is my sensor out of alignment?

[graham.hobart]

Hi,
Since owning my FLI 8300 I have struggled to get a decent shot. Most of them have hideous gradients despite using different scopes/adaptors/flatteners/correctors combos.
The other day I tried again and started shooting light box flats then sky flats and then wondered why the illumination was totally asymmetrical on each filter.
(This was with SX filter wheel and 31mm filters). The SXFW seems to be lining up the filters ok so tonight I bought the camera in and shot with filter wheel and no scope just ambient light 1 sec subs at -30.
As you can see I have a horrendous gradient across the sensor.
Does this mean my sensor needs re-alignment?
Please advise.
These are lum, RG shots, bright being lum. Ha and blue failed to load so I will try and shrink these and post later. AstronomiK filters.
Thanks
Graham

[graham.hobart]

added blue filter _ 7nm Ha

[Shiraz]

Hi Graham

if the CCD looks fairly well centered when you look into the front of the camera, then it should be lined up OK.

One possibility is that you might have light leaking into the system somewhere - the Ha image should be very much darker than the others, but it doesn't look like it is. could light be getting into the filter wheel or the camera?

Running a sensitive camera in ambient light with no optics may well overload the CCD and mask what is going on. Do you have any of your night-time sky or lightbox flats that you could post? A dark could also be useful - these symptoms could possibly be caused by Peltier cooler issues.

[graham.hobart]

Good point Ray, later I will post some Lum frames and a dark and some flats. I originally thought about a light leak as the problem seems to shift around on each filter but I can't work out where

[rally]

Graham, couple of Q?s

I am assuming these are stretched images ?
Just how far have they been stretched - most auto stretching will create a black point and a white point and stretch to fit.
So even a small gradient will produce such a horrid result, when in fact that isnt really what exists - just a small gradient.


When you say ambient light, unless you have some perfectlly diffuse light its not likely that the light source will not have a big gradient, so I would expect a gradient to appear - from reflections, from directions of original light source from shadows - particularly the camera body itself shadowing the directional light.

Can you supply a link to some of your orginal (scope attached) flats (full sized FITS with all metadata (before any processing at all) for us to have a look at.

Cheers

Rally

[graham.hobart]

Hi Rally, none of these were stretched, they were FITs taken in Maxim Dl then converted to JPEG and compressed to fit the IIS requirements only.
The camera was pointing away from any point light source so the black band at the corner is what I am concerned about.
When the shutter is closed the darks seem ok (no obvious issue with sensor) so I wonder whether the incidental light is not orthogonal with the sensor or if I do have a light leak.
The other issue which you can see when I post the other frames later, is that assuming the TAK 85mm baby Q 44mm image circle is much bigger than the KAF 8300 sensor size, there appear to be areas not illuminated by the scope?
I know its hard when you can't see the frames but I am at work and will post tonight. Cheers
Graham

[graham.hobart]

another worry is that even if there is a normal gradient, you would (I think assume ) that it would be illuminated centrally and drop off at the edges- this seems not to be the obvious case with my images. :help::shrug:

[PRejto]

Graham,

Have you tried doing any tests without the filter wheel? I would do that just to rule it out. Try taking some flats and see how they look in comparison. Good luck tracking this down.

Peter

[SpaceNoob]

What do the stars look like? That would be a good way to rule out tilt. Perhaps use a bahtinov mask if you have one, it will be very obvious. If the stars match, my guess would be a sensitivity issue across the sensor.

[gregbradley]

Just saw this. I never had any issue with gradients with this camera nor any tilt. But I do see with the many different setups I have used over the years different gradients. For example my Honders setup has a little black bar near the top. I think its from the MMOAG pick off prism. The CDK and Tak BRC250 tend to have a brighter centre in flats, APOs with reducers can get funny reflections.

31mm filters are a little small. I believe they work on QSI cameras because they positioned the filter wheel so close to the sensor. But the SX filter wheel is probably a fair way off from the sensor with adapters etc.

On a faster scope this may be made worse as the angle of light is going to be worse.

I thought 36mm were the go for the KAF8300.
I was sharing a filter wheel with a larger camera so I was using 50mm square filters. FLI promotes how they get their sensors extremely orthogonal. A tilted sensor would be more odd shaped stars in one corner or side of the image not a gradient. A gradient implies something is blocking the light. Are you filters fully secure and they are not loose and flop forward or back? (ie Astronomik filters are 1mm whereas everyone elses are 3mm).

The best suggestion is Peter's. Try taking flats (not overexposed as that complicates any evaluation) without the filter wheel and see if its still an issue. That then nails it down to where the problem is.

Greg.

[gregbradley]

Read your post again.

Ah! You can't take 1 second exposures for flats with a shuttered camera. The flat will pick up part of the shutter.

I found 3 seconds was about the minimum exposure for flats. Although when I did get shutter effects they didn't look like that.

Also you flat box seems way too bright. Those shots would make bad flats as they are badly overexposed. See if you can turn down the light box a lot and try 4 second exposures and keep turning it down until you get about 25,000ADU for a flat with no badly overexposed areas.

Its not the full story but part of it. Too small filters may be another but I would imagine that would mainly cause vignetting. Is the shutter working fine? Have you inspected its operation?

Greg.

[Flugel88]

Greg makes a good point here Graham i was getting similar looking flats with my Atik 383l when taking short exposure flats turned out to be the shutter and also the prism from my OAG.
I found taking flats of atleast 3sec aiming for around 25,000 ADU solved this fro me.
Also had to raise my OAG prism a little.
My 1.25 filters are to small for my 8300 chip i get very distinct concave dark corners in the very edges of my flats but i don't notice them in your flats.

[graham.hobart]

Thanks all for your help, I will try without the filter wheel. It does sound a bit like filter wheel issue as the shutter and camera look Ok and it is a far way off the sensor with the adaptors. Sorry I meant 36mm filters not 31mm
Will keep you posted.
Graz

[graham.hobart]

Looking at my flats- it looks like it could be the shutter smearing across the flat as it is not a uniform vignetting image. Oh Bugger!
Will get there eventually with this camera I'm sure!!:D

[Atmos]

I usually aim for a minimum of 5 second exposures with my QHY9 so as to make sure that the shutter doesn't leave any artefacts. The longer the better at times, takes longer to get the flats but you will have better quality ones.

[gregbradley]

Yes it catches us out occasionally. Especially if you are doing dusk flats and get a bit impatient and take some while its still too light and the exposure time goes below about 3 seconds. Its true of any camera with that type of leaf shutter. I see the same with the Proline. With exposures of 3-4 seconds its no longer an issue.


Greg.

[graham.hobart]

Thanks everyone for sterling advice as per usual for this forum. Will try again once the showers have moved on ( maybe Dec sometime!)
cheers
Graham