Assistance Requested: Artefact

[Stephane]

Hey folks,

I am trouble shooting a large circular artefact with my newly acquired rig. The artefact is not entirely removed with flats. Its center appears to be the same as the center of my optical train, although this could be coincidental. My setup is with a colour zwo2600mc process GPU coma corrector, and an OAG. The focuser is the Esatto. My focal length is 800mm. No filter.

Image1: Uncalibrated sub
Image2: Calibrated sub
Image3: Starless integration of 4 calibrated subs insanely stretched.

Images have had background subracted in order for the artefact to become visible.

Any help or trouble shooting suggestions would be much appreciated.

Regards,
Stephane

[Dennis]

Hi Stephane

Sorry to hear of your problem, I had a somewhat similar result when my camera frosted up. The artefact then slowly disappeared when I set the temp to +1 Deg C.

If that is not the case, try taking Flats at different exposures just to test if a longer/shorted Flat exposure might resolve the issue.

I also wonder if there might be a light leak or a bad internal reflection?

Good luck.

Dennis

[Nikolas (Nik)]

seems like internal reflection of the image train what is new in thios setup?

[oska (John)]

To me that looks exactly like the filter wheel rotating in both directions problem. Given no filter wheel, I would look at anything that allows the slightest of misalignment/vignette over time. Try getting reasonable focus and locking the image train and reshoot tests, including cal.

Also test without the CC, image train locked again.

The first one does look very light leaky.

[Dave882 (David)]

Can’t say for certain but I had something a bit like this a while back that plagued me for a few weeks and then disappeared. I put it down to condensation that was appearing during the session (hence not corrected by flats). I dried out the desiccant tablets in the 2600mc and make sure the anti-dew function is on all the time and haven’t seen it since.

[Stephane]

Thanks for the responses! I've done a series of little experiments which clearly indicate that the OAG, camera, and CC are not causing the artefact, but rather light coming into the tube (not a light leak though as the artefact disappears when the tube is covered even in broad daylight).

Dennis, I've determined that it is not a light leak, but perhaps an internal reflection? Any suggestions for identifying the origin?

Nik, I think you are right. When uncalibrated, it's a centred dark disk (rather than bright ring). Any suggestions where & how to look? The tube is different with a new focuser, secondary mirror, tube, spider, primary mirror mask and primary cell.

John, thanks. I have indeed test without the OAG and CC. The artefact is still present. So those pieces of equipment are innocent. It must be originating somewhere in the tube.

Dave, great suggestion. The artefact however remains even when not cooling.

[Nikolas (Nik)]

Does the tube have an internal baffle? did it have one before? Is the secondary mirror smaller or the same size? How different is the new focuser? Have you tried a dewcap of sorts in front of the tube? Is the tube shiny inside? Is the focusing tube shiny inside?

[Startrek (Martin)]

Hi Stephane,
Sorry to see your issue
Trial and error process unfortunately
One thing I can confirm is that the GPU coma corrector will introduce Vignetting into your image
Mine did on all my Newts 6” f6 , 8” f5 and 10” f5
Flats resolved my Vignetting easily

Maybe send an email to Diego and attach your photo , Diego is the Newt Guru
I’m sure he would be happy to advise

Good luck
Martin

[Dennis]

Originally Posted by Stephane

Thanks for the responses! I've done a series of little experiments which clearly indicate that the OAG, camera, and CC are not causing the artefact, but rather light coming into the tube (not a light leak though as the artefact disappears when the tube is covered even in broad daylight).

Dennis, I've determined that it is not a light leak, but perhaps an internal reflection? Any suggestions for identifying the origin?
>snip

Hi Stephane

I assume the corrector is anti-reflection multi-coated to minimise any internal reflections from the optical elements?

If so, are there any other exposed reflective surfaces or components in the optical train whose profile might look like the unwanted artefacts?

These can be vexatious problems to track down.

Cheers

Dennis

EDIT:

I forgot to ask about the length of your Flat Frame exposures. I read somewhere that certain CMOS sensors can be non-linear with Flat exposures of less than 2 seconds. The advice was to take Flats of at least 3-5 seconds (Histogram peak around 30-50%) and then take equivalent Darks (i.e. 3-5 secs) for those Flats then Dark Subtract them from the Flats. This will give you a say 5 sec Flat Frame calibrated by an equivalent 5 sec Dark frame (same Gain, Offset, Temp.). I guess you could call these Flat_Darks as the Flat came first and the Dark followed it to calibrate it.

[Startrek (Martin)]

Originally Posted by Dennis

Hi Stephane

I assume the corrector is anti-reflection multi-coated to minimise any internal reflections from the optical elements?

If so, are there any other exposed reflective surfaces or components in the optical train whose profile might look like the unwanted artefacts?

These can be vexatious problems to track down.

Cheers

Dennis

EDIT:

I forgot to ask about the length of your Flat Frame exposures. I read somewhere that certain CMOS sensors can be non-linear with Flat exposures of less than 2 seconds. The advice was to take Flats of at least 3-5 seconds (Histogram peak around 30-50%) and then take equivalent Darks (i.e. 3-5 secs) for those Flats then Dark Subtract them from the Flats. This will give you a say 5 sec Flat Frame calibrated by an equivalent 5 sec Dark frame (same Gain, Offset, Temp.). I guess you could call these Flat_Darks as the Flat came first and the Dark followed it to calibrate it.

Dennis,
Good point
The TS Optics GPU coma corrector which Stephane and I use is a high end 4 element coma corrector with no internal reflection to produce a coma free flat field. It’s the best CC I’ve ever used !!
However in saying that , a faulty manufactured coma corrector is possible??

Stephane,
I’ve been using the 2600MC for nearly 3 years now
Here’s my details on Calibration ( Flats and Bias work best on my 2600MC )

Capture software APT ( Flats Aid Tool )
LED light panel ( Huion A3 dimmable light panel )
I take 40 flat frames for each session
White t shirt and heavy duty elastic band
I take 60 Bias frames ( minimum exposure time on 2600MC is 0.000032sec but 0.1 sec will work ok as well
I don’t use Master Bias from a library, I take Bias every session , takes 5 minutes to do.

Broadband
No Filter (2600MC )
ADU Target of 19,000 to 20,000 works best
One layer of white T Shirt stretched over end of OTA with elastic band
Panel dimmed to lowest
Histogram ADU range is usually around 8,000 to 34,000 and exposures between 0.1 and 0.5sec
These Flats work well ( 40 off )

Narrowband
L Extreme and Antlia ALPT dual band Filters ( 2600MC )
ADU Target of between 19,000 and 20,000 works best
One layer of white T Shirt stretched over end of OTA with elastic band
Panel dimmed to lowest then bump it up a notch or two
Histogram ADU range is usually around 8,000 to 34,000 and exposure between 3sec and 5sec
These Flats work really well ( 40 off )

Hope the above is useful

Cheers
Martin

[Stephane]

Thanks guys. I’ve taken note of all your suggestions and appreciate your input. I still haven’t pinpointed the cause, but the darker region in the center could be due to the large secondary mirror with some light pollution gathering around the outer edge.

I’ve been taking flats >2 seconds. The CC is multicoated. I might experiment with the ADU, but I don’t believe this is flats related.

Thanks again.