help with elongated stars

[red dwalf]

hi all,

i seem to be having trouble with elongated stars, i`ve tried everything i can think of and am now struggling to what it can be,

i have tried the following,

tried very hard to get my focusing as good as possiable using either a bantinov mask of the FWHM index in nebulosity 3 which i usually manage to get below 1,

camera square into the focuser, 

focuser tightened up as to not allow slop,

measured the distance from the rear of the reducer flange to the ccd chip (67mm) which i think is correct for this setup.

my equipment is as follows,

William Optics GT81 with reducer / flattener fitted which has a 10mm adapter on the back of that, then a 15mm t2 spacer, then a starlight express filter wheel which is 29mm, and finally a Atik 314L+ ccd camers which has 12mm from front of camera to ccd chip, so total distance around 66mm.

the only thing i can think of that might be the problem is the distance but i`m hoping someone else can shed some light on my problem, here is a picture which is one 6 minute sub in jpeg form.

thanks for your help.

[swag72]

As these look to be across the image and not corner specific I would be looking at a guiding issue. The small Sony chip really shouldnt struggle with chip illumination, even if ypur spacing isn't bang on. How long are these subs?

I'd be taking a few images at perhaps 4s and looking closely at the stars in those subs. If they are round then looks like some form of guiding issue I would suggest.

[AndyUK]

I see this is a 360s exposure...  As Sara's suggested, I'd also be looking at this being a guiding issue as opposed to spacing or tilt.  With the stars being pretty much the same shape across the entire image, I'd personally suspect you may have some differential flexure between your imaging and guide scope. 

I see you use a finder-guider, nothing wrong in that , but I'd personally be looking at checking that everything's clamped down, and also looking at cable management.  I used to let them all hang free, but it's surprising what a difference it makes. 

If this is one exposure of a run, and you have a number of them, if you put them all into Deep Sky Stacker and simply register and compute offsets, you'll be able to see any drift you have by looking at the dY and dX columns...

[James]

I've had a similar problem recently that drive me round the bend trying to sort out that it was... turned out one of my filter holders had a faulty thread meaning that when I screwed in a filter it went in at a slight angle. It was barely visible but as f/4.3 it doesn't take much... I only figured it out when I coincidently twisted the filter holder round and the problem went the same way...

Odds are it's guiding but don't rule out tilt yet

James

[red dwalf]

thanks for the replys, 

i used to guide using a st80 and a zwo 120 mm camera on a dual mount bar, but since i have been getting these strange star shapes i`ve lightened the load and now try guiding using a altair astro 10x60 guider with the same camera, it has lightened the mount weight as i have a smaller guide scope, lost the dual mount bar and have lost one of the counter weights for the mount as well so balancing the mount is better but these star shapes still keep happening.

i have been letting the cables dangle put i have too noticed that these dragging on the pier or sheer weight of them does make a difference to the way PHD 2 performs so have been trying to shorten them and clip them to the pier. 

one strange thing i seem to notice also is that the stars don`t look as bad in RGB filters, maybe thats something to do with the lesser amount of RGB light getting through i don`t know.

the above picture was in Luminance channel but i also get it in Ha to some extent.

[red dwalf]

I've had a similar problem recently that drive me round the bend trying to sort out that it was... turned out one of my filter holders had a faulty thread meaning that when I screwed in a filter it went in at a slight angle. It was barely visible but as f/4.3 it doesn't take much... I only figured it out when I coincidently twisted the filter holder round and the problem went the same way...

Odds are it's guiding but don't rule out tilt yet

James

thats interesting to know, something to consider.

[Davey-T]

Have you looked at the guiding graph to see if it's gradual drift or a sudden jump.

Dave

[Sp@ce_d]

Could be a slight rocking. Mine came loose at the foot to scope fixing. Worth checking.

Sent from my iPhone using Tapatalk

[Thalestris24]

Hi

I think it could also be a seeing issue - poor seeing or too short a guide cam exposure can result in odd-shaped stars. It's worth monitoring the guide star during exposures, at least initially.

Louise

[andrewluck]

I had elongated stars that turned out to be caused by camera fan vibration.

Andrew

[red dwalf]

thank you all, all advice gratefully recieved regarding this problem,

camera fan vibration is something i`ve not thought of, hopefully not, it`s a new Atik 314 and i don`t think you can get a picture without the fan being on.

poor seeing is a possibility i quess as it`s not been the best conditions lately but this problem has been going on for a while, i usually set my PHD exposure time to 2 seconds which gives me a good star exposure with the ZWO 120, infact i`ve been amazed at how good it is as it can certainly penetrate cloud to a certain degree and still stay locked on.

should i maybe increase the exposure time in PHD to maybe 3 seconds ?

i will also check the mount bolts although i believe they are very secure as i`ve been getting the PA as good as possiable to try and help fix this problem.

Have you looked at the guiding graph to see if it's gradual drift or a sudden jump.

Dave

my PHD graph is maybe not the best, but using PHD 2 there are a few different settings that show different graphs, some look great and bob along the line lovely, but on other settings it bobs up and down but never over the +1 or -1 line but does look sharp movements rather than smooth.

[ianaiken]

Did you sort this?

I have exact same problem in same direction.  I think it's down to focuser slop on mine.  There are other reported cases the same.  At that f ratio too it only takes a little.

[red dwalf]

I did, I had a bit of slope in the camera as it wasn't 100% parallel, weight of the filter wheel and camera to blame and the horrible compression fittings on the focuser, also I download the latest snap shot of PhD 2 version 2.4. Orc 3 and the guiding is a lot better with the zwo 120 camera now. I also did away with focusing with the bantinov mask and now use fwhm in nebulosity 3 and get the focusing as good as possiable,

I'm a lot happier with it now and images are miles better.

I also put in 3 x 1 millimeter delring spaces and used some proper calipers to measure the distance from rear of the reducer to front of camera, now it's spot on.