Effect of Dew Shield Length on Dew Prevention

[@RobB_VK6ES]

I literally applauded when you showed the UPB mounted to the OTA. Now that you have popped your DIY cherry so to speak ;) you might try re-purposing one of the ADM dovetail clamps to carry the UPB underneath the OTA. Sticking it up the sharp end will allow the OTA to sit further back, reducing the moment radius for windage and RA balance. I find that with the supplied stick on feet all I need is a velcro belt (with buckle) to retain the UPB. A simple spreader bar on the remaining clamp should be sufficient to be usable with the handle.

[@Aero19612]

HaHa. The problem with mounting it there is I would have to take it off every time I put the telescope on the mount. There's a handle at the rear that only lets you slide the OTA into the mount from the front. Don't you like my rubber bands?

[@RobB_VK6ES]

@@Aero19612 Bummer, I should have known a smart fella like you would have thought of that. Put it down to more retarded design from C. Like how my CGX has an offset when gripping Losmandy style base plates.

[@michaelmistaken2863]

James Lamb Rubber bands, Velcro and Duct tape bind the hobby together. :)

[@IndependentThinker74]

Looks like a quick and easy set up. Keeping it simple!

[@Aero19612]

Yep. The 3" homemade dew shield with the dew strap at the end of the tube worked like a charm. Thanks for watching!

[@horaciodortona574]

Thanks for sharing this video. I live in Miami and dew on the corrector is a big problem. Your video gave me some insight into how to approach the issue. My solution thus far has been to sideline the C8 and set up the refractor which is OK but I want to use the SCT for Mars which is in a prime position for observation. Again great video and clear skies to you.

[@Aero19612]

Great, Horacio, I’m glad it helped. I have been using the homemade 3” overhang dew shield with the dew strap wrapped around the outside on numerous high-dew nights with no problem. However, I suspect you deal with bad dew conditions more frequently than I do. Good luck and Clear Corrector plates!

[@davidbalogh520]

Thanks, for me it was a very insightful introduction to dealing with dew.

[@Aero19612]

Thanks for watching, David! It's been a learning experience for sure. There are some great comments/observations/recommendations from viewers. Look through those if you haven't already.

[@AmatureAstronomer]

Bought a Celestron 6” Schmidt Cassegrain optical tube. Arrived yesterday. Bought a 16" aluminum dew shield from Celestron for dew and local street lights. Supposed to come Sunday. Hoping for the best. Guess I'll use my refractor on windy nights.

[@Aero19612]

Congrats on the new scope! Yes, the SCTs are wind magnets. The refractor is a good fallback scope on windy nights. Thanks for watching!

[@denodan]

It was always was fir a dew shield to work on a refractor the dew shield needs to be 2 1/2 times as long as the aperture. A dew shield and heat strap can get away with heat dew strap, without one needs to be 2 1/2 times Aperture

[@hbmike47]

James, Glad to hear about your success. If you end up felt lining the inside you can make it with the black side of the poster board facing out so it won't stand out quite so much :D It was interesting to hear that the dew strap alone worked that one night when it had failed in other positions but, with the wind going, you might not have had a dew problem anyway. I didn't look too carefully at your graphs. Good luck with the continued fine tuning of your rig. Clear skies!!

[@Aero19612]

Hey Mike. Thanks for your input. You've been a big help to me and those who've read your comments and suggestions. HaHa. Looks pretty ugly. I'll probably end up just cutting down the Astrozap. Two for the price of one is always a good deal, right!? Not sure what to make of the overhanging dew strap as an "answer". As you note, the wind alone might have done the trick.

[@clintkennedy8374]

Liked and subscribed I’m enjoying the info on this channel

[@PeterJahans]

Hi James, haven't seen any updates on this topic from you since this video and because galaxy season is approaching and I'm getting ready to set up my 8" SCT again, can I assume you still like your DS3 solution? Last year I constructed a dew shield from 1/16 neoprene foam, similar in size to the Astrozap, and mounted it over a dew heater strap just behind the front metal ring. This worked to prevent any dew formation, but even the slightest wind messed up my guiding. I have ordered the new Celestron corrector lens heater that just came out, have you seen these? Reviews I've seen are positive. Still, I think I'm going to cut down my dew shield to about 3-4" in length and use it to create a warm air reservoir above the corrector and also to block just a little more light from the neighbor's house from entering the OTA. Fingers crossed my results this spring will be better. Cheers - and thanks for so many excellent and informative videos!

