Comments on the Use of a Dew Strap and Dew Shield with an SCT

[@Tedterranova]

I really love your content. Your videos on the Celestron dew rings convinced me to stick with my dew straps. Thanks for taking the time to collect all this data and share it with us in such an informative way. I still wasn't able to tell from your testing if the strap should be on the dew shield, heating the air in front of the glass, or on the tube heating the air in the tube. Thanks again for all the work you do.

[@Aero19612]

Hi Ted. Thanks for watching. I’ve heard different thoughts on that. I put mine around the foam dew shield just in front of the corrector plate so that it can heat the air in front of the corrector plate. I just don’t see how the (seemingly) small amount of heat these dew straps generate can heat the OTA tube and the air behind the corrector plate. I am planning to get a second dew strap and place it adjacent to the one I have. One thing about that stupid Celestron dew ring is that it definitely kills dew. It’s just the other stuff it does that’s the problem. Good luck!

[@Cardnim]

Never watched one of your videos before, but thoroughly enjoyed this one. Refreshing to see a YT video which uses accurate science and logical reasoning to analyse the problem. Great job, very helpful and thanks for posting. Clear Skies from Ireland!

[@Aero19612]

Glad you enjoyed it! Welcome, and clear skies from Texas!

[@jamestickle3070]

Your videos are always well considered experiments and data. I find myself also having to try different options and hacks to find ways to help my mount and optics do what they should. I’m glad you are doing your thing.

[@Aero19612]

Thanks for watching, James! Yes, this seems like it should be easier but I keep having to deal with "stuff"

[@danjensen9425]

A few months back I came across your outdoor setup , a 2 part series. I just got my celestron c8 and cgem2 mount. So been three months of a few nights of imaging . I put a focal reducer corrector on it before first full moon images . So happy with results . Then Orion Nebula just one image came out nice enough for me . Now I know more . So out to a bortle 4 and m51 . Stacking lights darks flats and bias , about 20 each . Now I’m hooked, scheduled time and budget enter my head . So here in Northern California I can drive to a bortle 4 or a bortle 2 . Duh the bortle 2 right . No , 4000 foot elevation and 80% humidity and took dew shield off for bhatinov mask focusing and instantly a dewed up situation. Now I go back to your videos . And I find very useful information on not only the dew problem , have dew heater now , but also getting into autoguiding . I wondered about size of guide scope and use of focal reducer . Did you post image results of m51 . I also have a spacecat for nebula imaging . Two very different scopes . I can probably get away with one camera for guiding , just a guess , but not the guide scope. Any thoughts thank you for your efforts on helping us newbies

[@Aero19612]

Yep. You're hooked. Glad you've been watching my "learning fest" video series! Also, I'm happy to hear you have a SpaceCat. That's an awesome scope and a good counterpoint to a C8. Dew is the big issue for me right now. I've been doing some recent imaging with a shorter-length simple dew shield combined with the dew strap. It's been working well. So far. I've been stealing a night here and there that I share between M106 and M51, so no final results yet. With any luck (and no dew), I should finish the Lum filter tonight. Then I will devote a night each to R, G, B, and Ha as the moon becomes increasingly bright. The forecast suggests I'll be done next week. Forecasts have been wrong in the past though. Once I'm done, I'll do a video comparing these results with those I obtained when I first started this hobby with a DSLR. Should be embarrassing! Either because of lack of progress or how poor the original images were. I'll also have a video update on my findings relative to dew prevention. Thanks for watching, Dan!

[@Aero19612]

I think it would be perfectly fine to use one guide camera for both scopes. A small OAG like the ZWO OAG would be very good for the SpaceCat. At that focal length, you won't have any trouble finding a guide star assuming your guide camera is sensitive enough. If you use a guide scope with your C8, you can get away with a guide camera with a smallish sensor such. If you go with an OAG for that scope as well, you may want to use the Celestron OAG (larger prism), and then you'll want a guide camera with a larger sensor to make use of the larger prism. If you always use the focal reducer, that will help in finding guide stars. Try our different options in Stellarium. It's free a vert useful for looking at fields of view for the main scope and OAG with difference camera sensor sizes.

