Can SCT thermal imbalance look like a collimation issue?

[badhex]

Soooooo might have made a rookie mistake here but I am here to learn! 

I have only really used my C5 a few times last year and for the 2020 great conjunction and the rest of the time it's been sat in a case and occasionally moved, partly because I never got on with it, having also a very fine (now two!) refractors. 

Wanting a brief high power doubles session the other night on a whim I got out the C5 only to find that the views were absolutely hideous, completely impossible to get anything like sharp focus.

I had initially assumed not enough cooldown time, but when out of focus it seemed to be terribly miscollimated - definitely not concentric circles, and in one direction (not sure if intra- or extra- focal) the circles themselves seemed almost teardrop or pear shaped. 

Unfortunately, at this point assuming it *must* be miscollimated, I made the mistake of adjusting the collimation screws - to no avail -  rather than just being patient and waiting to see if cooling was the issue. Bear in mind I've never collimated an SCT but figured it can't be that difficult it you adjust carefully and methodically. Regardless, I did not make it better - nor worse. 

All in all feeling a bit silly and in the cold light of day wondering if I took a collimated, but thermally unstable, SCT and foolishly miscollimated it for no good reason. 

 

[Zermelo]

Like this?

 

 

 

[NGC 1502]

Perhaps tube currents, more cooldown required. Your comment “teardrop & pear shaped” is a good description.

Edited February 17, 2022 by NGC 1502
Correction

[badhex]

15 minutes ago, Zermelo said:

Like this?

 

 

 

So, nowhere near as bad as that but yes, quite similar. I guess that is sort of a relief? Except for the knowledge that is now almost certainly miscollimated!

Thanks for the reference. Not something I've encountered before but  this is the first SCT I've owned (I used to have a lovely Mak 102 years ago) and have apparently forgotten that CATs take a lot longer to cool.

TBH, I was in a bit of a rush due to having only a short window in which to observe. 

[badhex]

12 minutes ago, NGC 1502 said:

Looks like tube currents to me, needs more cooldown. Your comment “teardrop & pear shaped” is a good description.

Thanks, yes as per the link above but not quite as bad. On the few occasions it has had use previously it was outside cooling for a good while prior to use - will have to test again as soon as it is clear. 

[NGC 1502]

3 hours ago, badhex said:

Thanks, yes as per the link above but not quite as bad. On the few occasions it has had use previously it was outside cooling for a good while prior to use - will have to test again as soon as it is clear. 

A suggestion that may help. To reduce cooldown time, are you able to store your scope in a cool place?  If you can do that then the scope is already some of the way to achieving full cooldown.

Full cool down is achieved when the scope is at ambient temperature.  So if it’s sub-zero outside and the scope is at comfortable room temperature then cooldown will take far longer.

[badhex]

7 hours ago, NGC 1502 said:

A suggestion that may help. To reduce cooldown time, are you able to store your scope in a cool place?  If you can do that then the scope is already some of the way to achieving full cooldown.

Full cool down is achieved when the scope is at ambient temperature.  So if it’s sub-zero outside and the scope is at comfortable room temperature then cooldown will take far longer.

Unfortunately not. If anything the storage space is probably one of the warmer spots of the house 😬

I do have a balcony where I usually put whatever scope I'm using outside for a good while before a session so it can get down to ambient outside temperature, and this usually works out well. If I don't have as much time, for either of the fracs I leave the focuser end cap off (2") and pointing upwards, to allow the warm air to rise out more quickly. The two fracs are pretty quick to cool down anyway. 

Unfortunately the SCT design obviously does not lend itself to letting warm air out of/cold air circulate in to the scope body very easily, and I really just rushed things. I'll have to be a bit optimistic and put the C5 outside if it even hints at being clear so I can ensure it has a couple of hours to cool. 

[dweller25]

Hello @badhex

To improve the thermal stability of your C5 when you take it from your warm house to your balcony just wrap it with TWO layers of reflectix - there will be no need to cool the scope down.

It’s cheap, effective and can be easily removed if required 👍

Edited February 18, 2022 by dweller25

[badhex]

On 18/02/2022 at 08:55, dweller25 said:

Hello @badhex

To improve the thermal stability of your C5 when you take it from your warm house to your balcony just wrap it with TWO layers of reflectix - there will be no need to cool the scope down.

It’s cheap, effective and can be easily removed if required 👍

Interesting! Might have to give this a go if I fancy a quick session and haven't left everything out prior - thanks!