Evoguide 50 w Flattener - what is going on in this image?

[vineyard]

Hello,

I just got one of the SW EvoFlatteners (gave up waiting for the Starizona one).  Took it for a spin earlier this week.

The image below is 60 lights of 60s - darks, flats & dark flats applied.  Darks & lights done at the same temperature, exposure time, gain & offset.  Flats & dark flats similarly.  Stacked in DSS, and then all that's been done is ABE in PI.

There is vignetting for sure.  And I think this camera & scope combination is quite under-sampled.  But the flattener seems to be working, although top right corner & maybe bottom left corner still have some curvature (vignetting means they'd get cropped out anyway).

But what is going on with the 4-lobed flower-shaped halo?  Is it pinching - but the star shapes seem ok?  Is it a stacking artefact? (I also attach the stacked TIFF & a single light FITS file - if I do ABE on the single FIT the ABE background that's extracted is a different shape from that in the stacked TIFF, and also the single FIT doesn't seem to show a halo although perhaps its there but just v faint?)

Any ideas?

I like the wide-field this flattener potentially allows me to capture (also took some of the Perseus double cluster & M45) but not if this halo will always be there.

Thank you & stay safe,

Vin

(PS - I didn't use any filters, so could it be something to do with that and the chip - some IR sensitivity?)

(sorry the files are so big, didn't want to upload compressed files in case the compression created artefacts too!)

Autosave.tif M31 EvoFlat 201009 60s neg10 gain125 offset16_Light_001.fits

Edited October 11, 2020 by vineyard
added PS

[happy-kat]

What camera did you use?

[vineyard]

Oops sorry ASI294MC Pro.  I've used that w my larger frac without those halos (but w filters).  Cheers

[vlaiv]

Can you be more specific in what you are asking - I don't really understand, or rather can't interpret.

This in particular:

45 minutes ago, vineyard said:

But what is going on with the 4-lobed flower-shaped halo?

Are you referring to the overall background of the image or star shapes in the corners.

Overall background is combination of two things - first is stacking in DSS and second is ABE process.

Stacking in DSS sometimes leads to very weird effects - like in this image - there seems to be vignetting, but if you took flats, there shouldn't be any. I had similar issue with DSS and red channel - red channel seemed to be clipped around 0 and that created color casts mixed with vignetting like in your image.

Second issue is when trying to apply automatic background extraction thingy from PI. This process can probably suffer from edge effects - and this creates that 4 lobed color pattern in the image - best seen when image is small:

If you are referring to cross pattern on the stars, best seen in this crop (although it is slight effect):

That is astigmatism:

Field flatteners often have some astigmatism in far corners of the image and this can be related to spacing or tilt if it is not the same in every corner or one line of cross is longer than the other.

 

[Stuart1971]

Looks very much to me like the flats have not calibrated correctly...have you processed the image with all but the flats, to rule this out, or prove it’s correct...? 👍😀

[vineyard]

Apologies @vlaiv, I meant the overall shape (I didn't even notice the slight astigmatism in the corners!).

Yes I think you are right on the combination of two different things - stacking & ABE.  Here is the same TIF run through DBE (downsampled just for size here) - the halo effects are less.  Still there but less - and I think its the vignetting that may be exacerbating those, because its basically along the edges of the largest rectangle that would be formed if you stayed inside the vignettes?

Do you think that would not happen if I used a different stacking program (eg PI instead of DSS)?  Or is it better to just crop & take out in post-processing (or use a smaller chip like an ASI183)?

I also thought flats should remove this but it looks like it didn't?  @Stuart1971 here are two images - both without flats or dark flats applied, one run through ABE, the other DBE).

On the flats, when I was taking them (on someone's light box) I did notice the channels were quite different - see attached screen-grab from an example flat FITS.  I just couldn't get them to align w different exposure times - I wasn't sure if again that was from not having any filters in the optical path.

Eventually I need to figure out a way of putting filters in - right now it looks like one way would be to thread a 2" in front of the OTA (w a ring-step down) - maybe that would also remove the vignetting as it would reduce the aperture slightly?  Otherwise a 1.25" further along the path, but there doesn't seem to be a natural place for it (it'll need another adapter!).

Cheers all!

 

 

[Stuart1971]

Hmmmm, well the second one looks better even thought you have much more vignetting showing...🤔🤔

[vlaiv]

4 hours ago, vineyard said:

Do you think that would not happen if I used a different stacking program (eg PI instead of DSS)?

Yes, please do that. I'm confident that PI will do much better job at calibrating and stacking subs than DSS.

[vineyard]

You were right!  Here are two images - weighted-batch-pre-processed & then integrated in PI (linear fit).  Only difference between them is DBE (the first one) and ABE (the second one).  Both downsampled for size.  Halo is still there a bit which I guess must be there b/c of the physics of the vignetting (?) but its a lot better than w DSS.  (I couldn't dither as the Evo is normally my guidescope, so I wonder if that might have helped a bit).

It took about 4x the time to stack though but if it produces better results, then so be it - lots of cloudy  nights ahead so plenty of time to work on the double cluster & M45 captures as well 😆

I think in the PI stacked versions, the ABE seems the better image to do further processing on (which is definitely not the case w the DSS versions)?

