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Why is my telescope full of water?

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#1 WadeH237

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Posted 03 December 2018 - 08:31 PM

I keep my imaging scope set up in the back yard all of the time, but I don't have an observatory.  I keep it under a Telegizmos 365 cover, with heating rods underneath.  I check check underneath it around the pier periodically to ensure that the heaters are running and that there is no condensation, at least where I can reach without removing the cover.

 

My last imaging session was about 4 weeks ago.  We have a forecast for clear nights through Wednesday, so tonight I went out and uncovered the scope.  The mount was dry, the outside of the OTA was dry, and everything seemed fine.

 

When I powered up the mount and positioned it parallel to the ground for access to the dust cover, water started running out of the tube vents.  When I took the dust cover off, I found that the outside of the corrector was dry and there was water pooled inside of the tube.  I positioned it so that there was a vent at the low point to let all of the water drain out, and I have the dew heater running warmer than usual to dry the corrector.

 

I've kept my system outside like this for about 5 years, and this is the second time I've had this happen.  In this case, the OTA is an EdgeHD 8.  It happened once before to my C11 (non EdgeHD, without vents - I had to remove the corrector plate that time and dump at least a quart of water out of the OTA).  It has never happened with a refractor mounted, and it's never happened with my small RC.

 

This is not a case of the TG cover leaking.  For one thing, I use different covers for the 8" and 14" scopes.  Also, I've encountered seriously inclement weather with driving rain and worse in the field - and the covers don't leak a drop.

 

It has to be condensation that's getting into the tube, but I don't know what conditions are allowing it.  As I said, the exposed surfaces under the cover are dry.  It's only inside the tube.  If I remember right, the time that it happened with my C14 was also in around the end of the year.  I wonder if there is something about significant temperature swings (15 to 20 degrees F with high humidity) that's trapping cold air inside the OTA.

 

Any thoughts on what's actually going on?  And even better, any way to prevent it (short of bringing in the OTA)?

 

Thanks,

-Wade



#2 jhayes_tucson

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Posted 03 December 2018 - 09:11 PM

Wade,

Wow...that’s no it good.  I think that your idea about the tube trapping cold air is a good one.  With wide temperature swings and high humidity, you could definitely be getting condensation inside the tube.  The outside of the tube might be getting enough heat and air circulation to dry off faster than the inside of the tube.  Running fans and using desiccant would probably solve the problem but would also require periodic checking to make sure that everything is ok.

 

Good luck with it...

John 


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#3 mvas

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Posted 03 December 2018 - 09:37 PM

The temperature of the OTA is falling below the Dew Point of the warm moist air trapped inside.

The Dew Point is only 44° F for 50° F air at 80% RH.

How do you control the heat to keep the OTA above Dew Point of the warm moist air trapped inside?

How do get the warm moist air out the OTA while it is covered with a Waterproof Telegizmo?

Tempest fans ?


Edited by mvas, 03 December 2018 - 10:01 PM.


#4 gfstallin

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Posted 03 December 2018 - 10:53 PM

I wonder if a heating element within the OTA, even just within the baffle tube, set to a timer to run for a few hours per day during winter months or the typical times of highest RH would solve the problem. 

 

While I would say to just bring the OTA inside to avoid the problem (my likely plan if I go the permanent pier route), I don't think I've ever heard/read about your issue, particularly with a vented OTA that should allow for some air exchange. It doesn't seem like something that should happen with the regularity required for water to pour out of an OTA. 

 

 

 

George


Edited by gfstallin, 03 December 2018 - 11:00 PM.


#5 WadeH237

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Posted 03 December 2018 - 11:02 PM

So I've got the OTA dried out now.

 

Upon closer inspection, the inside of the corrector was covered completely with dew, and so was the secondary mirror.  The primary mirror was completely dry.

 

After about two hours, the dew heater was making some progress on the corrector (enough that I could see the reflection of the secondary in the primary - which is how I discovered that it was also completely covered in dew.