[@the27thMONKEY]

Very useful video, first few clear nights in a while (moon still out though) and I've found my dew heater is losing the battle against frost forming. Up to now I've gotten away with just the heater but now I'm going to 3D print a dew shield. This video will help me to decide where exactly I need to place the heater with the shield for maximum effectiveness! Thank you!

[@Aero19612]

My dew conditions may be less severe than yours, but I found that a homemade poster board dew shield that projects 3" beyond the end of my C9.25 works well with the dew strap wrapped around the dew shield hanging half way over the end of the SCT. Good luck!

[@steppen57]

This is very informative. I also live in Texas, and having challenges with dew and my 8 in Celestron. I already have the Dew Shield which doesn't do much, and I am researching the heating straps and controller. Just about to get one and try the combination as in your video. Thanks for sharing!

[@Aero19612]

Thanks for watching! This was one of the "success stories" for me this past galaxy season. I use a homemade poster board dew shield that overhangs the OTA by about 3" and I wrap a DewNot dew strap around the dew shield so that it overhangs the end of the OTA by 1" or so. I power the dew strap off of the Ultimate Power Box. I have not had dew problems since adopting this approach. Good Luck!

[@steppen57]

@@Aero19612 Thank you! Just received the dew strap and power box. It will be cloudy the next few days. Will let you know my results.

[@Aero19612]

Excellent. Good luck!

[@CuivTheLazyGeek]

Thank you so much for this - this is extremely useful. Dew has caused me all sorts of trouble with my SCTs when imaging in the mountains. And is one of the reasons I switched to a Newt. Still, I have a C9.25 I want to take to darker zones for visual, and knowing how to protect it will be very useful to me.

[@Aero19612]

Thanks, Cuiv. I'm not out of the woods on this issue yet, but thanks to the great comments and suggestions I'm getting, I feel like I'll finally have a solution. There's no room for lazy geeks in this hobby (present company excepted)!! We all have to work hard at it.

[@MrBebopbob]

Hi James. As a former aerospace thermal analyst, I would make the following recommendations. 1) If possible , put the the few heated inside the few shield and on the forward cell (biggest chunk of metal in contact with corrector plate capture features. 2) Add additional insulation strip around OD of few heater. The foam dew shield (if installed over the few heater, should improve the fraction of heat going to scope). In Texas, on a clear night, radiant heat loss to "clear" sky will suck a lot of the available heat away. it is all about differential thermal resistance. Good luck and thanks for all the interesting and informative videos. Bob

[@Aero19612]

Hey Bob. Maybe we crossed paths. I worked with a few aerothermal engineers in my Navy missile engineering days. Totally agree! I want to test a "short" (maybe 8" extension) dew shield with the dew strap inside (radiating toward the center). Mike, a commenter here (and my dew prevention patron saint), has said the goal is to heat the air in front of the corrector plate. There is a solution! I just haven't found it. Fortunately I've got you, Mike, and others providing great ideas...Thanks for your suggestions, as always!

[@MrBebopbob]

@@Aero19612 Hi James. I started my career working on Trident II in the 80s. I worked for Hercules/ATK until my retirement a few years ago. Where do/did you work? A shorter dew shield will still provide a significant reduction of the view factor between your corrector plate and the sky (which is always way colder and is the predominant heat loss from things facing up at night). Optimally, you would want the inside of your dew shield to be just above the dew point temperature to avoid having unnecessary convection currents.

[@Aero19612]

I worked with the Applied Physics Lab in Maryland. I visited several ATK facilities over the years. I believe there's one in Utah (unless I'm mistaken) and one in Elkton, Maryland. I'm still working. I moved back to Dallas to join some friends in their structural engineering firm. Congratulations on your retirement! I find that work keeps interfering with my astrophotography. Yep, I gotta go with a short dew shield. As you say, even a short one (8" maybe) will reduce the view factor. More experiments!

