Problem with Flats - BIG Doughnut!

[old_eyes]

Hi All,

I am havin problems with flats. I had previously been getting noise problems which meant I was getting horizontal lines on my calibrated and integrated images. This was traced to problem flats. Those noise problems now solved, but another issue has become obvious. I am getting flats like this (master flat debayered and autostretched in PI):

And I am pretty sure flats houldn't look like that.

The dust mostes are fine, that is what I would expect to see, but what is htis damn great doughnut across the image. Almost looks like an Airey disc.

I cleaned all the optics in the train (well reasonably well anyway). It stays.

I thought it might be a light pollution filter screwed into the front of the 2" nosepiece on the front of my flattener. Took it out. Still there.

Perhaps my flat panel was uneven or too bright? Tried skyflats (this image is from skyflats with a 7 sec exposure). No change.

If it calibrated out of all images it would be less of a problem, but often when stretching you get a ghost of the ring showing up clearly in the image.

Going back through the archive it seems to have manifested when I switched from DSLR to a cooled CMOS OSC camera. It is hard to be sure as my DSLR flats weren't great.

Any thoughts?

Could it be the flattener (this one from FLO https://www.firstlightoptics.com/reducersflatteners/ovl-field-flattener.html)?

Could it be the flattener to chip distance? I have it reasonably close according to CCDInspector, but there is some coma on stars in the corners. Could an error give rise to this effect?

Could it be the scope itself?

I have looked hard at the camera window and chip with a handlens, but I can't see anything.

The scope is an Equinox 80 ED Pro, 500mm FL f6.25. Camera a QHY 168C OSC.

If I can get rid of it I would like to as it turns processing into more of a struggle than it should be.

All suggestions welcome.

 

[michael8554]

So the 80 ED and Flattener were okay on the DSLR, the only change is to the 168C ?

Michael

[old_eyes]

9 hours ago, michael8554 said:

So the 80 ED and Flattener were okay on the DSLR, the only change is to the 168C ?

Michael

Hi Michael,

I think so, but I am not absoluttely sure. DSLR images were often noisy and plagues by gradients from amp glow etc. Plus I was working with a poorly guided rig that I had to set up a strip down every night so I never had an excess of data. The same problem could have been hiding in poor images.

When I was using the DSLR I shovelled all the light subs, darks and flats into DSS and let it do its thing. I never made master flats. What I will have to do now is to try and reconstruct what the master flat would have been using the same processing as for CMOS flats.

I will come back when I have done that.

[michael8554]

To save yourself a lot of work, take some flats without the Flattener.

The spacing would only need to be approximate for this test, shove in whatever spacers you have.

Michael

Edited April 6, 2020 by michael8554

[tooth_dr]

I would suggest taking your flats at night / in the dark to eliminate light leakage. 

[old_eyes]

34 minutes ago, michael8554 said:

To save yourself a lot of work, take some flats without the Flattener.

The spacing would only need to be approximate for this test, shove in whatever spacers you have.

Michael

Yes that was going to be my next test

[old_eyes]

5 minutes ago, tooth_dr said:

I would suggest taking your flats at night / in the dark to eliminate light leakage. 

I generally do when using the EL light panel. I take flats at the end of an imaging session. Obviously sky flats are taken when there is still some light, but without a filter wheel there is less chance of significant light leakage. If there were I would not expect the radially symmetric pattern I am getting. More of a gradient from one direction.

[old_eyes]

OK. This is a masterflat using the same Pixinsight processing sequence and parameters using a set of DSLR flats from previous years. 

Now there is some horizontal banding, but I seem to detect a similar doughnut pattern in the data.

If true it would suggest scope or flattener.

Reinforces the need to test without the flattener, which I will do later today.

[Notty]

Following.  Same scope same flattener and similar wierd flats, similar post!

 

[old_eyes]

Some interesting results from experiments yesterday and imaging last night. However it is a nice sunny day, and I am not going to spend it crouched over a hot computer. Me for the garden chair with the latest Hilary Mantell.

results later today 😉.

[old_eyes]

OK - not later today. A few days later 😉.

A series of masterflats created using the Flats Wizard in NINA. Processed in PI using matched dark flats and no bias. Debayered and then autostretched with STF.

Done during daylight, so there may be some light leakage. Whole set done at -10C rather than my usual -20C as it was a warm day and the cooler could not get down to -20C. It was stable at -10C.

This is the normal flat with calculated chip to flattener distance and telecope in focus.

Take the camera/flattener combination off the telescope. Not sure why colour is different on this one, but I was just allowing STF free rein to do the stretch.

Remove the flattener. Now we have the bare camera and no doughnut.

This next one is the the camera mounted on the telescope without the flattener. I had to bodge this a bit with some packing as I don't have a T2 to 2" nosepiece adapter.

Finally I increased the chip to flattener distance by a couple of mm using the next size up T2 extension ring.

What do I think that all this is telling me?

The problem is not the camera. The flat from the bare camera on its own is pretty smooth.

The flattener on its own is not producing the effect, but I am not sure what I would espect to see anyway.

Telecope and camera with no flattener seems better than with flattener (but focus wrong).

The interesting one is changing the chip/flattener distance. Increasing it slightly seems to reduce the intensity of the effect. A later test that night showed it was still possible to come to a focus with the increased spacing, and a quick check with CCDInspector claimed that the curvature was reduced.

Have I just got the spacing completely wrong? I will need to go back and check carefully the OVL flattener backfocus and the spacers. I'd also like to try the rig without the flattener just to see how much coma I get, but for that I need a T2 to 2" adapter