[@Aero19612]

Hey Peter. Haha. I went "dark" as I was struggling with my CGEM last Galaxy season. With the EQ6-R, I've been doing a lot of imaging with the C9.25 in humid conditions that would definitely dew up the corrector plate. I am pleased to report the that 3" (overhang) dew shield (I cut my Astrozap foam dew shield down) combined with the dew strap (I wrap it around the end of the scope with it overhanging by about 1" then wrap the dew shield around it) is working great. Not a single minute lost to dew! I think I did see something about the corrector heater, but I didn't pay much attention since my current setup is working just fine. Any wind will always be a problem for an SCT. Perhaps a bit less so now for me with my EQ6-R and its much reduced RA/DEC backlash. Good luck and thanks for watching!

[@marvinwhisman3333]

What program are you using to track the temp and dew point? It looks like very valuable info to have when determining the effects of different heater/shield combinations. As always, another great and informative video from you.

[@Aero19612]

That data and plot are coming from the Pegasus Astro Ultimate Power Box v2. It's expensive, but pretty awesome. Thanks for watching, Marvin!

[@starman6092]

Thanks for the informative video on dew prevention. I will have to scrap my dew shield for the moment to use my bahtinov mask. Electronic focusers are very expensive. However, my dew straps and the adjustable dew shield that comes with my ES ED102 will have to suffice.

[@Aero19612]

Thanks for watching! I’ve never had a dew problem with my ES ED102. I do use a dew strap with the dew shield fully extended. My big dew problem is with the SCT (much larger diameter glass). The dew shield works for the SCT, but it’s a wind magnet. Good luck!

[@foxglovemead]

Have you considered a sealing 6” wide foam “filler” around the dovetail and a dew strap attached directly to the OTA with Velcro also on the outside? A homemade dew shield with three bands of matching Velcro on the inside and up the sides? You would have three lengths of sliding dew shield in one e.g. vary the length as conditions change? Simon

[@Aero19612]

That sounds like too much work for me! I'm a rubber band and duct tape kind a guy. Actually, I did consider something like that by just extending the "slots" at the base of the dew shield (where it fits around the top and bottom support bars). That way I could slide it down to vary the overhang. In the end, I opted for the paper solution. So far, It would appear that an adjustable length is not needed. Dew is prevented seemingly independent of length (as long as there's a dew strap there to heat the air in front of the corrector plate). So far, so good with the 3" extension as wind is concerned. But I've been having some of the most wind-free days I can remember. Thanks for watching, Simon!

[@michaelmistaken2863]

Great science, thanks for making and sharing your data and graphs! Have you tried the dew strap sandwiched between the external rim of the corrector plate and the inside of the dew shield tube, with the dew shield “clamping” the strap to the metal of the plate? My thinking is this might conduct heat more directly into the corrector plate and reduce convection losses to ambient air. I guess the dew shield size might have to increase slightly to fit snuggly over the tube+strap diameter, and might be harder to keep the shield firmly attached.

[@Aero19612]

Good thought, Michael. I'm not sure that would work. I have tried wrapping the dew strap tightly around the end (but no dew shield) and I got dewed over pretty quickly. I have also tried attaching the dew strap inside the dew shield, but nothing contacting the heating element so it's just radiating heat. That didn't work either. So I went back to the dew strap wrapped around the dew shield as shown in the video and that works just fine. Your mileage may very. Thanks for watching!

[@michaelmistaken2863]

​@@Aero19612 Looks like you've found your "just right" combination, which is all that matters. *thumbs up* I guess that once we have "just enough de-dewing" for a given situation the benefit of tinkering would only save a spot of power - and ultimately that's not really important with a mains cable. :)

[@Aero19612]

HaHa. Yeah, power is ok. Not worried about draining a battery. My next step is to mount a counterweight under the front end of the OTA so that I can slide the body back and create a more balanced geometry. As it is, the OTA sticks out too far in front allowing the wind to use it like a wrench on the DEC axis. Always something to mess around with...

