Artifacts around bright star - what could it be?

[SkyViking (Rolf)]

Hi All,

Does anyone know what these dark doughnuts are that seem scattered around the star (Canopus) in this image? They only appear on the very brightest stars. They also seem to radiate from the star.
They don't look like regular dust artifacts, and collimation and focus is perfect. Could it be something to do with CCD microlenses??
I have opened the camera and found no dust apart from a few regular grains that I already see on my flats as the usual big doughnuts, nothing like this.
The image is a 20s exposure through my Luminance filter. I haven't had a chance to check through other filters yet.

Any ideas?

Regards,
Rolf

[tonybarry (Tony)]

Hi Rolf,

I have not see this artefact before, but if you slew the scope very slightly, do they remain centred on the star or do they move paradoxically ?

If they move opposite to the slew direction, then they are internal reflections from something. If they remain trained on the star then they are part of the camera.

Regards,
Tony Barry

[multiweb (Marc)]

Never seen this before TBH. Have you tried different filters? Also offsetting the star in the FOV.

[RickS (Rick)]

How many pixels across are the artifacts? I doubt they are dust external to the sensor or they would appear larger.

[SkyViking (Rolf)]

Thanks for your input guys. It looks like it might be clear tonight, so perhaps I'll get a chance to narrow it down more.

I plant to test how the artifacts appear as a function of:
1) Chip temperature
2) Filter
3) Focus
4) Star's position in FOV
5) With/without coma corrector

[Astro_Bot]

Hmmmm. They're elongated radially away from the star, so that ought to be a clue.

My first guess was refraction and reflection, and a resultant interference pattern, around specs of dust or around impurities/anomolies in a glass surface or coating. After more thought, I'm not so sure.

Edit: Was the star centred in the 'scope at the time?

[multiweb (Marc)]

Originally Posted by SkyViking

Thanks for your input guys. It looks like it might be clear tonight, so perhaps I'll get a chance to narrow it down more.

I plant to test how the artifacts appear as a function of:
1) Chip temperature
2) Filter
3) Focus
4) Star's position in FOV
5) With/without coma corrector

The pattern is not unlike light scattering you get at high magnification on an eye piece. Maybe the corrector?

[2stroke (Jay)]

Probably the coating on your Luminance filter Try no filter to rule it out

[Regulus (Trevor)]

Rolf, i am hoping it is a simple fix, like your filter.

i have a similar artifact appearing in a Vixen VMC110 when I throw a star out of focus for collimating. It is not as numerous as the ones you display, but still annoying.
I have ended up doing a very close inspection on the primary mirror and found a worryingly large number of very, very fine pinholes in the mirror coating with some large enough to present an appearance of 'raindrop spattering' in the unfocused image.
As I said, I hope your problem is just a faulty filter.

Good Luck - Trevor

[gregbradley]

I would take a shot, then wipe the sensor glass and take another and see if the pattern has moved. That would indicate dust. Bright shots show up dust the most. Sometimes you can't see it on the CCD window either.

Otherwise some sort of fault in the coatings or mirror sounds likely.

Greg.

[SkyViking (Rolf)]

I think I have found out what caused artefacts on bright stars in my CCD images. Thanks to everyone for your suggestions.
I had already blown all surfaces with compressed air, both front and back of all filters and the CCD chamber window itself. This made no difference.
However, I then determined that the pattern appears confined to the central area of the chip. I made an image consisting of multiple superimposed frames of Sirius at various positions on the chip (see link below) This illustrates where on the chip the pattern appears. Some of the Sirius frames used were taken in focus and others out of focus, to illustrate that there is no difference.

Today I decided to take the camera apart again and take an extra look at the CCD chamber window. It turns out it was full of small spots on the surface - predominantly concentrated inside a circle in the centre, so this is consistent with where the artefacts appeared in the Sirius test images.
These spots must be transparent because they are NOT visible on flats and were completely invisible to the eye but did show up when I pointed a flash-light sideways across the glass. I suspect they may be a result of the condensation that regularly forms on the window surface every time I cool the camera. It usually lasts 1/2 hour and then disappears. Perhaps over time this repeated condensation tends to accumulate some residue on the glass which appear transparent but still refracts the light and produces these artefacts on bright stars.

It's cloudy at the moment so I can't test if the cleaning of the glass window has helped, but I'm pretty confident that this must be the root of the problem.
Since I cannot prevent the condensation I'll just have to open the camera and clean the glass every now and then, probably not more often than every 6 months or so. I'm happy with that.

I have uploaded a set of test images here: http://www.rolfolsenastrophotography...Camera-Testing

[Paul Haese]

It's certainly something that everyone who operates a QSI in a humid environment ought to take note. While I have never had this problem and do operate the camera in a humid environment lots of nights of the year, I am going to take a bit of a look at the frames to ensure I don't get the same problem with the condensation causing these sort of rings. I am surprised that Kevin has not put out an email to all the QSI customers about what the approach should be with cooling but glad it turned out to be a problem that can be solved.

[gregbradley]

You can get dew heaters on the ccd window on FLI cameras. My Proline has them but I never use it as I have not had a problem with dew.
Perhaps QSI can implement something like what FLI has.

Greg.

[SkyViking (Rolf)]

Cheers guys, just to be clear there is never any dew while I'm imaging, but it forms readily if I cool down from ambient to operating temp of say -25C. This repeated condensation/evaporation cycle on every cooldown obviously leaves some residue on the CCD window which was what caused my problem.
The solution is to cool the camera to for example 0C and after a little while, when the electronics have heated the interior, it is safe to set the desired operating temparature - no dew will then form on the window.
I think this is mostly a problem in humid areas where dew is more likely to form.

[billdan (Bill)]

Hi Rolf,

I had a similar problem with my QHY12 on humid nights running at -25C, I changed to -15C and never had another problem.

I was thinking about using a handheld vacuum pump and install a valve instead of the QHY phillips head plug that the dessicant screws in. Then I could suck the moist air out and the valve should retain the vacuum.

However running the CCD chamber at -15C, I found the images are no different than using -25C. ( Well that's with only 5 to 10min subs).


Regards

Bill

[stevous67 (Steve M)]

It's a standard problem with my STL. As a standard procedure, I cool to zero for at least 30mins, then to my set point after that. The moisture/ frosting issue never then results. Peter Ward fitted additional heating to the window, but this only helps after 15-30mins of being on. It's been excellent ever since.

This may be a common issue amongst other brands and models. It must be quiet a severe environment in there! Not much moisture has to get in to cause grief.

Great you worked it out Rolf.

Steve