Thank you again @vlaiv- I would be lost & tearing my hair out without these sorts of tips & guidance so they are highly appreciated.

[EDIT: the ABE version seems slightly 'warmer' in tone than the DBE version so I'm not sure yet which one I'll work more on]

 

Edited October 11, 2020 by vineyard
added [EDIT: ]

[happy-kat]

Have you tried a stack with no calibration files as I'm seeing uneven tone across the image.

The flat file shown further up had a dark round middle area.

[vineyard]

Just now, happy-kat said:

Have you tried a stack with no calibration files as I'm seeing uneven tone across the image.

The flat file shown further up had a dark round middle area.

I've tried without flats & dark flats (in the thread above).  If you mean try w/o darks even, sure can try that w DSS to see what happens - will post later.  Cheers!

[happy-kat]

If you use no calibration files in theory I think your final stack would be noisy and have typical vignetting and possibly amp glow depending on camera, but I wouldn't think it would show the large splodges of uneven tone.

EDIT If it did then next test is remove the flattener and try again

Edited October 12, 2020 by happy-kat

[vineyard]

Hi @happy-kathere are two images.  Both stacked in DSS w/o any calibration - the first is DBE, the second is ABE.

Its a good idea re trying it w/o the flattener - I will try and do that next time I'm using this for wide-field AP (I will also try it with an ASI178 chip size).

Cheers!

[EDIT: the DBE one does suggest edge effects form vignetting given the way there are straight lines from top left curved vignetted corner to bottom left?]

Edited October 12, 2020 by vineyard

[happy-kat]

The first is more like what I would expect plus there is a dust bunny or two though the vignetting  is very strong but I thought your camera sensor was no bigger than a APS-C sized sensor.

The second is very odd again.

(I am an enthusiast not highly experienced imaginer)

 

[vineyard]

You're quite right, the ASI294MC is only APS-C sized so I am a bit surprised as well to see so much vignetting (haven't seen reports of that w the Starizona).  Its interesting to see the left hand side is a straight edge between the vignetted corners, but the right hand side if a curved one.  The halo/tone effects are more pronounced on the left hand side so I wonder if that's connected. Could there be some tilt in the system (which could also explain the slight curvature in the far top right and bottom left corners)?  Will check if I made any newbie errors when I break the system down!

I'm v much an enthusiast like you - more energy than skill at this point, but as long as that's accompanied w curiosity it's all good

[ollypenrice]

What I'd do first is measure the level of vignetting without flats applied. Take sample ADU values from the background sky in the corners and in the middle (which you won't be able to do in this image 😁). This will tell you the true level of uncorrected vignetting. Actually I guess you could just measure it using the flats, in linear form. That should work if the flats are correct but a sub of fairly 'empty' sky would be best. In my most vignetted rig I have a 23% fall-off corner-to-centre. This sounds catastrophic but flats do correct it. That will tell you the level of vignetting you're dealing with.

My experience of ABE is that it will not work on galaxy images. Galaxies have faint outer glows, sometimes scarcely visible in the image, and ABE will measure the sky within these glows and conclude that it's too bright due to some kind of gradient. It will then pull down the background where that outer galaxy glow was, and create a dark ring around the galaxy. When using DBE keep well away from the galaxy and look on the web for very deep images which will reveal where the faintest glow will be on your own image. Keep the markers away from there as well.

Olly

Edited October 12, 2020 by ollypenrice
Typo

[wimvb]

14 hours ago, vineyard said:

(I couldn't dither as the Evo is normally my guidescope, so I wonder if that might have helped a bit

No, that wouldn’t have helped. This is purely a calibration issue.

[vineyard]

3 hours ago, ollypenrice said:

What I'd do first is measure the level of vignetting without flats applied. Take sample ADU values from the background sky in the corners and in the middle (which you won't be able to do in this image 😁). This will tell you the true level of uncorrected vignetting. Actually I guess you could just measure it using the flats, in linear form. That should work if the flats are correct but a sub of fairly 'empty' sky would be best. In my most vignetted rig I have a 23% fall-off corner-to-centre. This sounds catastrophic but flats do correct it. That will tell you the level of vignetting you're dealing with.

My experience of ABE is that it will not work on galaxy images. Galaxies have faint outer glows, sometimes scarcely visible in the image, and ABE will measure the sky within these glows and conclude that it's too bright due to some kind of gradient. It will then pull down the background where that outer galaxy glow was, and create a dark ring around the galaxy. When using DBE keep well away from the galaxy and look on the web for very deep images which will reveal where the faintest glow will be on your own image. Keep the markers away from there as well.

Olly

Hi Olly, I did a quick look at a couple of flat FITS and the drop off from centre to corner is about 18%.  Thanks re that on ABE vs DBE - I do notice ABE galaxy images as having a dark ring around them, this explains it.  I had done a quick DSS+ABE stab at the Perseus Double Cluster image I took with this rig and it was comparatively a lot less weird than this galaxy one, so I wonder if that also helps explain that.  I'm going run that Perseus image through PI instead of DSS and let's see what emerges.

Will use that tip for DBE galaxies as well!

Thanks again!