 

I removed the secondary mirror and used a hair dryer at a distance to dry it off (Before doing so, I tested the hair dryer on its low setting and found that it was pretty close to the ambient temperature in my house.  It actually felt cool.  I definitely would not apply any heat to cold optics.)  After drying the secondary, I directed the air into the empty secondary assembly.  It took about 30 minutes, but it did clear the corrector.  There is no now indication of moisture in the OTA at all.

 

The things that are interesting to me are that the primary mirror was dry, while the secondary and inside of the corrector were completely covered.  It was also interesting in that it seemed like the air displaced from inside the tube when I dried it, was considerably colder than ambient.  I didn't measure it, but it was very noticeable.

 

When the scope is covered, it is in the Astro-Physics Park 3 position, with the scope pointed north and the counterweight shaft down.  I live at about 47 degrees latitude, so the scope is pointed up about 43 degrees.  It is covered with a Telegizmos 365 cover that reaches from the top of the OTA, to about 6 inches from the ground.  It is open at the bottom, with two 50 watt rod heaters that are on all the time (when covered).  The cover is arranged in such a way that air can circulate freely.  It has the stock Celestron dust cover installed over the corrector.  There is nothing to force air to circulate through the OTA.

 

Given that the parts that were in direct contact with the top of the TG365 were the only things covered in dew, I have to wonder if there is radiative cooling happening that is making the OTA much colder inside than the rest of the environment under the cover.  Thinking back now, after this happened to my C14 a few years ago, I started putting a blanket over the scope before putting on the cover.  I know that the C14 didn't experience the problem again, but I have no idea if it's because of that or not.  I put the EdgeHD 8 on the mount in September and the last time I had the cover off was about 2 weeks ago (for a possible session that I didn't do).  Everything was dry at that time.



#6 WadeH237

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Posted 03 December 2018 - 11:07 PM

I wonder if a heating element within the OTA, even just within the baffle tube, set to a timer to run for a few hours per day during winter months or the typical times of highest RH would solve the problem. 

 

While I would say to just bring the OTA inside to avoid the problem (my likely plan if I go the permanent pier route), I don't think I've ever heard/read about your issue, particularly with a vented OTA that should allow for some air exchange. It doesn't seem like something that should happen with the regularity required for water to pour out of an OTA. 

 

 

 

George

I have a friend with a 3D printer.  I'm thinking that I might work with him to see if we can design a desiccant holder that could replace the secondary mirror, plus a holder for the secondary mirror to protect it when it's out (or maybe I will just see if Starizona will sell me one of the assemblies that they provide to protect the secondary mirror when Hyperstar is installed).

 

I have considered bringing in the OTA, since we can easily go months in the Seattle area between clear night skies.  If I do that, though, then I'm probably going to use it less often.  The whole point of having it outside set up is that when an opportunity to image arises, I just need to remove the cover and go.  Also, I have a pointing/tracking and tracking model that may or may not work properly after removing and reinstalling the OTA.



#7 choward94002

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Posted 03 December 2018 - 11:09 PM

I keep my imaging scope set up in the back yard all of the time, but I don't have an observatory.  I keep it under a Telegizmos 365 cover, with heating rods underneath.  I check check underneath it around the pier periodically to ensure that the heaters are running and that there is no condensation, at least where I can reach without removing the cover.

 

My last imaging session was about 4 weeks ago.  We have a forecast for clear nights through Wednesday, so tonight I went out and uncovered the scope.  The mount was dry, the outside of the OTA was dry, and everything seemed fine.

 

When I powered up the mount and positioned it parallel to the ground for access to the dust cover, water started running out of the tube vents.  When I took the dust cover off, I found that the outside of the corrector was dry and there was water pooled inside of the tube.  I positioned it so that there was a vent at the low point to let all of the water drain out, and I have the dew heater running warmer than usual to dry the corrector.