[@wanderingquestions7501]

What I really want to learn about is how dew heaters interfer with the NSAs mind surveillance program. Ya know it’s just something I wonder about. Please correlated effects of multiple dew heaters running. Could check if there’s a DC under-voltage for that processor. Going to the AC/DC converter basically gave you more base DC voltabe. DC voltage can drop fast in undersized wires. In my RV I have a separate fuse for my radio/TV console that also has auxillary USB and cigarette lighter outlets. That wire is only 7 feet long from a 10 amp fuse and it cannot change my laptop via inverter. The current just isn’t there through a 14 gauge wire. Being at dew point does not guarantee dew formation. The atmosphere can be at 800% humidity and not precipitate clouds. What is need is nucleation points for dew drops to form same with surface. It may be that night dew did not form cuz the air mass was low on dust/pollen particulates. Did it rain recently; washing out the air? What about a heated wire mess shield? Put 3 heaters around the base of the mesh above the OTA. It’s America; more is better.

[@Aero19612]

Yeah, that must be the main factor and I must be on the ragged edge of the power provided. After I recorded that video, I tried the UPBv2-to-mount power with the PA cable (again) and a slightly shorter heavier gauge cable I bought. Both started the mount up like a charm with no hint of the issue I observed the first time I tried with the PA cable. May give the heavier gauge cable a try tonight. I can leave the AC adapter nearby for "emergency" re-start if it lets me down. As for the dew heater issue. I think the dew problem is "solved" with one dew heater and a very short dew shield. I just need to move the dew strap to the inside face of the dew shield for maximum effect. But for now, it appears to be working.

[@wanderingquestions7501]

James Lamb With the heater inside the shield, I wonder if wrapping the dew strap in aluminum foil would allow it to radiate more...

[@Aero19612]

Yeah, you could have an open-ended foil shape that would direct the heating toward the center. I would wonder about stray light reflections off the foil though.

[@louisrosner7902]

Another great video. Love the fact that you finally raised the power ox to the OTA. Interesting mass correction device with all those washers. I just mounted my large Telrad to the other side and that seem to equalize the mass on the OTA. Also find that I cannot get any kind of guiding with the longer Astrozap dew shield even in winds as light as 5 mph. Best guiding with the dew shield is about .8, .9 rms. Just wondering if perforating the dewshield with a hole punch will allow some of the wind to go through but still allow convective currents to raise the hot air off the corrector plate. Possibly scalloping the end of the dewshield with a pinking shears may also be effective. What guiding rate are you using with oag. . Reading that it might be appropriate to use a faster rate such as .25 or .35 sidereal with this type of guiding With my image scale of .55 arcseconds per pixel.. Any thoughts.

[@Aero19612]

Thanks, Louis! Yeah, the Astrozap is just a non-starter for me too. It's very well made and fits like a glove, but it's a sail and lever arm combined. I would not recommend perforating the wall of the dew shield. I suspect you'll get more turbulence from that than benefit, not to mention the potential for weird stray light reflections. My results to date suggest cutting down the length of the Astrozap is a viable option. Hmmm. Hadn't heard/read anything about guide rate with an OAG. I've always used 0.5 x sidereal (with or without an OAG). Also, not sure that matters that much. PHD2 knows the guide rate. When it calculates a correction of X arcsec, it determines the pulse width (in milliseconds) based on the guide rate. If you use a smaller guide rate, PHD2 will just use a longer pulse to compensate, and vice versa.

[@mrspock2al]

Thanks for sharing your results. I'm going to replicate your experiments up here in Michigan and find my "sweet spot". I can buy a lot of poster board for the price of one commercial dew shield. Seems like some black velvet and a can of waterproof, spray varnish would finish off a diy shield. Then add the dew strap. I've also seen metal dew shields which I assume are aluminum. Has anyone experimented with different materials?

[@Aero19612]

Great, Alan! Share your findings when you collect the data. Not sure about other experiments with other materials. And I'm not sure if it matters a whole at least with dew prevention. The Astrozap shield is a nice thick, lightweight, insulating foam. It's just too darn long (if the wind blows)! Of course, I'm experimenting with thick paper basically and it's working great. I might be concerned about added weight of an aluminum shield, but it would/could be very thin and may not weigh much more than the Astrozap. Your idea of pimping out your diy dew shield sounds very functional and durable.

[@DanPechiu]

Did putting the dew strap on the inside of the dew shield provide better results in temperature delta, and/or dew prevention? Why not use a Celestron dew ring instead of a shield, if wind causes guiding issues?

