Illumination/vignetting changes with light pollution - Forums

Topic starter

Jan 25, 2023 · Edited: Jan 25, 2023

Hello,

I have recently purchased a new telescope and got out to grab a test frame on my balcony (bortle 7-8 with heavy local light pollution) and noticed strong vignetting in the corners.

This caused me to look back at my old telescope to see if it was as noticeable (which it was - to some extent) compared to the new one. This lead me to go further back to when I visited a dark site with the same telescope and imaging train and to my surprise there was almost no vignetting. I'll attach some pictures.

The two above images are taken within a month from each other with the exact same setup and configurations - nothing has changed between them except the first one is from my bortle 7-8 balcony and the second is from a bortle 2-3 dark site.

Third picture is with my new telescope (TS ONTC 8" f4.5) but the exact same imaging train. Taken yesterday from my balcony. (All the images has had an auto strech applied)

Full imaging train:

Old: 200PDS > TSGPU CC > ZWO OAG > 11mm spacer > 1mm spacer > ZWO EFW (1.25" filter set) > ASI 294MM pro
New: TS ONTC > TSGPU CC > ZWO OAG > 11mm spacer > 1mm spacer > ZWO EFW (1.25" filter set) > ASI 294MM pro

I guess my question is: why does light pollution play such a big role in illumination and vignetting?

Clear skies!

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andrea tasselli

Frequent contributor

Jan 25, 2023 · Edited: Jan 25, 2023

I wouldn't put it that way. Barring strong nearby (very nearby) sources of illumination more or less straight into your imaging train what LP does is to render it much more apparent since, after all, most of the sky patch you're recording is essentially made of background sky with few and far in between bright subjects. The brighter the sky the more apparent the vignetting will appear. This is why you can get away with non carrying out field flattening correction when you shot in really dark skies (Bortle 1-2) if the subject isn't very bright and isn't very large (relative to the sky covered).

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atlejq

Jan 25, 2023 · Edited: Jan 25, 2023

Don't know about the light pollution issue, but ZWO 1.25" filters are too small for the 294MM in this configuration at least. I had significant corner vignetting both with my 200PDS and the 8" f/4.5 ONTC with these filters. The new ZWO filters are made for the ASI1600 sensor which is slightly smaller.

Changed the filters to Antlia 31 mm and the vignetting in the corners is nearly gone.

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Linus

Topic starter

Jan 25, 2023

Don't know about the light pollution issue, but ZWO 1.25" filters are too small for the 294MM in this configuration at least. I had significant corner vignetting both with my 200PDS and the 8" f/4.5 ONTC with these filters. The new ZWO filters are made for the ASI1600 sensor which is slightly smaller.

Changed the filters to Antlia 31 mm and the vignetting in the corners is nearly gone.

I suspect you have slightly more vignetting than me as the GPU has no Barlow effect as opposed to my coma correctors (ES and Paracorr).

I see, I was under the impression that 1.25" was well enough when I bought them. using the tools at: https://astronomy.tools/calculators/ccd_filter_size it says minimum size is 26mm 1.25" = 31mm so should be good with 5mm to spare. My filters aren't ZWO branded, they are Optlong for LRGB and Antlia for SHO.

atlejq

Jan 25, 2023 · Edited: Jan 25, 2023

Yeah I did the same calculation and got the same answer. It is probably correct, but it is close to the margins and then add manufacturing inaccuracies etc.

I'll add a before and after image as reference.

1.25" filters:

31 mm filters:

Linus

Topic starter

Jan 25, 2023

Thanks for the comparison @atlejq, might be time for an upgrade then... Did you have any issues with the corners on the first one not calibrating out with flats?

atlejq

Jan 25, 2023

Thanks for the comparison @atlejq, might be time for an upgrade then... Did you have any issues with the corners on the first one not calibrating out with flats?

I did not manage to get rid of them with flats, but I may have done something wrong. I accidentally scratched one of the ZWO filters so I anyway had to upgrade. So please try the flat calibration before you buy anything.

Linus

Topic starter

Jan 25, 2023

Thanks for the comparison @atlejq, might be time for an upgrade then... Did you have any issues with the corners on the first one not calibrating out with flats?

I did not manage to get rid of them with flats, but I may have done something wrong. I accidentally scratched one of the ZWO filters so I anyway had to upgrade. So please try the flat calibration before you buy anything.

What did you end up upgrading to? 36? 2"?

atlejq

Jan 25, 2023

Thanks for the comparison @atlejq, might be time for an upgrade then... Did you have any issues with the corners on the first one not calibrating out with flats?

I did not manage to get rid of them with flats, but I may have done something wrong. I accidentally scratched one of the ZWO filters so I anyway had to upgrade. So please try the flat calibration before you buy anything.

What did you end up upgrading to? 36? 2"?

31 mm.

dkamen

Jan 25, 2023

My newt (TS Photon) is open at the bottom end, to let air circulate. In a high LP setting, especially if there is direct line of sight to some LED in another appartment or the road, this allows stray light to enter which ruins flat field correction. Other parts of the imaging train may be leaky, too. But that one definitely is.

Solution for me was to cover it up when taking flats (and do both dark and dark flats with the camera capped instead of attached to the scope). No light leaks during flats and darks => even the worst vignetting is corrected.

Cheers,
D.

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andrea tasselli

Frequent contributor

Jan 25, 2023

My newt (TS Photon) is open at the bottom end, to let air circulate. In a high LP setting, especially if there is direct line of sight to some LED in another appartment or the road, this allows stray light to enter which ruins flat field correction. Other parts of the imaging train may be leaky, too. But that one definitely is.

Solution for me was to cover it up when taking flats (and do both dark and dark flats with the camera capped instead of attached to the scope). No light leaks during flats and darks => even the worst vignetting is corrected.

Cheers,
D.

I have the same scope (mine is just faster) and the issue is with the collimation donut, which isn't a donut (paper reinforcer) at all but simply an area without coatings which makes it very handy when collimating in daylight but it is troublesome when taking flats and at night if there is any light showing off from the bottom up. I found that even moonlight reflected off the grass would cause issue with darks. But I never remove the camera from the scope, I just cover the back end with a showercap.

Marc-Antonio Fischer

Jan 25, 2023

Im living in b6 and doing a lot of imaging there. I know the exact "problem" its because these images with much LP are extremly quick in background limitation. I mesured it and afted around 10 secs with f4 i am limited, means the background is "saturated" and further exposuretime basicly does not improve imagequality. Under dark skies you can exposure waay longer. Next time try a 10 min RGB exposure with bortle 1 and you will see you will get the same vignetting, because you are limited