 

I've kept my system outside like this for about 5 years, and this is the second time I've had this happen.  In this case, the OTA is an EdgeHD 8.  It happened once before to my C11 (non EdgeHD, without vents - I had to remove the corrector plate that time and dump at least a quart of water out of the OTA).  It has never happened with a refractor mounted, and it's never happened with my small RC.

 

This is not a case of the TG cover leaking.  For one thing, I use different covers for the 8" and 14" scopes.  Also, I've encountered seriously inclement weather with driving rain and worse in the field - and the covers don't leak a drop.

 

It has to be condensation that's getting into the tube, but I don't know what conditions are allowing it.  As I said, the exposed surfaces under the cover are dry.  It's only inside the tube.  If I remember right, the time that it happened with my C14 was also in around the end of the year.  I wonder if there is something about significant temperature swings (15 to 20 degrees F with high humidity) that's trapping cold air inside the OTA.

 

Any thoughts on what's actually going on?  And even better, any way to prevent it (short of bringing in the OTA)?

 

Thanks,

-Wade

Oy ... been 'kinda there once (hoar frost inside and outside of the OTA), never want to do that again ... :(

 

Water inside the OTA can only come from one place: humid air inside the OTA that condensed out when the OTA exterior got cold and inside the OTA the humid air dropped below dewpoint.  TG covers can enhance this as they don't breathe; warm (moist) air comes in during the day, cover cools down, condensation on the inside of the cover ... same with your OTA (and what happened with me, except it got cold enough that the dew froze into little crystals) ...

 

Solution, as John mentioned, is to keep the dewpoint at bay by using dessicants and heaters and watching how things are going with monitoring ... when I'm using my TG365's they have one of these [https://www.amazon.c...ssicant plug in] to keep it dry, one of these [https://www.amazon.com/VIVOSUN-Waterproof-Seedling-Hydroponic-Standard/dp/B00P7U259C/ref=sr_1_3?ie=UTF8&qid=1543896381&sr=8-3&keywords=hydroponic+heating+pad] to keep it warm and one of these [https://www.amazon.com/ThermoPro-TP-60S-Hygrometer-Thermometer-Temperature/dp/B06XKH666P/ref=sr_1_14?ie=UTF8&qid=1543896415&sr=8-14&keywords=humidity+wireless+monitor] to squawk like a chicken if it's getting close to dewpoint.  Put 'em all together in the TG365, wrap it up tight with bungee cords at the bottom and wait for nightfall ...

 

Oy ...



#8 choward94002

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Posted 03 December 2018 - 11:23 PM

So I've got the OTA dried out now.

 

Upon closer inspection, the inside of the corrector was covered completely with dew, and so was the secondary mirror.  The primary mirror was completely dry.

 

After about two hours, the dew heater was making some progress on the corrector (enough that I could see the reflection of the secondary in the primary - which is how I discovered that it was also completely covered in dew.

 

You'll want to make sure to get the OTA nice and warm, beyond just getting rid of the dew, to prevent the growth of mold inside your OTA.  Do some searching here for "mold" and "moisture" and "OTA" but it's basically all around, thrives on optical surfaces and is decidedly no bueno ... only way to prevent it is to keep things dry, which your OTA right now isn't ...

 

I removed the secondary mirror and used a hair dryer at a distance to dry it off (Before doing so, I tested the hair dryer on its low setting and found that it was pretty close to the ambient temperature in my house.  It actually felt cool.  I definitely would not apply any heat to cold optics.)  After drying the secondary, I directed the air into the empty secondary assembly.  It took about 30 minutes, but it did clear the corrector.  There is no now indication of moisture in the OTA at all.

 

The things that are interesting to me are that the primary mirror was dry, while the secondary and inside of the corrector were completely covered.  It was also interesting in that it seemed like the air displaced from inside the tube when I dried it, was considerably colder than ambient.  I didn't measure it, but it was very noticeable.

