I swapped from my Canon 550D to an ASI294MC Pro camera recently. I've been working my way through getting used to that (along with the occasional hickup as I try and get observatory automation sorted).
One that I've not got to the bottom of is a large bright area appearing in all of my light images. The attached image is a jpg exported from an original FITS file. I was not getting this with the Canon.
The tube is a Skywatcher ED100 (slightly cutdown). I do normally have a diagonal in the path but get the same issue with it removed (the observatory is a little smaller than it should have been and camera cables can hit the sides in some spots without the diagonal).

I can't see any unusual obstructions in the tube. With the front lens removed all of the sensor plus some is visible looking back up the tube.
I run a dew heater strap on the front and have pondered if the center of the tube is not getting heated sufficiently but the bright spot is a little to consistent to convince me of that.

No obvious signs of in in darks (and I've not got the drivers for my flat flap working yet but doubt I'd spot it in a flat anyway).

Any thoughts on what else I can be checking?

Bob,

Do you see this also when you take flats? And if you don't see this with the Canon camera then I doubt the dew heater can be blamed for this. To what temperature do you cool the ASI294MC Pro? Do you see this pattern also when cooling to a higher temperature? Or if you cool first to, say, 0º C and then further down?


HTH, Wouter

I've not got my flat setup working yet. I've seen it at -10, -5, 0 and 5C. I tried for -15 earlier but it's struggling to hold that so I've not got images at that.
I'm still learning my way around this camera but the bright spot isn't making sense to me.

Hi Bob,

I recently wrote a starters guide to the ASI294MCpro . The guide references some really good resources for information regarding calibration. You can find it as a sticky in the CCD section of the forum. The 294 is particularly picky about white balance and offset. Flats can be quite important in getting rid of any visual axis artifacts. Try to take your flats at exposures longer than 2-3 secs as the sensor is not linear below that and you can end up with odd clipping in your calibrated images.
I had something similar once related to a residue of a lens pen cleaning solution which I could not see unless i looked really closely but showed up as a persistent artifact and seemed to encourage dewing too. All disappeared once I realised what it was . It is a great value camera though and I have never regretted buying it.

Mike

Thanks Sparticus. I think it was your guide that I used to adjust my gain etc. Very handy thanks.

I will need to get myself sorted for flats by the sound of that. I've built a dark/flat flap for the refractor but now worked through the driver setup for it yet.

I've just been out swapping tubes to see if the bright spot happens with an entirely different optical setup. Gone from a 100MM refractor to an F4 153mm reflector. That tube does not have autofocus and does not seem to guide as well on the HEQ5Pro so I could be in for an interesting time getting it setup.

Does your camera have the bright spot in the original images? Am I chasing something thats normal? It is not something I expected and I'm trying to work out if this is a camera issue or something to do with my refractor.

Bob

Hi Bob,

It is definitely not a common feature of this camera. What was your exposure time for the image that you posted? This was an uncalibrated fits? If so the exposure must be pretty short as I can't see the usual amp glow. I can usually see this in exposures of 10 secs at 120 gain if I look hard enough. Just in case you cropped the image to better show the bright spot post an uncropped jpg so we can see how big the area is.

It would be good to see an image of a relatively featureless bit of sky as the galaxy interferes with the bright spot area. Alternatively you could do a ccd analysis following the instructions in SharpCap (not sure but you might be able to download a trial version just to test this). Just to gather more info about the sensor performance. There are some guides on You Tube how to do this. You can do a sort of flat pointing at a white wall. You could do a quick master flat by this method and see if the bright spot shows up.

It is sometimes a matter of chipping away getting bits of evidence together to pin down what it is. Check every bit of glass that is in the light path. However if you have changed scopes and the problem persists I would be looking at the camera itself. If you have not yet tried it with a different scope this would be useful in identifying the cause.
Performing the "indoor" type flat removes any prospect of dewing issues being the problem although unlikely if this is consistent across many nights.

Cheers

Mike

Try taking an image at daylight, with the telescope aperture closed, to see if there is some light leak.
Else the suggestion of Spartacus is a good one, a thin film of oil or other solvent on an optical surface will produce scattered light that can create such a central light blob. Check surfaces like the entrance window or any filter in the beam....

Looks like you have internal reflection on your optical train, have light shine directly into your objective lens maybe?

I think that one was a 300 second, 150 gain image. It was a straight export from Gimp. The amp glow thing had me wondering as well. The amp glow pattern show in on the companies website is really clear in darks but I've not spotted it in any images taken of the sky using as far as I can tell the same set of settings. I'm doing some testing tonight with an F4 Newt and now seeing the spot. I have done a clean of the lens and had the diagonal out of the path in the refractor so it seems unlikely either is the issue.
Is there a chance that a 100mm 900mm refractor is too narrow for this sensor? Interference from the tube is about all I can think of at this stage (although it looks fine to my less than knowledgeable eyes)

The F4 didn't guide particularly well on my mount in the past so I'm not keen to swap back to using it (and need to work out where the camera would need to sit in relation to the coma corrector).
I've also got an F12 Maksutov Cassegrain and I'm wondering how it would go with this ASI294MC. That might be my next experiment. Weight wise combined with the focal length may make it a bigger challenge to do the kind of work I want to do (and I'm somewhat interested in the wider views).

Derpit, when you say aperture closed do you mean the front cover on the tube? There is a light leak around the focuser tube. I should be able to try that pretty easily even with the tube off the mount.
By the same token I'm thinking I can get around the not yet developed driver for the dark/flat flap and just bypass some wiring to get the flat light source running.

There is a free version of Sharpcap, I'll have a look and see what it can do. Feature comparison between the free and the paid versions is at www.sharpcap.co.uk/feature-comparison

Yes, front cover closed, that's what I meant. The tube doesn't have to be on the mount for that. If there really is a light leak, that could be an issue.
Else you can look into the tube with (only) the camera removed. Are there any 'shiny' tubes? Especially close to the camera, and/or a part that is not used with the DSLR. (Else the reason you don't see it there could be that it is more infrared which a normal DSLR won't see)

I strongly doubt it's software related....

This is definitelly not ampglow or software problem. Look like stray light. Check if you have stary light shine directly into the tube and/or light leak in your optical train. You'd be suprised to see how much reflection there can be from a 40W lightbulb from 20m away shining into the scope. It can easily overpower all the nebulosity in the nebula.

Hi Bob,
I am not familiar with Gimp. So sorry to be a bit pedantic but, is the jpg uncalibrated ? e.g. straight from the camera or have you calibrated it with a dark? The light spot is nothing related to amp glow as minhlead says but if it is uncalibrated where is the amp glow? At 300sec exposure it should be really obvious as a flare in the right upper area. If they have been calibrated with darks this will remove the amp glow. However, once you start calibrating this can lead to odd effects creeping in. Ensure any calibration frames/masters are done at the same gain and offset as your lights. Having said that the artifact is not typical of calibration problems that I have seen.
The glass window that protects the sensor should checked as this was the source of the artifact that I mentioned. Give it a good clean just in case.
On a different note guiding an F12 scope unless using an OAG can be pretty testing but even poor guiding should not have an effect on the bright artifact that you are seeing.

Mike