600 sec test image - oval stars - different directions

[rubecula]

I took a 600 second test image the other night to see how good the guiding was.  I know I need to do something about the light polution but I am mainly interested in the star shapes.  They seem quite good in the centre of the image (round the over-exposed Aldebaran) but towards the corners they are egg-shaped.  What's foxing me is that the egginess is in different directions.

Could anyone advise me as to whether this is a tracking polar alignment/image rotation issue or does it indicate some sort of physical hardware issue.

[Daniel-K]

was this with the 8" EDGE ?

[MarkyD]

With what little knowledge I have could it be that you need a coma corrector?

--

Mark

[rubecula]

Yes it was with the 8" EdgeHD and the 0.7 focal reducer

[Daniel-K]

Could it be that you have the spacing wrong between the sensor and focal reducer 

[Davey-T]

Wrong spacer distance can give eggy stars that radiate from the centre or could be focuser tilt or bit of both.

Dave

[Thalestris24]

Hi

Could also be some field rotation if your PA is not so good.

Louise

[ollypenrice]

Corner crops would help. At the resolution on here it looks like the top is fairly good but the bottom is elongated downwards and slightly outwards?

For me the chief suspect would be tilt but chip spacing might be a player. Off the top of my head I'd be doubtful about PA because that normally creates a rotary distortion. Tracking affects the whoe image equally.

Olly

[Uranium235]

+1 for tilt, you will need to look at how the imaging train is attached to the focuser, check for any slop.

[Oddsocks]

Olly, Dave and Rob above and I think it is tilt. 

I put the image though CCD inspector, the absolute numbers it came up with are meaningless because the image is too highly compressed and contains jpg compression artefacts, CCD inspector needs DSLR raws, FIT or 16bit TIF files before meaningful numbers come out of it but the 3d view clearly showed a major tilt down and to the left with some residual field curvature.

Assuming you have the dedicated Celestron #94242 0.7 Edge reducer have you set the camera sensor to be exactly 105mm from the reducer and how is the camera mounted?

Celestron recommend the #93644 two-section long "T" adaptor, the second-part extension is not used with a DSLR and is unscrewed from the "T" adaptor body, the remaining short part is screwed directly to the reducer and DSLR "T" ring thereby setting the backfocus at exactly 105mm and giving a sag free mounting for the camera.

(As per the Celestron Edge reducer document: http://www.celestron.com/media/796173/94242_reducer_inst_sheet.pdf )

Reading your signature list it seems you are using a slim OAG with nosepiece attachment so the problem is most likely sag where the nosepiece attaches to the rest of the imaging chain, the safety-undercut nosepiece attachments are notorious for being a sloppy fit in compression ring holders, you may need to wrap the nosepiece with some thin foil or plastic film so that the nosepiece cannot sag under the weight of the camera.

[rubecula]

Thanks all for the very helpful responses.  Having read your comments I think it must be some looseness in the imaging train (photo below). Having got some of the components stuck together previously I've been reluctant to tighten them too much in case they become permanently welded together.  But a response to another of my questions suggested using boot polish on the threads to stop them sticking together so I'll have a go at using that and really tighten them up.

Regarding the back focus I'm pretty sure it's exactly 105mm.   It's made up like this:

Sensor to T-ring                  = 55mm

Extender tube                     = 14mm

OAG                                   = 16mm

Blue Fireball SCT-T2 adapter = 20mm

Although with needing a light polution filter I'm wondering about getting a filter wheel.  But that's another issue.

Thanks again for your help

[knobby]

Old thread I know but how did you get on with this ? My setup works out at about 108mm but not used in anger yet ?

[rubecula]

knobby, it turned out to be a faulty Focal Reducer.  FLO kindly worked with me to get a new one from Celestron.  All works fine now. Now trying to master dithering.