mand Posted December 5, 2023 Share Posted December 5, 2023 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? Link to comment Share on other sites More sharing options...
Carbon Brush Posted December 5, 2023 Share Posted December 5, 2023 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. Link to comment Share on other sites More sharing options...
PeterCPC Posted December 5, 2023 Share Posted December 5, 2023 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. 1 Link to comment Share on other sites More sharing options...
doublevodka Posted December 5, 2023 Share Posted December 5, 2023 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. Link to comment Share on other sites More sharing options...
bosun21 Posted December 5, 2023 Share Posted December 5, 2023 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. Link to comment Share on other sites More sharing options...
dweller25 Posted December 5, 2023 Share Posted December 5, 2023 (edited) 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 1 Link to comment Share on other sites More sharing options...
ollypenrice Posted December 5, 2023 Share Posted December 5, 2023 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 1 Link to comment Share on other sites More sharing options...
AKB Posted December 5, 2023 Share Posted December 5, 2023 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 1 Link to comment Share on other sites More sharing options...
bosun21 Posted December 5, 2023 Share Posted December 5, 2023 I've just ordered myself a roll of Reflectix and the tape for my Maksutov. Looking forward to fitting it and trying it out. Link to comment Share on other sites More sharing options...
Gfamily Posted December 5, 2023 Share Posted December 5, 2023 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. 1 Link to comment Share on other sites More sharing options...
michael.h.f.wilkinson Posted December 5, 2023 Share Posted December 5, 2023 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 Link to comment Share on other sites More sharing options...
mand Posted December 5, 2023 Author Share Posted December 5, 2023 Thanks guy Think I’ll look for space in the garage for storage. Bit more hassle for a Quick Look, but worth it. Link to comment Share on other sites More sharing options...
Neil H Posted December 5, 2023 Share Posted December 5, 2023 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 Link to comment Share on other sites More sharing options...
Gfamily Posted December 5, 2023 Share Posted December 5, 2023 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. 1 Link to comment Share on other sites More sharing options...
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