 

That makes sense; warm, humid air will rise while colder, less humid air will fall ... so your corrector probably got dewed just because it was stored with the corrector up as you mention later ... also makes sense that the air felt "colder", again that's air that had all the moisture squeezed out of it due to the cold.  If the air had been able to heat up it likely would have taken some of the water with it ...

 

When the scope is covered, it is in the Astro-Physics Park 3 position, with the scope pointed north and the counterweight shaft down.  I live at about 47 degrees latitude, so the scope is pointed up about 43 degrees.  It is covered with a Telegizmos 365 cover that reaches from the top of the OTA, to about 6 inches from the ground.  It is open at the bottom, with two 50 watt rod heaters that are on all the time (when covered).  The cover is arranged in such a way that air can circulate freely.  It has the stock Celestron dust cover installed over the corrector.  There is nothing to force air to circulate through the OTA.

 

That's a bit stranger, with no OTA vents and (I assume) your visual back plugged there's no good route for air exchange ... but remember there's still a gap between your corrector plate and the OTA tube ... that's a very tiny one though, so stumped how the air exchange could occur ...

 

Given that the parts that were in direct contact with the top of the TG365 were the only things covered in dew, I have to wonder if there is radiative cooling happening that is making the OTA much colder inside than the rest of the environment under the cover.  Thinking back now, after this happened to my C14 a few years ago, I started putting a blanket over the scope before putting on the cover.  I know that the C14 didn't experience the problem again, but I have no idea if it's because of that or not.  I put the EdgeHD 8 on the mount in September and the last time I had the cover off was about 2 weeks ago (for a possible session that I didn't do).  Everything was dry at that time.

 

That could easily happen with the TG365, it's not very "shiny" on the exterior side so you'll get some radiative cooling to cool down the cover and since it has a very small R-value conduction from the cover to the OTA would certainly cool down the OTA over time.  Putting a blanket between the inside of the TG365 and the OTA would definitely interfere with the conduction.  I put a bath towel over my OTA/ mount before I put on the TG365 but for a different reason: I discovered that the metal edges of the mount/ OTA would wear little holes into the sides of my $300 TG365, so putting a towel between it and the mount solved that problem ...



#9 freestar8n

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Posted 04 December 2018 - 02:12 AM

I had heard about someone who picked up a C11 in an observatory and found it full of water - and there was no clear path for how water got in.

 

There are many systems for collecting dew as drinking water - and they are fairly elaborate and usually involve a large collection area and some kind of funnel or collecting system that effectively focuses the water into a smaller container.

 

It is very hard for me to imagine an metal cylinder with a small vent in it - somehow have enough connection to outside air that a lot of moisture can come in - while at the same time the collected moisture at the bottom of the tube - which presumably has the full area of the cylinder - doesn't evaporate and escape.  And if the tube in either case was blocked from the sky - there wouldn't be much radiative cooling anyway.

 

So somehow it gets hot during the day and the moist air goes in - then the tube lining cools way below dew point and water collects - then things heat up again and again more moist air comes in than evaporates out.

 

It just seems like this would be a known way to collect dew as water.

 

This isn't to say I don't believe it happened.  It's just that saying "the tube went below dew point and water collected" doesn't seem enough to explain all that's involved.

 

If the OTA were sitting up with the secondary on top, then the curvature of the secondary would have the geometry of a dew collector, where it forms on the secondary then drips from the bottom of the curvature into the baffle - where the water was then somewhat blocked from escaping.

 

Was the OTA in a home position just pointing up at an angle?  Maybe it just trickled down the collector and then in a narrow stream to the bottom - where the collected water had little surface area to evaporate.

 

It's certainly easier to explain if the water formed outside the OTA and then dripped into the ota through a small gap where it was then trapped and simply collected.  In that process there is no need for moist air to be sucked in - but then not evaporate as things heat up.  So maybe it formed between the corrector and the lens cover and gathered at the edge of the tube where it then leaked in - and was trapped.

