SCT cooling

[mand]

My sct lives inside in a warm room. I understand that I need it to cool down when using it outside to get the best viewing.

But what if I’m looking at a planet with the scope still indoors but viewing through the open door?

 

[Carbon Brush]

The glass surfaces do not change size/shape with temperature - at least not enough to worry.
A tube does change size (length) with temperature so you move the focus a little to compensate.

The temperature drop causes convection currents in the tube, which will cause an unstable image until everything has reached equilibrium.
That is the reason for the cool down time.

HTH, David.

 

[PeterCPC]

Just to add to what David has said. If you are looking through an open door you are going to have air currents disrupting the view - I recommend setting it up outside.

[doublevodka]

There's some quite good info here, yes it's targeted at reflector users, but the principles are very similar - https://www.skyatnightmagazine.com/advice/thermally-optimise-your-telescope

The last picture shows quite accurately the effect of air currents, being indoors with an open door will amplify this effect as the warm air inside will be replaced with the cold air outside and cause turbulence.

This is most easily seen by yourself by looking at the moon with your scope, the view will appear to shimmer which is partly down to the seeing conditions but also down to the cooling of the scope and room.

Ideally, if possible, you want to get the scope outside to cool in advance of observing, roughly 30 mins for a 4 inch scope and add 10-15 mins per inch larger.

[bosun21]

When I take my 150 Maksutov outside the views are terrible due to shimmering and I even wait for an hour to check the alignment of my finder scope. I give it another half hour before observing/imaging the planets. The difference is night and day. SCT's require slightly less time but still require a cooling period to reach equilibrium.

[dweller25]

When you take a warm SCT/Mak outside on a cold night it will have internal thermal temperature differences between the warm mirror and the cooling aluminium tube assembly which is exposed to the cold air.

This causes turbulence at the eyepiece.

One way to deal with this is to leave the scope outside for a couple of hours so the mirror cools to the same temperature as the outside air, in other words there is no thermal gradient between the mirror and outside air.

Another way to deal with the thermal gradient is to wrap the OTA with two layers of Reflectix which you can get from B&Q, this allows you to use your scope straight out of the door.

 

Edited December 5, 2023 by dweller25

[ollypenrice]

50 minutes ago, dweller25 said:

When you take a warm SCT/Mak outside on a cold night it will have internal thermal temperature differences between the warm mirror and the cooling aluminium tube assembly which is exposed to the cold air.

This causes turbulence at the eyepiece.

One way to deal with this is to leave the scope outside for a couple of hours so the mirror cools to the same temperature as the outside air, in other words there is no thermal gradient between the mirror and outside air.

Another way to deal with the thermal gradient is to wrap the OTA with two layers of Reflectix which you can get from B&Q, this allows you to use your scope straight out of the door.

 

The 'Insulate rather than cool' solution is popular here on the continent but never seems to get much exposure in the UK.

Olly

[AKB]

1 hour ago, ollypenrice said:

The 'Insulate rather than cool' solution is popular here on the continent but never seems to get much exposure in the UK.

I'm a huge fan. 

A couple of layers of foil-backed bubble wrap seems to work well.

Did it for my Hyperstar, and now for my RASA...

Tony

 

 

[bosun21]

I've just ordered myself a roll of Reflectix and the tape for my Maksutov. Looking forward to fitting it and trying it out.

[Gfamily]

One issue with Cats (such as SCTs and Maks) is that the light travels 3x through the OTA, so the effects of any internal thermal currents are triply emphasised. 

As Olly and AKB say, insulation can help reduce the effects (and is often mentioned on other forums) - though in both cases using a Hyperstar or RASA will by their design tend to show less effects as the light only travels 2x through the OTA. 

[michael.h.f.wilkinson]

I store my SCT OTA in a garage that has little in the way of heating, but may well add insulation to the OTA for added thermal stability. It is certainly worth trying on the Tri-Band SCT I use for solar, where tube currents can be a real problem

[mand]

Thanks guy

Think I’ll look for space in the garage for storage. Bit more hassle for a Quick Look, but worth it.

[Neil H]

Hi i keep my MAK in the outdoor shed so it's cold , if you want a cheap Cooler there is one on ENS I think it's about £25 

[Gfamily]

When I had a chance to go to the Mount Palomar Observatory in 2001 (regrettably, just as a visitor I have to say), I think I was most struck by the cold cold air streaming out of the vents between the dome and the visitors' gallery. 

It struck me as (belatedly) obvious that it would make sense to keep the dome air conditioned to roughly the forecast temperature for the upcoming evening, so that there wouldn't be thermal air currents between the dome air and the outside air once the dome is opened.