[@hbmike47]

James ! Glad to see you are making progress on this particular stuff. I like your videos because you really attempt to analyse the problem and then develop some experiments to improve your understanding. My impression is that you're impressed with the performance of the dew strap and shield when you located the strap where I suggested, but are now puzzled because in the tabletop bench test you only measure a very modest rise in temperature. Now you wonder if all the work is the dew shield and if the heater is contributing that much. Let me assure you, it is. Your bench test measurements are not capturing the dynamics of how the system prevents dew formation. The system works because you are creating a column of air that has an insulating effect in front of the collector. That column of air is warmer than surrounding air because it gets a small amount of the radiant heat from the collector as it cools. What you didn't capture in your bench test is the accumulated heat for the entire column of air. Also, the heater will be more efficient when the ambient temp isn't already at 23. With that in mind, the most efficient location is inside the dew shield at that base just in front of the collector with the hot side of the heater towards the center. This takes advantage of the insulating material of the dew shield. Also be sure the seal on the dew shield is tight around the scope, as any air gaps there will create a chimney effect and will draw the heat out the top. How short a dew shield can you go? I suggest if you try making a short one that you use a cheap yoga mat, because they're made of flexible foam that will help insulate the column. Just remember you are heating the air, not the collector plate.

[@Aero19612]

Yes. Some progress. Thank you for your excellent comments on this topic. You're really helping me to get my head wrapped around this issue. Interesting suggestion regarding the need for a good seal on the back end. The dew shield fits well, but it's worth a check. Thanks, Mike!

[@raymondb8596]

Wow - nicely done! 1st impression of you video is A+

[@Aero19612]

Thanks for watching, Raymond!

[@seyahneimad]

Very informative. Thank you. I’m trying to figure out the best dew strap, controller, and power tank arrangement for my 6SE. New to the hobby and it seems like every time I start to find my groove on a night of planetary imaging, that’s when the dew sets in. I have a shield, but it’s not enough for Florida nights.

[@Aero19612]

I ended up with a homemade dew shield (made from poster board and overhangs the OTA by 3") with a DewNot dew strap wrapped around the outside and 2" back from the end of the dew shield. I can get through a full night's worth of imaging on a bad dew night now. But I'm sure Florida brings a whole new level of pain as far as dew is concerned. Good luck!

[@paultadrick8801]

Hi James, I have the same setup with a few minor differences. I use the pocket power box set to auto and I have the Edge HD version of the ota. I've always had the strap under the mounting bars like your original setup but I always use the dew shield with it. The dew shield doubles as a shade from my neighbors lights so it's a must for me. Maybe something simple like wrapping an Ace bandage around the strap would help hold some heat in and provide better contact. The good news is I haven't had any dew in 2 years using both the strap and dew shield combination but it's much more sensitive to wind.

[@Aero19612]

Thanks for the feedback, Paul! The dew shield is a huge help from a dew perspective, but I, too, like the stray light control it provides. It's just the DARN wind. If you have the dew problem solved and can tolerate the added wind sensitivity, sit back and enjoy a nice adult beverage. I feel like I'm close to a long-term solution. I'm leaning toward mounting the dew strap inside a much shorter dew shield. Mike (fellow commenter to this video) has provided some great insight along these lines.

[@MrProulx]

Have you tried using a material called Reflectix and wrapping your telescope with it? It can be found at any home building center. I have not had dew or frost on my SCT using this and a heater. I have never used a dew shield.

[@Aero19612]

Hey Matt. I have considered insulating the telescope tube to minimize focus change but had not considered it for dew control. Interesting idea. Right now, my shortened dew shield and heat strap are working very well. Thanks for watching!

