Should I be worried

[AndyKeogh]

Hopefully someone can interpret the attached CCD image and shed some light on the cause of all the artifacts.

Its a sort of 1st light test image. No darks no flats etc It is one of a set of 21 images.

I was testing my new camera, filter wheel and off axis guider on a Celestron C8 unguided. The first set was 5 seconds each with a Bhatinov mask to check if focus changed as each filter was selected. This was then increased to 30 seconds as the OOB images were a bit faint.

I then shot a 30 second image with each filter and no mask.

The main star is Arcturus and the image below was taken at 01:07 through the luminance filter and binned 1x1. The same effect can be seen in all images though it is brightest in the L & R images

There appears to be many reflections of the corrector plate across the whole image and then repeated on a smaller but more intense scale around Arcturus.

Your thoughts would be appreciated.

Regards

Andy

[steppenwolf]

Andy, what CCD do you have and does it have a square sensor? Is this a cropped image?

First thought is that this is a reflection from the optical flat in front of the sensor to the filter and back and again to 'infinity'

[AndyKeogh]

Hi Steve, I thought I had posted the image train. Need to improve my reviewing .

Full train is:-

C8

WO SCT DDG

Starlight Xpress AO-LF ( not in use yet)

Starlight Xpress AO-LF OAG with Lodestar

SBIG FW8-8300 Filter wheel with Astrodon filters

SBIG ST-8300M camera

Just checked the spec for the KAF-8300 image sensor and they are

5.4μm square pixels.

The image is not cropped just a medium resolution Jpeg.

Interesting thought on the reflections since the AO-LF and the filters are also posssible reflection surfaces.

Andy

[steppenwolf]

I think the first thing I'd do is take the AO-LF out of the image train and make up the missing path with a spacer (something crude would be fine for testing) as this will help to narrow down what surface is reflecting what!

[AndyKeogh]

I'll try that. I will also try the eighth filter position as it is empty. If I cannot try tonight then it will be next weekend before I get another chance.

I'll also replace the oag with a spacer as it mates with the AO-LF

Thanks for your help so far.

Andy

[roundycat]

If I were to shoot Arcturus for 30s I think my camera would burst into flames. A bit like throwing the car off Beachy Head to see if the air bag works. Severe is the way I would describe that test.

What did you hope to achieve?

Dennis

[FraserClarke]

Wow. That is quite a reflection pattern I'm not sure quite how you'd generate that from two plano bits of glass (i.e window and filters). The ST-8300 probably has microlenses on it, so maybe they could contribute to the grid pattern of the reflections? I'm not sure what the optical layout of the AO-LF is? I guess that may have some more complex optics in it which could generate this pattern too. I'd start by taking that out probably...

I'd agree with Dennis though -- a 30s exposure of a Arcturus is a pretty severe test of an optical system!! Whatever effects you see here may well not be a problem for any "normal" observation.

[AndyKeogh]

Well Dennis & teadwarf, with hindsight the test may be deemed severe.

I had just finished some focus tests using a bhatinov mask and to get good spikes with the OOB filters I had cranked the exposure unto to 30 seconds .

The order of exposure once the mask was removed was

OIII, SII, Ha, B, G, R, L. The reflection pattern was not prominent until I used the R filter.

I will get some T spacers and try swapping things around next weekend.

Until then work comes first.

Andy

[AndyKeogh]

Well spacers ordered from Flo today so I should be able to test at the weekend

[AndyKeogh]

The problem has been solved:hello2:

Followed Steve's advice and removed everything in front of the filter wheel and replaced it with 17.75cm of T-tubes and a canon T adapter. Swapped the SBIG interface to a Canon type and this allowed me to swap between my 50D and the SBIG ST-8300M.

At 30s exposure Picture from 50D did not show the artifacts but the SBig still did. The filter wheel was in an empty position so there was now no extra glass in front of the 8300M.

Even reducing the spacing to 50mm did not improve the situation.

It was only while I was preparing some revised images that I realised that CCDops ( the SBig camera software) automatically applies a stretch to the displayed image and saves that stretch with the image when an extension other than sbig is used.

So the image at the start of this thread without the stretch now looks like this

This also explains why my darks also looked noisy/ Guess its time to read the manual.

Thanks for everyones comments.