180mm Mak - use of thermal blanket avoids cool down time?

[markclaire50]

This is a scope I'm seriously thinking of getting for double star, planetary( when the come back!), lunar observation. I hear lots of people saying it's a pain due to cool down time. But I'd like to know if

A. THIS IS AS BAD AS 'PEOPLE' SAY with respect to waiting time and performance degradation? 

B. CAN WRAPPING A THERMAL BLANKET ALLOW QUICK USE OF THE SCOPE (and double up as dew shield and stray light protector)? 

Thanks

Mark

[ashortfallofgravitas]

Wrapping it in a thermal blanket would make your cooldown time worse

[markclaire50]

1 minute ago, ashortfallofgravitas said:

Wrapping it in a thermal blanket would make your cooldown time worse

Hi

I've heard though that the idea isn't to cool it down, but to stop it cooling, to prevent air movement inside tube and thus image is sharp? 

[ashortfallofgravitas]

1 minute ago, markclaire50 said:

Hi

I've heard though that the idea isn't to cool it down, but to stop it cooling, to prevent air movement inside tube and thus image is sharp? 

It would slow its cooling but whether or not that would reduce air movement in the tube I have no idea

[John]

Best approach would be to store the OTA somewhere that remains close to outside temperature. I've seen folks use large waterproof boxes kept in sheds / greenhouses / garages etc to store the OTA.

 

[RayD]

6 minutes ago, markclaire50 said:

Hi

I've heard though that the idea isn't to cool it down, but to stop it cooling, to prevent air movement inside tube and thus image is sharp? 

I would have thought the idea is to stabalise the temperature at the ambient temperature, so adding a thermal cover would be an almost pointless exercise apart from preventing the OTA heating up in the day time.  My Telegizmo 365 cover has a thermal type lining for this purpose.

[Stu]

9 minutes ago, markclaire50 said:

Hi

I've heard though that the idea isn't to cool it down, but to stop it cooling, to prevent air movement inside tube and thus image is sharp? 

That is exactly it. You are reducing or preventing convection currents within the tube which stabilises the view.

In my experience, even with long cooling times you still do get tube currents, particularly when night time temperatures continue to fall as you observe.

I have an Intes 178mm Mak and even with time for cooling I get tube currents, so I am going to experiment with insulating the tube. Several members on here do the same with good results.

[JamesF]

My understanding (and I may have got it completely wrong) was that the thermal jacket was an attempt to stabilise the temperature of the OTA once it is already at ambient temperature.  So if the OTA is kept in an observatory at ambient temperature the jacket would then slow down the effects of variation in ambient temperature over the course of the night and reduce the effect that result from the lower half of the OTA being warmer than the upper half.  I didn't think it was really supposed to help that much with taking the OTA from (say) a 20C+ house to sub 5C observing site.

I'd have thought that if the OTA is being moved from inside the house then one of those SCT-style cooling fans (the ones that fit in place of the visual back, or even into the visual back) might be the best bet.  My brain is coming up blank on who makes them at the moment though.  Once I have my observatory finished I was thinking about 3d-printing one for my C9.25.

James

[Peter Drew]

This is a hotly debated subject on CN. Those that use this form of insulation are adamant that it works, those who don't say it won't! For what it costs I suggest it's worth a try.   ?

[JamesF]

1 minute ago, Peter Drew said:

This is a hotly debated subject on CN. Those that use this form of insulation are adamant that it works, those who don't say it won't! For what it costs I suggest it's worth a try.   ?

Hah!  "hotly debated".  I see what you did there

James

[Gfamily]

14 minutes ago, Peter Drew said:

This is a hotly debated subject on CN. Those that use this form of insulation are adamant that it works, those who don't say it won't! For what it costs I suggest it's worth a try.   ?

One thing I would say is I've read more posts from people who say "that won't work" (on theoretical grounds) than I have from people who say that they have actually tried it and found it didn't worl.

I think 'try it' is the best advice, but bear in mind that you can't tell whether poor seeing is due to thermal currents or due to just generally poor atmospherics.

I've heard the advice but not tried it. 

 

[Stu]

39 minutes ago, JamesF said:

My understanding (and I may have got it completely wrong) was that the thermal jacket was an attempt to stabilise the temperature of the OTA once it is already at ambient temperature.  So if the OTA is kept in an observatory at ambient temperature the jacket would then slow down the effects of variation in ambient temperature over the course of the night and reduce the effect that result from the lower half of the OTA being warmer than the upper half.  I didn't think it was really supposed to help that much with taking the OTA from (say) a 20C+ house to sub 5C observing site.

I'd have thought that if the OTA is being moved from inside the house then one of those SCT-style cooling fans (the ones that fit in place of the visual back, or even into the visual back) might be the best bet.  My brain is coming up blank on who makes them at the moment though.  Once I have my observatory finished I was thinking about 3d-printing one for my C9.25.