 

Frank


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#10 carolinaskies

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Posted 04 December 2018 - 09:33 AM

Given you are using rod heaters under the cover I suspect what is happening is the Edge vents are allowing moisture laden air to condense inside the tube.  Hot air rises (whether laden with moisture or not)  and essentially the top of the unbreathing cover stops the rising so the OTA is bathed in the moisture rich air.  The heat does not penetrate the OTA so therefore the interior is cooler, creating a draw for the warm moist air.  The corrector glass is the highest point in the park position so this is the point of collection.  It then runs off the corrector and pools inside.  It becomes a further heat and moisture sink to keep the interior cooler than the exterior. 


Unlike refractors, SCTs are not particularly air tight because they have many holes drilled in them to secure components together and the corrector is a free-floating optic secured by a ring but often there is no air-tight seal.  Even end-caps are often less than full sealing.  

Using dessicant may help some, but if you are getting huge swings in humidity they become ineffective as they don't have time to cycle between being dry and moisture laden.  

  


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#11 WadeH237

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Posted 04 December 2018 - 10:25 AM

Given you are using rod heaters under the cover I suspect what is happening is the Edge vents are allowing moisture laden air to condense inside the tube.  Hot air rises (whether laden with moisture or not)  and essentially the top of the unbreathing cover stops the rising so the OTA is bathed in the moisture rich air.  The heat does not penetrate the OTA so therefore the interior is cooler, creating a draw for the warm moist air.  The corrector glass is the highest point in the park position so this is the point of collection.  It then runs off the corrector and pools inside.  It becomes a further heat and moisture sink to keep the interior cooler than the exterior. 


Unlike refractors, SCTs are not particularly air tight because they have many holes drilled in them to secure components together and the corrector is a free-floating optic secured by a ring but often there is no air-tight seal.  Even end-caps are often less than full sealing.  

Using dessicant may help some, but if you are getting huge swings in humidity they become ineffective as they don't have time to cycle between being dry and moisture laden.  

  

I am thinking that you are right.

 

It's a pretty rare occurrence, so I suspect that it's a particular pattern of humidity and temperature change.  In 5 years, it's only happened twice (but I've not had an SCT on the mount for the entire time, since I change OTAs periodically).

 

I should just build a roll-off roof observatory.  I have the space for it (and my wife has OKed it), but I've got very little construction experience.  I could probably manage building a pre-fab shed, but I'm not sure that I could do the modifications to make it a roll-off roof...



#12 carolinaskies

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Posted 04 December 2018 - 12:00 PM

I am thinking that you are right.

 

It's a pretty rare occurrence, so I suspect that it's a particular pattern of humidity and temperature change.  In 5 years, it's only happened twice (but I've not had an SCT on the mount for the entire time, since I change OTAs periodically).

 

I should just build a roll-off roof observatory.  I have the space for it (and my wife has OKed it), but I've got very little construction experience.  I could probably manage building a pre-fab shed, but I'm not sure that I could do the modifications to make it a roll-off roof...

The roll off roof shed is pretty simple.  But you might consider a SkyShed or NexDome as they don't take a lot to setup/maintain.  



#13 Noah4x4

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Posted 04 December 2018 - 12:01 PM

Pesky cat next doors. Marks its territory everywhere. Today, it's dug up my crocus bulbs to do its business. Good job I keep my scope indoors.  More seriously, moisture has an astonishing ability to penetrate where it shouldn't.

 

I laid heavy duty (outdoor) cables under my wooden decking sealing a single joint inside an (alleged) waterproof connector box with tight grommets around the cables and yet my RCD system kept tripping. Opened up the box after a week of frustration yet no rain and it was dripping inside with moisture which I can only attribute to humidity (or cat). Eventually, had to run a single unbroken cable inside sealed conduit before the problem ceased. 


Edited by Noah4x4, 04 December 2018 - 12:15 PM.