[@foxglovemead]

James interesting stuff that you do. The cable extension vs no cable extension; I would expect a dew shield controller to use a current source to limit the current so the full on current would be constant. When/if you turn the duty cycle down then the reduction will be what you expect regardless of using/not using the extension. So it performs (essentially) the same. But if you had used a "cheap (unsophisticated)" dew shield controller then the extension could be significant. When you put the dew heater on the outside of the dew shield you put it against an insulator. If there was not insulator you would expect half the heat to go outside and half to go inside. As it is only a smaller percentage goes to the inside. If you had put the strap around the SCT metal before the corrector plate, then any small variations would even out as the tube acts as a heatsink and conducts from hot tot cold areas. If the tube radiates some heat out and some inside then like before the efficiency is reduced. But because you are heating the air inside the SCT it is constrained so it's more efficient than heating the air outside. If you wanted to really "nail it", you might remove the corrector plate and position a heat belt close to the outside tube but separated from it. The heat would go into the air and relatively small amount would be wasted heating the tube. I suspect most users would prefer the simple approach so they would be OK with using the band around(below) the corrector plate and using a bit more current. If you stop to think, what does a dew shield do? It has two effects; it stops stray light entering the tube. So it needs to be opaque and most are black. Also it reduces the off axis "cold" striking the corrector plate. You can't cover the plate - because you don't get an image, but you can cover the incident "cold" striking the plate. If you shorten the dew shield it's effect is reduced and a rule of thumb is that it should be as long as the diameter of the tube. Half as long will be less effective. Twice as long will show little improvement. So for a reasonable length, the main issue is the area of the shield that is subject to wind. If you put holes in the dew shield the surface area would be reduced but so would the shielding. Half the area would also be half as effective e.g. much like reducing the effectiveness using a shorter dew shield. There is one potential solution: block the cold (light) without increasing the wind resistance. One example of this in your house might be a slatted blind near a window - it blocks the light (somewhat) but has little resistance to wind (compared to a blackout shield). So a dew shield made of slats (or tubes) would block the cold (light) but offer little resistance to wind. Something to experiment with? In your backyard you have a solid fence. It blocks stray light but is also affected by the wind. If you also wanted to block light but wanted to limit reduce the wind resistance, you might try a blind e.g. two layers with gaps but with light blocking without the wind resistance. Similarly a dew shield needs to be constructed to solve the main problem without introducing a second problem. One limitation of a dew shield is cost. If the cost was "astronomical" then no one would buy it. You could look for a material that has the desired properties but has the effect you want without the drawbacks e.g. a very open cell foam supported on a frame (like fabric on a lampshade). Simon

[@Aero19612]

Thank you for the thoughtful comments, Simon! Your comments about cable length and power supply totally agree with what I found. As for the dew shield: I tested a shorter-length "paper" shield last night with the dew strap wrapped around the outside. It worked very well. No dew at all. Its length is 8.5" from the end of the corrector plate (9.25" aperture), whereas the Astrozap is 14" long. There was a bit of wind, but guiding went OK. If I can confirm this shorter shield length works for dew prevention, I will cut down the length of the Astrozap shield and mount the dew strap inside the shield, just in front of the corrector plate. That's my plan. Thanks again for the comments!

[@Dennis-tf2cs]

You can purchase heated dew shields. There is also a dew heater strip for SCT's that mounts right inside the rim near the corrector plate. The combination of the two has never failed me.

[@Aero19612]

I bought things in the wrong order: dew shield then dew strap. I like the idea of a dew strap inside the rim. I agree, my setup has been working great. Thanks for watching!

[@fredmercury1314]

What if you put the strap on the inside of the shield, up against the from of the scope, creating a pocket of warmer air inside the shield?

[@Aero19612]

That would be better. I tried to do that but didn't have high confidence that the dew strap would stay attached to the surface and not drape into the field of view sometime during the night. Since the outside mounting has been working for me, I just stayed with that method.

[@TurboTalksTech]

James, you talk about the air being heated, but the whole point is to try and get the heat in to the glass. I'd recommend putting the dew strap as close to the corrector as possible on the cell, then use the dew shield on top as an insulator to radiate more heat inwards (as well as the physical protection aspect). Direct contact to try to get the warmth in to the glass is the idea.

