Poor seeing or OTA issue?

[IanH]

Hi,

It was a lovely clear sky here last night so I had the Celestron C11 out for a session with Jupiter.   I've had the scope for just over a year, and have managed to collect a few good eyepieces as I've gone along. 

The results last night were in the main much, much better than I remember from last year - thanks to the eyepieces, with a lot of excellent detail and colour, but there was a strange effect:

- the moons had moviing flares coming off the top and sides like a viking helmet!, while the bottom of the moons were in perfect focus. Also, I would say that the top half of Jupiter was a bit less cleanly focused than the bottom half. This was seen in all eyepieces.

The OTA was well cooled - it lives in an unheated shed and was brought outside about 4 hours before observing, the temperature drop over the observing period was only about 5C. Also I have a lymax cat cooler, which was on it for a good hour before observing.

So, do you think this is this local seeing? I am looking at air across South West London and Slough. Or the jet stream or maybe collimation - I have tried the star test and it looked fine, but the seeing was typically poor then, so it was hard to tell.

My biggest worry, having read around a bit, is that it's mirror turbulence - the effect of a warmer layer of air just above the surface of the mirror. If that is the case, is there an easy way to solve it? Some people have gone of the length of drilling holes in the back and installing fans etc.? With my diy skills I'd probably knacker it completely.

All advice appreciated,

Cheers, Ian

By the way,   I posted this first in deep sky observing - obviously the wrong place - so apologies to the moderators.

[ChrisLX200]

Yes, fans will help with this problem, and it does sound like boundary-layer effects. However atmospheric 'seeing' can play wierd tricks sometimes. All you can do is log the observations and see if you can come to a conclusion whether it's local seeing or related to temperature and your OTA. I did chop a hole in the back of my SCT and install a fan for this reason...

ChrisH

[Peter Drew]

From your description it looks like a classic case of poor seeing, particularly as you judge the star test as looking OK. A good test for poor seeing is to defocus a star or planet and see what the expanded disc is doing, if it's flashing or agitated then it's either poor seeing or the optics haven't reached thermal equilibrium. Or both!  

[Robp]

I had a problem last night with water on the primary on my 200p (Dew?). Not sure if this can impact C11's but I had to get the hair dryer out!

Someone else with more experience may be able to step in and let you know if this could produce the problems you had?

[Robp]

Might even have been a frost instead of dew, hard to tell

[stevepeverall]

I noticed a similar thing last night Ian.

I was walking home about 6pm last night under a beautifully dark, clear sky last night and was expecting a good nights seeing.

I was doing mostly low power observing and couldn't bring stars into pinpoint focus as they normaly appear. Looking at jupiter under low power, the bottom of it appeared in focus but the top skewed. This phenomenon disappeared under high power. There was no dew on my ep or objective lense and  was convinced that there was something wrong with my scope.

I got my other scope out to compare and noticed the same thing. This continued up until 10pm when i finally called it a day. So i guess it must just have been atmospheric conditions.

Just to the naked eye though the sky looked perfect.

[lensman57]

I noticed a similar thing last night Ian.

I was walking home about 6pm last night under a beautifully dark, clear sky last night and was expecting a good nights seeing.

I was doing mostly low power observing and couldn't bring stars into pinpoint focus as they normaly appear. Looking at jupiter under low power, the bottom of it appeared in focus but the top skewed. This phenomenon disappeared under high power. There was no dew on my ep or objective lense and  was convinced that there was something wrong with my scope.

I got my other scope out to compare and noticed the same thing. This continued up until 10pm when i finally called it a day. So i guess it must just have been atmospheric conditions.

Just to the naked eye though the sky looked perfect.

Hi,

Could this cause ablong shape stars, while imaging?  recently I have had more than my fair share of this happening even though the guiding graph is flat.

A.G

[stevepeverall]

Hi,

Could this cause ablong shape stars, while imaging?  recently I have had more than my fair share of this happening even though the guiding graph is flat.

A.G

Not sure to be honest A.G. as I don't image.

I've never noticed anything like this before while observing visually.

[IanH]

I hope its seeing. It was similar on Thursday, which was the other nice clear sky we've had recently. It was wierd how good the image of jupiter was at the same time. I will monitor this as I go along. And when the OTA is out of warranty I might have a go at installing a fan.

[Ludd]

I like your "viking helmet" description.  This is exactly how Jupiter's moons were looking yesterday evening in my C8.  I was reasonably confident that the scope had cooled down OK, so I slewed to Capella, cranked up the magnification and tweaked the secondary to get a nice star image.  (Wish I had bob's knobs - I don't like fumbling around with a screwdriver near the corrector in the dark while peering through the EP!)

Back to Jupiter, and everything looked a whole lot better but still not quite the level of crispness I've seen before, so I'm sure the atmosphere was having some effect.

[IanH]

I'm glad to hear, I'm sorry to say, that you saw the same thing!

Now I think of it, the moons with their flares scaled up and down with the changes of magnification, if it was a local effect caused in the OTA, maybe the aberrations would have stayed roughly the same size or at least varied less?

I suppose we have to put up with british seeing as well as British weather.