_Astronomer]

So I am having major problems with the dew ring. I am using the 9.25 Celestron just got it and thought it was a bad scope but it is the ring. If I can send you a looped video of 6 frames that I blink you will see how it gets worse as I turned up the heat. I started 0 the 10 % 30% 40% 70% and 100... So now I have a ring and wont use. But will like you to see the video. Thx

[@Aero19612]

Sure. Send the video to the email address in my "About" section. I saw something interesting the other night and am planning to make a video about it. There may be some clues on how to "fix" it (assuming this is what you're seeing).

[@Steve_The_Ignorant_Astronomer]

Sent you the video , hope this helps in your investigation and we can solve this problem

[@Steve_The_Ignorant_Astronomer]

James could not send video , did you ever get images ? @@Aero19612

[@Aero19612]

Thanks! Yes. I got the images. Your effect seems much more noticeable than what I'm seeing. Maybe because you're pointed at a star cluster at native focal length(?). I am planning a follow-up video on this topic with some other observations, so stay tuned.

[@markdeslauriers241]

I have star distortion when the ring is on high. When i turn it off stars within 10 minutes the stars are back to normal. I can do this on night on my 9.25.

[@marchoude2303]

I just purchased the celestron dew ring but haven't used it yet. Will I still need to use a dew shield?

[@Aero19612]

You can get by with a shortened dew shield. I cut mine down to project beyond the OTA by about 3”-4”. Maybe could try with no dew shield but you may have to dial up the power on the dew ring. Some people, including myself, see star bloating and streaks on some bright stars at high power.

[@JoseLausuch]

It seems after 2 nights of usage at full power, it destroyed my corrector plate.. my stars now look elongated without even switching it on... Is there a way to revert that back?

[@Aero19612]

Hmm. I’ve never heard of the high heat setting actually permanently damaging the corrector. I used full power for many nights until, I cut things back. No permanent damage in my case. You should verify if there is not another cause. First, take some very short exposure (like 2 seconds) images with the fan off and then with the fan on. If the correct is in fact deformed, the stars will be elongated with the fan off. If they are elongated when the fan is on, then you have fan vibration problem. The very short exposure will eliminate guiding as the source. If you’re still getting elongated stars, check collimation. Good luck!

[@sevenskiesastro]

Excellent summation. Pixel peeping don't help anyone. Maybe we just should enjoy taking astrophotography pics rather than trying to beat hubble.just too many variables seeing being one of the most influential. Clear skies

[@Aero19612]

I find this hobby to be challenging enough without worrying about how a star looks when you zoom in and fill screen. But I guess each person pursues the hobby in their own way. For me, I'll take a dew-killer solution even if it gives the brightest stars a few spikes. Thanks for watching!

[@Microtonal_Cats]

I think people who see distortion, it's from tightening the screws too far down, and THAT is bending the glass.

[@Aero19612]

I thought that when I first heard of the issue and some people have loosened the screws and see an improvement. But...there are effects that don't come from the attachment. I need to put out a follow up video once I've tested a potential fix.

[@RobB_VK6ES]

The discrepancy is obvious James. Analytical mind vs the paranoid mind :)

[@Aero19612]

Also helps not to stress about artifacts that can't be seen unless you go looking for them. Haha. Thanks for watching, Rob!

[@nikaxstrophotography]

too much maths for this little black duck.

[@Aero19612]

Haha. I do the math so you don’t have to! Some viewers will want to know where I got my numbers/conclusions, so I provide the extra detail for them. It’s perfectly OK to ignore the equations…. Thanks for watching!

[@nikaxstrophotography]

@@Aero19612 Mate I tell you now I would NEVER do the math I don't see the point I don't understand the reason why some get all mathematical when it comes to explaining something that seems elementary without all the squiggles and numbers which don't mean much to the lay person. In all honesty how many people would understand the maths behind it rather than the elementary explanation? I love your videos don't get me wrong but I caome for the astronomy not a physics lesson

[@neilhankey2514]

This is why I have stay away from these Schimdt Cassegrains. Seem silly to point all of that glass at the night sky, especially when the trend is to use on axis optical correctors. It's just my preference. Great informative video as usual. The RASA's are really tempting telescopes but I'll pass for now.

[@Aero19612]

Thanks for watching, Neil! SCTs present a real challenge for dew prevention. But it is possible to image throughout the night with no dew problem. I was having good results with my dew strap, but this heated ring idea seemed really promising so I jumped in. Some people are seeing star shape effects, but I'm certainly not. Could be that people are over tightening the screws during installation.

[@neilhankey2514]

@@Aero19612 I think you're right, pitched optics / miss-algined are more likely the cause of their problems. I can not see of the heater ring causing this. As you discovered the thermal co-efficent of expansion for glass it really low.