[@Aero19612]

Thanks for the comments! My experience shows my C9.25 dew up if the dew strap is on the cell or on the tube behind the cell. Recent experience also shows no dew forms if the dew shield is used (alone? or) in combination with the dew strap. Recent testing/imaging with a half-length dew should has also been successful (reduces wind disturbance). This dew issue is actually quite complicated. Like you, I thought that heating up the corrector plate is the objective. And that may well work if you can heat it up enough. I have serious doubts whether that little dew strap has the heating capacity to heat up the cell (or tube) and also heat up the corrector plate. Mike (commenter in this section) has brought up an alternative view that heating up the air mass in front of the corrector plate is the objective. I have more faith in the dew strap for this role. As I understand the mechanism, higher temperature air has a greater capacity to hold moisture. When the air temperature drops to (or close to) the dew point, the air must get rid of the excess moisture. If you heat up the air, you maintain its capacity to hold onto the moisture. Also, some of the heat in the local air column will transfer some (probably very small) amount of heat into the corrector plate. My current objective is to find the minimum-length dew shield that achieves the dew prevention objective while reducing the increased sensitivity to wind. Then, I will attach the dew strap inside the dew shield next to the corrector plate to more effectively heat that local air column. Thanks for watching!

[@dfinchermd]

What a great bit of informational research and some solid technical responses. Based on reading through all, for my C8 Im tempted to forgo the dew shield initially (for dew) but I was considering buying one for the stray light reduction in my Bortle 8-9 zone. It wasnt mentioned so do you see any benefit in that by using the dew shields?

[@Aero19612]

Thanks for watching! A dew shield may help to reduce light from neighbor’s porch lights or nearby street lights. It won’t help for Bortle-type skyglow. If you have an SCT, killing the dew problem is a significant stress reducer. Good luck!

[@RobB_VK6ES]

Nearly missed my James Lamb fix. Power is off due to extreme weather here in WA. 3 cheers for the phone. It may be obvious why not put the strap between scope and shield ?

[@Aero19612]

I looked up the weather there. Looks pretty bad. I'll have to give your suggestion a try. Not sure there's enough overlap with the shield to go around the dew strap. But it's definitely worth seeing if it will work. Heck, even velcroing the dew strap to the inner surface of the dew shield might be better yet. That said, the dew shield works fine, maybe even without the dew strap, I just can't use it when there's any real wind to speak of. Ideally, I want a solution that doesn't involve the dew shield at all. Stay dry!

[@Aero19612]

...oh, and don't forget to bring your scope inside!

[@RobB_VK6ES]

@@Aero19612 I took the opportunity to make another dew strap for the guide scope today . Construction is ultra simple. After calculating the length of resistance wire I sleeved it with 180mm long sections of heat shrink tube. !80 being circumference of scope. To encase the folded up element I used 50mm wide cloth backed adhesive tape (AKA gaffers taper) top and bottom. It is not possible to solder to ni-chrome so I cut the ferules from automotive crimp terminals discarded the insulation and crimped on the power cable. Add a little velcro each end and some lashing stitches around the power cable and an RCA connector and it's done. Custom sized and rated dew strap for about $8 in materials and an hour to make. Highly recommended. I'll turn you into a DIYer yet :) Did you resolve the focus motor mounting issues? Having good success with the USB3 active extension. Night and day compared to the ethernet connection I had before. the only anomaly is the 2600 MC sometimes shows up as 2600 mono ??? not sure if it is driver related (ZWO 290 mono guide cam + ZWO 2600MC) or something to do with the USB connection. Very strange.

[@Aero19612]

Very industrious of you! Your skills are much more applicable to this hobby than my skills. I may actually try giving that a shot (instead if buying a second dew strap). I put the focuser back on with Loctite applied to that one screw that keeps backing out. Working so far. Once it backs out again, and it will, I'll take it off and see if the longer screws will fit. I also bought a M3 tap and will run through that problem hole again. Glad the cable is working for you. Seems like I saw/heard someone else make the comment about a ZWO OSC camera coming up as monochrome.

