Star aberration - opinions please

[Tommohawk]

Hi All

I'm having ongoing problems with a set up that has been reliable in the past and am a bit mystified re the origin of this. I'm seeing pretty gross aberrations around moderately bright stars - doesnt seem apparent at all on v bright stars eg Regulus, or on dimmer stars.

I'm wondering if this has to do with either some kind of mirror pinch issue, or possible something thermal - but I stress I've never had this before.

Mirror pinch seems unlikely - I'm  working at about 50 deg elevation, so there should be any sideways flop. I noticed yesterday that the pattern changes when I cross the meridian - so I wonder if I have some thermal currents either from the primary or possible even the secondary.  

The primary has no fan/heater, and although it has plenty of time to settle before I start, the temperature has been dropping so markedly that it may still be an issue. Last night was probably air temp around 15 deg earlier dropping to maybe 2 deg.

With all this extra furlough time I've been able to  set up nice and early and have everything ready to start imaging promptly, but as part of my routine I've had the secondary heater on maybe 2 hours before I start - maybe its overheating? Its only 70 ohm @ 12V so 2 watts give or take.

Here's a couple of FITS files either side of the meridian - grateful for any thought because I've wasted all the recent session with the fine weather and getting a bit despondent.

2020-04-14_22-09-20_L_-20.00_60.00s_139.00_64.00_1_.fits

2020-04-14_22-00-38_L_-20.00_60.00s_139.00_64.00_42_.fits

[Tommohawk]

Well - didn't seem to excite too much interest with this post!!! Anyhow, I've solved the problem - definitely down to an over-heated secondary.

As a rule I only put the secondary heater on as I start imaging - but due to a combination of later imaging runs with the lighter evening, and extra set-up time as a result of furlowing, I've been leaving the whole rig set up and ready to go for maybe 2 hours including having the secondary on, mostly so I dont forget it. 

So 2 nights back I got set up but only connected the heater as I started and... problem solved. In fact after an hour or so imaging with the heater the aberration was returning again, and resolved shortly after disconnection. In fact  because the humidity was low I just left it off for the final 90 mins and no probs at all.

So the take home message is, dont get the secondary up to egg-frying temperatures. As I mentioned its only a 2 watt heater so amazed it could overheat. I dont have a controller, but I do have a powertank with variable voltage outputs so I'll use that for now.  

Hope this post may help someone else at some point!

[alan potts]

Can you post Jpegs, I can't open them, not that I will be able to help you but others I am sure will be having the same problem,

Alan