Ring pattern in images with Newtonian telescope Generic equipment discussions · R8RO · ... · 171 · 6772 · 85

Andy Wray:

The scope is currently outside under my telegizmo cover and it's raining so I can't take a picture of it. This is the latest picture I have from last week when I had it in and was doing some modifications. The imaging train is the following: ASI294MM-Pro - 1mm spacer - EFW - 11mm spacer - OAG - TSGPU CC - focuser

It's probably worth comparing notes as it looks like our two setups are quite similar and I do not have that dark centre/light outside issue on my uncalibrated images.  My setup is:

ASI1600MM Pro (very similar in terms of sensor size and resolution and the same 6.5mm distance to the sensor
Camera screwed directly into EFW7 Mini filter wheel (no spacer)
I'm using the ZWO 1.25" screwed in filters
I actually use a 9mm spacer and then a few others to get me to the required 56mm backfocus
I use the Skywatcher coma corrector for F5 Newtonians
I also use the ZWO OAG V2 although I bought the helical focuser with it
Not important here, but I have the ZWO EAF 5V also having flipped the focusser by 180 degrees

What are the differences (apart from the obvious one of the coma corrector)?

You did not understand me, light goes trought the primary mirror, from the bottom up, you do not have any caps on the bottom of the telescope. there is a primary holder, there are gaps on the bottom, you can close gaps on the bottom, light will still go trought the mirror itself, with source shining from the bottom. Even if it is covered with reflective material, the sides of the mirror are not, the break the light that comes from the bottom of the scope and it eventualy gets trought sides and get into the tubus, this is your problem, i know, i have same problem, and you can test it puting a light source on the bottom of the telescope, even with you cap industral or not on, you will see artefact very clearly, brighter light source, easier to spot the artefact. Judging by the frames you have your light source comes not directly from the bottom but a bite sideways, so it is very important that no light shines on your entire telescope not only on the top. Keep that in mind and you will never have problems like this Clear sky my friend!!!

andrea tasselli:

I would disagree. Flats do not remove light pollution or local light pollution which I believe is causing this reflection. If the reflection moves from taking the flat to taking the light no matter how good your flats are it will not correct it. This is why I see the "embossed" effect on the final calibrated stack. This also explains the far right dust mote since it is by far the most extreme of them all. Being in the far corner and being darker than the rest it would be more susceptible to inconsistencies in movement. As to speak to the tilt. Yes, you are correct. I had a tilt issue when running this test since I had to resort to using the skywatcher original two screw "clamp" mechanic to attach the camera. So the top/bottom inconstancies are due to this.

All my calibration is done through Pixinsights WBPP.

Although, suppose you are correct in your assumption that somehow my flats are not good enough. I do not see any other way to do them. I literally take them within minutes of taking the lights and still they do not correct, any suggestions?

I should have been more specific and saying "it should correct ALL the irradiation disuniformities due to the optics and image train". As for calibration, as opposed on taking them, I'd venture to suggest that you do it manually, thus you'll be sure what is what. Specifically:

a.  Take a series of flats with hystogram below the mid-range threshold.
b. Take the same number of darks at the same gain of the flats (in case you change it from the ASI defaults) and exact same length.
c. If you're using flat panel to take the flat use NINA to take the flats+dark flats.

I'll reiterate: the only way you can be sure your flats are ok is if the correct themselves. Condition necessary but not sufficient, alas.

Retrun to the issue you describe I have 3 newtons and none has anything anything similar, with plenty of light pollution left, right and center.

On the processing side, first, using PI, create a master bias-dark from the dark flats (integration with no normalization and whatever outlier rejection method is most fitting). Then use the master bias-dark thus created calibrate the flats by just using it as a master bias. The proceed to integrate the stack of calibrate flat frames (integration normalization: multiply, rejection normalization: equalize fluxes). Sa as master flat. Then use a master-dark with integrated bias (so no bias removal) and this flat but no biases to calibrate the image.

andrea tasselli:
That a good forst step. Now I'd ensure that taking flats on a different tube orientation produces the same results. If not, something is tilting within you image train. If you think it already does, correct the issue and try again.

Andy Wray:
Apologies if you have already covered this, but have you recently taken any shots without a coma corrector in the train?  A 200PDS would not usually on its own create that dark centre effect, so just trying to rule the CC out of the equation.

Also, do you have any light polution sources that you supect could be affecting your imaging?  My scope sits just outside our kitchen window and my wife frequently walks in and just turns on the very bright lights (we have no blinds in the kitchen) and I still don't see that kind of effect.  Obviously I get gradients, but nothing like yours.

Andy Wray:

@Andy Wray Would it be possible to ask for some samples of your data? A light frame and a flat frame using Lum filter?

Sorry, just seen this request.  Here's a single flat capture and a single light capture from my most recent image.  I applied an STF stretch to each before saving:

Also:  I think I have the same tracing board as you and had to put it on its lowest setting and add a T-shirt on the scope to get anywhere reasonable exposure times.

Lastly:  the very dark gradient at the very top left is my OAG intruding a tiny bit into the image.

Tim Hawkes:
A  long shot probably.  I have had somewhat similar-looking vignetting problems in the past that turned out to be related to the darks rather than the flats.   I was using a masterdark comprised from frames at a slightly different offset value than the lights.  I was usinga ZWO camera with Sharpcap and didn't at the time know that the 'brightness'  referred to the offset value and hadn't been careful to match lights and darks with respect to this parameter.

Tim

Andy Wray:

Andy Wray:

@Andy Wray Would it be possible to ask for some samples of your data? A light frame and a flat frame using Lum filter?

Sorry, just seen this request.  Here's a single flat capture and a single light capture from my most recent image.  I applied an STF stretch to each before saving:

Also:  I think I have the same tracing board as you and had to put it on its lowest setting and add a T-shirt on the scope to get anywhere reasonable exposure times.

Lastly:  the very dark gradient at the very top left is my OAG intruding a tiny bit into the image.

Could you make these available for download? I'd like to take a look and compare in Pix.

I've never used Google Drive, but maybe this will work:

https://drive.google.com/drive/folders/1zGcZlSHd-77PygcELg8bVGPjYn_gAV5A?usp=sharing

• Unlike dust motes on filters, there is no secondary "hole" in the dark area in your flats. So if it's something in the imaging path, it is very close to or on the sensor.
• It is roundish, but not perfectly round.

Okay... Progress?