#14 dscarpa

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Posted 04 December 2018 - 03:55 PM

 On one odd night with very light fog my C9.25XLT dewed up inside. There was a fair bit of water. David  



#15 munirocks

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Posted 04 December 2018 - 05:13 PM

Is the tube actually sealed, or does it have vents that allow a continuous feed of humid air? It seems to me that if the tube is sealed then hot-cold cycles would only keep recycling the same water vapour. But because it's got a volume of water in it, this indicates that some conveyor mechanism is refreshing the air from outside to keep the new humid air coming in, to feed the condensation. I think that sealing it and using a dessicant would work. If you don't seal it, the dessicant will get saturated and you'll be fighting an ongoing moisture gradient.



#16 WadeH237

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Posted 04 December 2018 - 05:54 PM

The current mounted scope is an EdgeHD 8, which has vents.  The C14 that experienced this earlier is a pre-EdgeHD scope with no vents, but I doubt that it's air tight.

 

What I didn't expect is that the scope could get much cooler than the rest of the environment under the cover.  I was assuming that the heat from the rods would rise, and it would be warmest up where the OTA is.  That's why I'm wondering if having the scope in contact with the cover is somehow allowing the heat to escape through the cover at the contact points causing the scope to get colder than the air below.

 

If that's the case, the answer might be as simple as covering the OTA with something that has decent insulation properties before putting on the TG365.



#17 whizbang

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Posted 04 December 2018 - 10:03 PM

Folks have no idea just how darn damp it is here in the NW.

 

I like your insulation idea.  Maybe a solid block or three from Home Depot cut to size.  I think more insulation is better than less.



#18 RAKing

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Posted 04 December 2018 - 11:56 PM

I agree.  I think you should put some space between the inside of the TG365 cover and the top of your OTA.  A blanket might work, but I think some spacers of sorts (Kaizen foam?) might allow some air circulation and work better. 

 

The close contact of the cover right now might be preventing any of your warm air from reaching the top of the OTA.  So adding spacers should allow some circulation of the air from your heating rods to work all around the OTA.

 

Best of luck,

 

Ron



#19 freestar8n

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Posted 05 December 2018 - 08:52 PM

Folks have no idea just how darn damp it is here in the NW.

 

I like your insulation idea.  Maybe a solid block or three from Home Depot cut to size.  I think more insulation is better than less.

I think they have an idea of how damp it is in Dublin, Ireland - and I had my EdgeHD8 on cge set up under tg365 cover outside all the time.  The only problem was one time when the whole thing got blown over in a strong wind one night.  It survived with minor damage.

 

In the past I have had a heater underneath - but I have switched to just leaving everything open at the bottom so air is circulated and kept in equilibrium as much as possible.  This has worked well for me in southern NY, where there is a full range of weather conditions; Dublin where there is a smaller range and it is most windy and wet; and now Melbourne, Australia, where it is much warmer and generally drier - but still plenty of dew.

 

Some people tie up the bottom of the tg cover to keep the scope enclosed - but I leave it wide open.

 

The heater may play a role here in forming water - but it is still somewhat mysterious if moist air is needed to enter in large amounts - but it doesn't correspondingly evaporate and leave.

 

Frank



#20 Bowmoreman

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Posted 07 December 2018 - 07:33 PM

I am thinking that you are right.

 

It's a pretty rare occurrence, so I suspect that it's a particular pattern of humidity and temperature change.  In 5 years, it's only happened twice (but I've not had an SCT on the mount for the entire time, since I change OTAs periodically).

 

I should just build a roll-off roof observatory.  I have the space for it (and my wife has OKed it), but I've got very little construction experience.  I could probably manage building a pre-fab shed, but I'm not sure that I could do the modifications to make it a roll-off roof...

One other thought occurs to me. Maybe use a Cat Cooler running continuously to ensure timely air exchange inside the OTA?  Also, put some type of spacer/insulator between the cover and the top of the OTA so that you get less thermal conductivity to rapidly cool the upper end?

re: Rolloff... I can't recommend Scott Horstmann and team at Backyard Observaties highly enough... ;)

 

Dave




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