[@RobB_VK6ES]

@@Aero19612 Something I will always remember were the times in the garage with my father. Those early experiences later paid great dividends giving me confidence to explore other areas of interest. Don't be frustrated by lack of skills in a field, instead see as an opportunity to grow your skill set and at the end of the day enjoy the satisfaction and pride in "I made that" The longest journey begins with the first step. There endeth the lesson :)

[@jhk11774]

My problem is humidity and heat in louisiana. So when it is upwards of 90 degrees you use the dew heater?

[@Aero19612]

Hi Jennifer. You have it worse than I do here in Texas, but it is hot and humid here. Absolutely, I often need the dew heater when it is hot. Generally, when the dew point gets to within 5 deg of the outside air temp is when I'm at most risk for dew. Thanks for watching!

[@jhk11774]

@@Aero19612 thank you for the reply. I will have to order a dew heater strip. I have a dew shield and I recently got a mak 180 and haven't been able to enjoy it yet because of the dew problems

[@tchallalemou5307]

Had a thought. What about making a neoprene "sleeve over both ends of the telescope? I've got an 8 in Newt. Wondering if this would help w/ dew forming in addtion to my dew strap.

[@Aero19612]

It might indeed. Neoprene is flexible, but it seems like it is also heavier than the foam like materials used in my dew shield for example. I haven't imaged with a Newt before, so I'm not too clear on how a "dew shield" would help on the back end. Thanks for watching, Tchalla!

[@louisrosner7902]

All great stuff including all the replies. The problem I have with the dew shield is difficulty in taking flats. I use a lighted LED panel which I lay over the top of the dew shield. But any shifting of the dew shield and there are some internal reflections that alters my flat and I get so weird curves in my final image that are difficult to remove with processing. Any thoughts

[@Aero19612]

Interesting. I guess the flexible dew shield is shifting because of the weight of the light panel? Can you take the flats with the scope horizontal and the light panel very close to, but not touching, the dew shield?

[@RaysAstrophotography]

Does your dewshield comes with a heated strap and you are using an additional one on top of it?

[@Aero19612]

Hey, Ray! No, this dew shield is not heated. I just have one heated Dew-Not dew strap I'm using. Although, I think Astrozap has a line of dew shields that are heated, if I'm not mistaken. I would think that would be very effective as long as you don't melt the dew shield material.

[@RaysAstrophotography]

@@Aero19612 Thanks James!

[@leoncorns1450]

Really liked the science on this. I have a 8'' SCT and dew is a real problem in the UK. I don't want to spend too much money so I'm going to make my own dew shield from a yoga mat. I also have a wine fermentation strap that I will use to give me some heat. But I don't know where to place the strap/heater. Over the dew shield level with the corrector lens or in front of the glass so it only heats the air inside the dew shield? What would you suggest?

[@mrspock2al]

James - thanks for the video! I just purchased a C9.25 and I know the dew issue will bite me this summer. I haven't invested in a dew shield yet so your video and experience is very timely. Please continue with posting your "experiments". Does the heater cause any image stability issues? Also, thanks Mike for your contribution. You raise some interesting thoughts especially the yoga mat. Would you advise that I try that first before investing in a commercial product? Would you line the mat with black flocking of some type?

[@Aero19612]

Hey Alan. I've bee experimenting with a shorter "dew shield" I made from an $8 posterboard. I have the dew strap wrapped around it. No dew so far. I have a couple of good imaging nights coming up and will test further. Afterwards, I'll post a video on the results, so hold off on the $60 purchase if you can. No, I think the dew strap heat effects are nothing to worry about compared to seeing, mound gear harmonics, and of course, dew. Thanks for watching!

[@Praetorianguard2001]

Dude that was a lotta needless nerdiness…thanks!

[@kayedsss]

Professional ! As always.

[@Aero19612]

Thanks, Kayed! The SCT is the only scope I have a dew problem with.