[@Aero19612]

Hi Dan. My dew control approach has been working very well. Shorter dew shield wrapped around the dew strap. The dew strap is wrapped around the end of the scope and over hangs by about half its width. I then wrap the dew sh8eld around that. I power the dew strap with a Pegasus Astro UPBv2. I turn the dew strap power to max. That’s worked very well, even on the most challenging nights when the dew point equals the air temp. I don’t think the Celestron dew ring was available when I “solved” my issue. Could have been, but I may not have been aware of it. I am now! In fact, I just installed it 2 days ago. No clear skies. I will do some temperature tests and publish a video. I like the concept but have 0 interest in buying Celestron’s $300 dew controller. I’ll power it with the UPBv2. Celestron cable matches the UPBv2 power port. Stay tuned. But, as I said, the other method was/is working just fine. As for wind, yes, it’s an issue and why I cut my dew shield back. The difference in wind speed that’s a no-go is not a big difference for a 3” overhang and no overhang and even a little dew shield provides stray light protection. Thanks for watching!

[@DanPechiu]

@@Aero19612 I bought Agena's Dew Ninja, which has an inline rheostat, so it doesn't need a controller. I've wrapped it on the inside of my dew shield, so when I attach it to the scope, the dew strap abuts the plastic rim of the C9.25. Based on your research, having it in front of the corrector plate should work better. If it's clear out tonight, I'll give it a try. Thanks for all your work on shields/straps!

[@DanPechiu]

Follow-up: Putting the dew strap on the inside of the dew shield, causes bright stars to have weird diffractions, which look like two triangular spikes about 30 degrees apart on one side of the star. I've used the scope without dew protection one night, and stars were perfect (Pleiades). I then realized the Agena Astro Dew Ninja, has a yellow velcro strap and that was causing unwanted reflections. I will either spray paint the velcro strap black, or just use it on the outside of the dew shield.

[@JohnDoe-ol3yz]

Have you tested just a dew shield with no dew heater/zapper? I'm curious how well that would work.

[@Aero19612]

Yes. As I recall, the full length Astrozap dew shield with no heated dew strap worked well as far as dew prevention is concerned. All that extra length though is like handing the wind a wrench - the SCT, combined with the CGEM, was just not usable if their were any wind. Hence the effort to shorten the dew shield by augmenting it with a dew strap. Thanks for watching!

[@KenanSatir]

Great work, thanks for all informations....

[@Aero19612]

Thanks for watching, Kenan! So far, I've been very happy with a 3" overhanging de shield wrapped around my dew strap at the end of the SCT.

[@MikeShipman]

I looked at both of these videos; great experimentation. However, in the first video you mentioned wanting to do a test with just the dew shield, no heater. So far, your experiments have been related to heated dew shield vs heated/no dew shield effectiveness, and the effect of dew shield length on wind buffeting (guiding). It doesn't look like you've tested the unheated dew shield (at the various lengths also). Do you have any data you may not have included? Thanks for your study, it inspired me to build a shorter dew shield rather than buy a full length one.

[@Aero19612]

Hey, Mike: No. I didn't do testing without the dew strap. My objective was not to test various configurations, but to find a reliable solution for dew. The combination of the short (homemade) dew shield and dew strap is working for me, so I'm sticking with it for now. I hope the combo works for you. If you don't have a dew strap, then by all means, test it without the dew strap before purchasing one. Good luck!

[@nadirteymurov1]

Hi James. Do you plan to install small cooling fans behind primary mirror to speed up thermal equilibrium ?

[@Aero19612]

Hey Nadir. I have been thinking about hanging a small fan over the front end of the scope so that it can blow air across the corrector plate. If the short dew shield works with respect to the wind, I probably won't even try that. I wasn't planning to install fans behind the primary mirror. I would have to disassemble the OTA to get access to that area. Some SCTs, perhaps Celestron's EdgeHD series, have vents in the back plate to accomplish just that. Thanks for watching!

[@textandtelescope8199]

I have a RASA and it occurred to me that the cooled camera inside the dew shield may provide enough dew abatement on its own without an assist from a dew strap. What do you think?

[@Aero19612]

Hmm. Good one. The fan on the back of the camera might actually be very beneficial in keeping the air moving a bit. One way to find out! If it were me though, and I had a dew strap handy, I'd still use the dew strap. Belt and suspenders philosophy, you know.