James

Obviously there are limits James, but my understanding is that you don't have to wait for the OTA to reach ambient before insulating it. Adding the insulation prevents heat being radiated from the top of the tube and creating internal tube currents.

I believe @chiltonstar has done this with positive results, and possibly @dweller25 too.

[JamesF]

Perhaps it might be interesting to make a "hair net" of 1-wire temperature sensors for the OTA to measure the temperature during the course of a night's observing, then do the same with a thermal jacket on the OTA.

James

[Stu]

44 minutes ago, Gfamily said:

but bear in mind that you can't tell whether poor seeing is due to thermal currents

From what I've seen, I often get a strong vertical 'spike' in the defocussed image when the problem is cooling related, vs a more general instability when it is seeing related. 

[Gfamily]

1 minute ago, Stu said:

From what I've seen, I often get a strong vertical 'spike' in the defocussed image when the problem is cooling related, vs a more general instability when it is seeing related. 

Useful to know. I shall look out for it. 

Should make it easier to tell whether a thermal jacket makes a difference. 

I generally am either using a scope that's been 30 minutes or more in a cold car beforehand, or I'll try and move the scope outside an hour or so before properly 'setting up' 

[Stu]

13 minutes ago, Gfamily said:

Useful to know. I shall look out for it. 

Should make it easier to tell whether a thermal jacket makes a difference. 

I generally am either using a scope that's been 30 minutes or more in a cold car beforehand, or I'll try and move the scope outside an hour or so before properly 'setting up' 

An 8" or there abouts Mak can take a couple of hours to cool, so it is challenging if storing inside. I'm finding ways of keeping the scope outside to reduce this but it doesn't always stabilise easily.

[Paz]

10 minutes ago, Stu said:

From what I've seen, I often get a strong vertical 'spike' in the defocussed image when the problem is cooling related, vs a more general instability when it is seeing related. 

I often observe with a tube that hasn't fully cooled and I get this spike in the defocussed image. I've tried insulating my ST120 and 127 Mak and it does help a bit with slowing down the tube currents.

Another interesting test is if you can mount up an uninsulated scope, put it outside and immediately observe a target you may notice a good image for a few moments and then you will see the effect of tube currents kicking in. I've seen this when I have my ST80 all set up and mounted indoors and ready to go then I lift the whole thing outside and start looking straight away.

[dweller25]

I tried thermally insulating a C6 with two layers of reflectix that I store in the house (centrally heated) - the results were great - I started to use the scope within 15 minutes, no thermals were evident on a defocused star and the corrector did not dew up.

I also tried it on an Intes Micro 715 Mak and did not get the same  results - maybe I need three layers and the vent holes front are rear may need blocking up - have not tried that yet.

For what the reflectix costs it's probably worth a try if you store you scope indoors.

[John]

I've owned a few maks and cats over the years and generally the cool down times have been why they have not remained with me. It's my own fault for preferring to store my scopes inside AND wanting fast setup / tear down times AND liking to use high powers on targets.

I had a mak-cass 180 for a couple of months but I don't think I ever got it fully cooled down !

Great scopes when they are at ambient of course and a lot less expensive than an apo refractor with comparable performance.

[dweller25]

2 hours ago, ashortfallofgravitas said:

Wrapping it in a thermal blanket would make your cooldown time worse

Correct - but the primary goal is to stop the thermal gradient inside the OTA between the cold OTA walls and the warmer mirror.

[Stu]

I have some time this weekend so will try to have a go at wrapping my Intes. Noted about the vents, will make sure I add plenty of layers and cover the back and side vent too.

[dweller25]

@Stu

Look forward to your results ........

[GazOC]

I've had a 180mm Mak for around 10 years and it does take a long time to cool down but you've just got to think a little ahead if you think you might want to use it. 

If clear skies look possible that night I move the scope to my shed before I go to work or, if I forget that, as soon as I get home. Its usually good to go in 90 mins/ 2 hours.

To hear some people talk you'd think they stored the Mak next to their log burners and only had 10 minutes notice to plan an observing session ?

[JamesF]

3 minutes ago, GazOC said:

To hear some people talk you'd think they stored that Mak next to their log burners and only had 10 minutes notice to plan an observing session ?

Who are you and how long have you been spying on my house?

James

[markclaire50]

1 hour ago, GazOC said:

I've had a 180mm Mak for around 10 years and it does take a long time to cool down but you've just got to think a little ahead if you think you might want to use it. 

If clear skies look possible that night I move the scope to my shed before I go to work or, if I forget that, as soon as I get home. Its usually good to go in 90 mins/ 2 hours.

To hear some people talk you'd think they stored the Mak next to their log burners and only had 10 minutes notice to plan an observing session ?

Hi. I'm very interested to know your best memories of using your mak. Like jupiter and its moons, saturn, Mars double stars, e and f stars in trapezium. And of course, our Moon! ?