[@Astro-George]

You should create a sky box. Takes care of the wind.

[@Aero19612]

Yep. I've been thinking of "simple" solutions along those lines. But my idea of "simple" has yet to be achieved in any of my imagined concepts. Still on the table, though. Thanks, George!

[@HollomanUFOLanding]

So my Dew Nots arrived today and here are some inital findings - Ambient temp of my basement where telescope is currently stored = 72F Temp of Dew Not strip laid out on (plastic) table = 130F after 20 minutes Temp of Corrector Plate metal shield (inside and out) and OTA before applying Dew Not = 72F After 30 mins, temp of inside of the Corrector Plate metal surround directly adjoining gllass was 84F The outside temperature of the OTA ranged from a high of 84F at the Corrector Plate to 73F at rear cell end. After 1 hour temp of inside of the Corrector Plate metal surround directly adjoining gllass was holding steady at 84F and the OTA showed same temp ranges as 30 mins earlier. *****INTERESTING PART***** I had an intuitive nudge to move the Dew Not appx 1cm over the edge of the Corrector Plate metal surround. Almost instantly - as I went to take a baseline temp - the temp of the metal surround (inside) rose to 100F (yes 100F) consistently around the inner edge of the metal surround and then decreasing back to about 86F at the very edge opf the glass plate. PERHAPS MOST NOTABLE is the fact that I could instantly detect a pocket of very warm air right around the front of the glass - right where we want it, righ??!! This is all without the shield in place, just the Dew Not. I anticipate this effect will be even more marked with the shield in place. I'll do some experiments and measurements with the shield installed over the Dew Not and I'll pso the results here when I have them. I am very happy with these results so far!!!

[@Aero19612]

Wow! Your dew strap (same as mine, I think) seems to be heating a lot more than my initial measurements showed for my dew strap. Those results sound great. I hope to do some imaging tonight and I will measure the temp of the corrector plate and OTA tube. Thanks for the info!

[@HollomanUFOLanding]

​ @James Lamb Hey James & whomever else may be reading this down the line. Well I've been doing a LOT of experimenting and these are my results and conclusions based on the evidence I've seen. My overall take = * Ditch the dew shield (DS)- it's unnecessary, ESPECIALLY as it affects your astroimaging when windy * Use 2 Dew Not Strips (DNS), tight beside each other. One on circumference of Corrector Plate Shield (CPS) tight to the edge but not protruding over. The other strip butted up to it. Both strips completely cover the metal of the CPS. * The goal - IMHO - is to keep the glass temp above the dew point. Think front & rear windows in cars and how warm is used to keep them clear. * The DSs offer very very little benefit and many minuses. They absorb at least 5F, typically 10F, of heat produced by the DNSs, which is then wasted to the air. The DS do not create a pocket of warm air sufficient to prevent misting / dew on the Corrector Plate. They offer some benefit in reducing the loss of heat from the OTA to space in the areas of the OTS covered by the DS. However, the DNSs do this much more effieciently, raising the average temp of the OTA from the edge of the CPS to 3/4 back toward the rear cell (SCT) by at least 10F. * The DNS put the heat into where it's most needed - into the metal of the CPS, which in trun warms the glass of Corrector Plate. * With 2 DNSs installed, I was able to raise the temp of the Corrector Plate surround (where the Celestron writing is "Edge HD" etc etc) where it joins the glass of the Corrector Plate itself to at least 90F, 95F in places, which is more than enough to keep mist / dew off. Obviously I was not going to poke and prod on the glass itself but even putting by hand close to it I could feel the warmth from it. * I purchased a Y splitter cable with 2 female RCA jacks going to one male RCA plug, which then plugs into an adapter with one female RCA jack at one end and a male 5.5mm / 2.1mm tip positive plug that goes into my 5amp Lithium power tank. * The final piece of anti dew system - telescope enemy no.1 - is a cordless fan made by Dewalt, poweerd by their batteries that power their drills. This fan will be mounted on a table blowing over the middle top of the OTA during observing. More or less guaranteed no dew, aside from any of my heat measures. www.lowes.com/pd/DEWALT-20-Volt-Max-Jobsite-Fan-Battery-Not-Included/1000340891?cm_mmc=shp-_-c-_-prd-_-tol-_-google-_-lia-_-129-_-cordlessdrillsandcombokits-_-1000340891-_-0&placeholder=null&gclid=EAIaIQobChMI4pmbgOHI6wIVDODICh0zwwThEAQYASABEgKRVfD_BwE&gclsrc=aw.ds So, at least there is a solution for you where you don't need the DS, with its wind affected problems. Although unsightly and athestically unpleasing, the most practical way to imrpove my method of getting heat into the CPS would be to wrap the 2 DNSs in reflective foil, to help reduce heat loss and get as much of that heat back into the metal of the CPS where it's most needed. I won't bee wasting my DS, though. I intend to cut it down into a custom jacket that fits around the top and sides of the OTA and butts up against the edge of the first DS. This will help in reducing initial heat loss of the OTA before the heat from DNSs kick in. Hope some of that proves to be of some benefit down the line to someone.