[@textandtelescope8199]

@@Aero19612 And the moving air is coming off of a warm sensor. I have an asiair and have a celestron dew ring attached and it kept shutting off the asiair - suspect to much power demand. May pull the ring and go back to strap backup to the cooling fan per your suggestion. Thanks!

[@Aero19612]

Interesting. I just bought a Celestron dew ring. It’s on back order. I was wondering about the power draw. I’ll power mine off of the ult Power Box. Will have to experiment with current draw VS effectiveness. If the fan air is warm, that should certainly help. Let me know what you find out.

[@textandtelescope8199]

@@Aero19612 What is that temp-dew point measuring device that you have?

[@Aero19612]

@@textandtelescope8199 it’s the environmental sensor that comes with the Pegasus Astro Ult Power Box.

[@ColeRees]

Do you still like the 3” shield? Trying to figure out an optimal length and your video is helpful

[@Aero19612]

Yeah, I think so. I recently cut my Dew Zapper foam dew shield down to a 3" overhang and mounted the dew strap inside the dew shield: that did not work. The next night I went back to the 3" (overhang) cardboard dew shield with the dew strap on the outside (as shown in the video), and that did work. I'll stick with the 3" dew shield with the dew strap wrapped around the outside. That has proved to be a very effective combination. Thanks for watching, Cole!

[@mechkiller31st]

James, I'm curious to know (since i own a 9.25 myself) how much wind has been too much? With my heated astrozap dew shield wind around 5-10 KPH hasn't really screwed up many of my images, though I would think that wind direction could also play a factor as well. Love the videos and look forward to more!

[@Aero19612]

I agree with you. If I see "6 mph (10 kph)" for the wind speed as reported at a local weather station, I think "that's a good night." It's the higher wind speeds I'm more sensitive to. Also, keep in mind that the force generated by the wind is proportional to the square of the wind speed, so guiding can go to hell in handbasket pretty quick if the wind ticks up or gusts occur more frequently. And, also, the longer the dew shield is, the larger force and the greater effect the force has, particularly on the DEC axis. I've been casually searching the web for a USB-powered anemometer that I can put out next to the scope. That way, I could develop a better correlation of "good" vs "bad" wind and know when to shut things down or, better yet, not start things up. Thanks for watching!

[@mechkiller31st]

@@Aero19612 Ah ok that makes sense. For me if I hear 15MPH i dont even bother, 10MPH i am willing to try and anything under is a go for me. Though I admit i always have the dew shield on and set to low-med. I myself havent tried to see how much wind is too much without the shield on. Im currently working on making a tarp enclosure around the scope sort of like an EZ-up but without the roof, hopefully that will help break some of the wind i get.

[@TurboTalksTech]

James, I just made a short dew shield (badly) because my full size one didn't fit in my dome. But, coupled with a dew strap, seems to do the job. With regards to your fan suggestion, given the wind sensitivity you experience, I'd suggest you look in to it. There is a forum post below which goes in to this in detail. If my solution doesn't work out longer term, this is what I intend to try next. You should take a look. www.iceinspace.com.au/forum/showthread.php?t=182057

[@Aero19612]

Thanks for the link! Great to see others are pursuing this option and finding it works. From a dew perspective, the short shield + dew strap has been working for me. As you say, still need to see the response in the wind for this 3" dew shield. Last night was a rare night with no wind and risk of dew. And then the clouds moved in. Oh well.

[@brett1354]

If you want to improve your marginal conditions, remember that when you've got a breeze affecting any kind of dew shield, you'll get less dew anyways.

[@Aero19612]

Absolutely. I found that to be true during my testing. Was getting bad guiding with the 8" dew shield due to wind and it improved quite a bit when I removed the dew shield. And I didn't get any dew for the remainder of that night's imaging. Thanks for watching, Brett!

[@Ganymede3310]

if you have the power needed cant you just get 1 more dew heater maybe 2 hearters solve the problem

[@Aero19612]

Perhaps. When I started this, I had no reason to believe 2 dew straps would work. I knew the dew shield worked but it's a wind magnet. Hence the direction I went. In the end, I found a solution that is working: a homemade dew shield with a 3" overhang + 1 dew strap wrapped around the dew shield half-way overhanging the end of the scope. Thanks for watching!