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Old 05-10-2015, 12:04 PM
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Flats with temp compensating focuser.

Ok, so flats should be taken at the same focals point as light frames, but how do you achieve this when the focuser moves during the course of the night.

I never had dramas with this on a telescope with robofocus however the camera lens im imaging with is very sensitive to focus position changes (and apparently backfocus distance changes too)

So I'm having massive issues getting flat fields to work.

I will post some examples shortly but I get rings, in some images, or the bottom of the frame appears flat fielded but vignetting is still present at the top. I really dunno what to do.

I suffer typical light pollution for someone who is 35km from a major city and 4km from the local Westfield and Costco illuminated carparks.

Any thoughts.
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Old 05-10-2015, 12:11 PM
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It must be the image scale is shifting. I think that is called focus breathing where the image scale shifts when you change focal length (lens shrinking as it gets colder).

Best to turn off temperature compensation and instead refocus a few times if needed although that may also do the same thing.

What sort of focus arrangement do you have?

For my own interest how well does the temp compensation work?

How did you do your flats? dusk flats, illuminated panel?

Were they bias subtracted when making the master flat or set as a master bias subtract when flat is applied?

How good are your darks?

Slightly off quality darks and biases can play havoc with some setups although I have only found lens imaging pretty forgiving.

Greg.
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Old 05-10-2015, 12:33 PM
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I dont feel that its breathing as the image scale of the light frames does not change, it just seems that the vignetting pattern changes..

I might try again tonight with the focuser disconnected and see how that goes. Just rig it up manually.

The focuser is a home made arduino powered stepper motor focuser with a belt drive around the focus ring of my 300mm f/2.8 camera lens. The camera lens is mounted on a DSLR camera.

The flats were done off a laptop LCD monitor with a fully white screen image with the lens hood of the 300/2.8 just off touching the monitor to minimise stray light in a dark room (no lights on, at night time)

No bias subtracted from the flat frames, and no flat darks

Im using ICNR with the DSLR (In Camera Noise Reduction) that essentially takes a light frame, then immediately takes a dark and subtracts it from the light in camera. This is not optimal most of the time, but because the temperature changes night after night I find that darks of this method work best with a DSLR

I might take a series of flat darks (that will obviously contain the bias information) and subtract them from the flats to see if that helps at all.


The one thing that I want to test but is currently impossible for me is this.

This lens is reportedly (by someone VERY experienced with astro imaging using this lens) extremely sensitive to backfocus distance. Now you might think "well that shouldn't be a problem with a DSLR lens on a DSLR camera.. Right?" The problem lies in the fact that I've modified this DSLR myself by removing the AA filter and the UV/IR hot filter completely from the camera, and as a result, there is a discrepency (the focal point is 1.3mm further back than was originally intended.. As such, the spacing for the lens is wrong and I am suffering coma in the corners as a result (which I know for a fact did not happen pre-modding the camera. and I know for a fact isn't the sensor being tensioned out of shape etc because I have no visible differences in images taken with my 8" newt...


I've added some images to show the problem.

Next thought, I was thinking about wrapping the rear of the lens and camera in Al Foil as a light trap incase somehow during my modification I've induced a light leak somewhere...
Attached Thumbnails
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Click for full-size image (ngc1365_Flat.jpg)
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Old 07-10-2015, 04:52 PM
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Which lens is this?

Is the problem only with temp comp? Turn it off. I don't think temp compensation is that important. I have never used and gotten plenty of good results with lenses. Backfocus distance is important, I agree depending on the lens. Longer lenses less so than shorter ones.

Stopping down will reduce the coma.

Greg.
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Old 07-10-2015, 06:17 PM
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It's since I added temp compensation. You wouldn't think it's required but the 300mm f/2.8 with 5.2um pixels and an all metal lens body = a noticeable focus shift with every degree of temperature change.

J-P Metsavainio told me that with his CCD on this lens his original sensor back focus distance was 1mm too short and the results were abysmal, once he added a 1mm shim he went on to produce 6 APODs and countless other perfect images with it. I didn't think removing the AA filter and the Uv/IR block would make that much difference however it was definitely enough. Like I said, before I modded the camera it was perfect right to the corners.

That 3rd image of ngc1365 and 17 of its more distant friends shows the weird rings perfectly. I did capture a new series of flats that made a big difference, and Don Goldman is making me a new filter for my 450D that will perfectly replace the two filters I removed in optical distance and I should regain my perfect stars.
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Old 07-10-2015, 06:34 PM
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I can see so much potential there. Wow. The widefield view really helps to put things into context.

And, also, the vignetting can easily be taken care of by DBE.

H
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Old 07-10-2015, 11:31 PM
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I agree H, The Not too long, not too short perspective gives a very different look. You see a lot of images from the 50mm / 90mm / 135mm lens ranges Then a lot of images from telescopes ranging from 480mm to 650mm

This is that very sweet middle ground... Big enough front aperture at 112mm to suck down a lot of light and resolve reasonable detail, fast enough to go really really deep with short exposures and just the right focal length to really show off extended areas of nebulosity like lambda cent. Eta Car, Rosette/Cone make a nice two panel mosaic with this length... M45 becomes a floating blue island... All the favorites take on a new face with such a wide view...

My 450D is about to recieve a cold finger too to slam it down to -5C to make the most of it...
I'd say 20 minute Ha exposures at f/2.8 ought to go deep enough!

As a note - all the above images are series of 1 minute sub-exposures as the system sucks the light pollution in if I go over 1min per sub.. I can detect mag 13 galaxies in a 1min sub and with 20x1min stacked you can make out mag 14....

Here is a test from the new flats/flat darks.
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Old 07-10-2015, 11:48 PM
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Oh.. And DBE. Sounds like more crazy PI jargon. Lol. I'm oldschool H. Dss+PS CS6. I need to learn PI though. It seems amazingly powerful.
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