[@Aero19612]

GREAT REPORT! Lots to think about here. As I experimented with shortening the dew shield and placement of the dew strap, I'm coming to your findings regarding dew shield length: short is better. I might cut mine down to the width of the dew strap (2") and just let it serve as a support for the strap. I also bought some lightweight USB fans that I was going to mount on the side of the OTA to blow across the corrector plate. My current setup solves my dew problem so I never got around to testing this out. Watch out for buffeting from your "external" fan. I'm interested in how this works!

[@HollomanUFOLanding]

Thanks for your helpful video :) I don't think dew will be a problem at all when it's windy. The less windy it is the more likely dew will form & vice versa.

[@Aero19612]

Totally agree. But I want the best from both worlds: I want to have the dew shield AND not worry about its sail effect. By the way: I visited Holloman AFB many times but got there in a rental car rather than alien spacecraft.

[@HollomanUFOLanding]

@@Aero19612 hahah ;) A question for you please, James. In point 2 of your summary where you say that the combo of shield & strap worked well, did you have the strap around the circumference of the corrector plate or around the outside of the shield? I was planning on doing the former butw anted to ask you if that's what you did. Thanks

[@Aero19612]

I wrap the dew strap around the dew shield, in front of the edge of the OTA. The strap's primary goal is to heat the air in front of the dew plate. So far, this arrangement has provide dew-free nights of imaging. Once I get a chance, I'm going to cut down my Astrozap dew shield to this short length and then mount (somehow) the dew strap INSIDE the dew shield in front of the corrector plate. Read comments from Mike on this video (and my other Dew-related videos). He has provided very good advice along these lines.

[@HollomanUFOLanding]

@@Aero19612 Thanks for your reply. I look at it a little differently. For me, it will be about warming the circumference of the metal rim that protects the corrector plate, with the objective of ransfereing some of that heat to the plate itself which will go a long way to stop fogging / misting. Think about how a car's windscreen is warmed to prevent fogging / misting. For me, warming the air in front of the plate is not such a good iead because it will introduce a new problem of thermal turbulence and we have enough of that to deal with!!! I am sure there is more than one solutionm to this issue. Thanks for all your uploads and sharing your ideas. I'll test my methos and report back in due (dew) course.

[@Aero19612]

Give it a shot and post back. I tried that early on but got dew anyway. The region around the outer circumference was more or less dew free, but the inner circular region was dewed over. I doubt that the little dew strap can generate enough heat to warm up the corrector plate. The idea behind heating up the air in front of the corrector plate is to increase its moisture-holding capacity. And, again, the low power of the dew strap can't really create thermal turbulence-at least very small compared to what's going on in the atmosphere. The one thing I can say is that ever since I set things up as shown in the video "Effect of Dew Shield Length on Dew Prevention", I have not had a problem with dew. Good Luck!