Basic Elementary Question (s) on Flats and Calibration

[Roger Baylon]

Question 1-

When I want to use ACP to calibrate images at the time they are acquired, I have been copying an AutoFlat folder and a Calibration folder into the ACP images folder for that night.
Is this step necessary?
I seems to work, but maybe there is a much, much easier way?


Question 2-

In the attached image (M81), the ACP calibration has attempted to correct a large circular area in the lower left. Either the diameter of the flat correction is a little too small, or a little too large. But in any case it leaves a ring --instead of the uncorrected circular area -- as the net result.
Does this mean that I may have specified something incorrectly somewhere, like pixel size, etc? This "artifact" only shows up by setting the Levels gamma to .5 or below.

In the past, I have rarely used gamma settings below .7 to get a useful image--so maybe I am worrying about a non-issue.

Warmly, Roger

[Dick Berg]

Hi Roger,

Q1 - MaxIm does the calibration. So I'm not clear why it's necessary to copy the flats and calibration folders. If you haven't done this, try using MaxIm's Calibration Wizard (in the Process menu, and read the help file) to gather and generate the "masters" that can then be applied to each of your images automatically (if you've told ACP to apply calibration) without having to do the repeated copying of your flats, etc.

Q2 - It's a little hard to see but I think that your calibration images are not constructed correctly, because those kind of "defects" in an image should have been removed by calibration. I don't know if the real problem is an "offset" in the y-direction, but that's what it looks like to me. I did a little experiment once upon a time and generated fake flats with donuts to look at the impact of misalignment. Here are three tests of simple subtracting of FLAT2 donut from FLAT1, after offsetting one from the other. This is not too precise - just an illustration of what happens when the flat is not aligned correctly with the image. Here's a picture from the spreadsheet.

donuts.png

I tried to bring out the detail in your image, but you can see the similarity here. It helps to stand back from the screen some.
m81.png

I'll be the first to admit that this may not be the problem, but it is the first thing that came to mind when I saw your image.

[Roger Baylon]

Hello Dick,
Thanks for your wonderful explanation. I tried to reproduce this in Maxim DL with a careful calibration. After that, I think I understand clearly what you were suggesting.

The top of the circle is too dark i.e. under corrected by the flat, the middle point of the circle is a perfect correction, and the under part of the circle is too light i.e over corrected by the flat.

But here is the question: How could I possibly have created this effect? I think that the camera position on the scope was unchanged from the time that I took sky flats with ACP and the time that I took the M81 image with ACP later the same night.

Dithering?

Have you encountered a similar issue before?

Warmly,
Roger

[Eric Dose]

This happens whenever the image's illumination pattern onto the dust mote differs from the illumination pattern from your flat's light source. That is, the dust mote's shadow falls on a different spot on the chip during imaging than during the taking of flat images.

Typically this is from imaging near the moon, or near the horizon around twilight.

Bad news is that it is unavoidable under such imaging conditions. Good news it looks worse that its practical effect, especially for astrometry, or for photometry maybe other than of exoplanets.

[Roger Baylon]

Dear Eric,
You are making me feel better. "Unavoidable" is something that I can live with rather easily. It does seem that it is easy to patch up with PhotoShop. I was imaging at 30 to 50 degrees between 7PM and midnight., which may be relevant too.
You comment may explain why I have not seen this effect in all images that I have taken recently.
Warmly, Roger

[Roger Baylon]

Dear Eric,
I have imaged in a different part of the sky a couple of times. The bas relief effect seems to go away. Thanks for your encouraging message that the practical effect is not large.
Roger

[Eric Dose]

Yes, MaxIm often does its best to emphasize such features, to make them easy to detect visually, and they can look scary on the monitor. But yes, the actual ADU effect is usually quite small, and quite often in photometric practice (target apertures vs sky-background annulae) they end up mostly self-canceling as well.

[Bob Denny]

Thanks very much Eric!

[William Bristow]

That's sometimes called 'Bas-Relief' effect Roger.

It's quite common and nothing to do with dithering.

It may sometimes occur because some component in the image chain had a stationary dust mote within the optical path which projected the ring artefact onto the camera sensor when taking the live image, but the camera, or the object causing the artefact, or some other part of the image chain moved position by a very small amount when you took the flats, probably less than a mm.

The result is this 3D effect when you calibrate the image.

Usual "suspects" are slightly loose focuser draw-tube, camera attachment via a T2 push fit nosepiece which is not firmly clamped in the focuser, filter wheel that does not return the filters to exactly the same spot after a change, or just bad luck that whatever the object was it moved under gravity as the telescope was slewed to a different angle for the flats.

By the way, there is a neat little formula that helps you find out how far away the object casting the ring shadow is from the sensor, useful when you need to do a clean up and don't know where the object is in the image path.

D=Pdf

D = The distance of the object casting the ring shadow from the sensor in mm.
P = The width of the dust donut in pixels.
d = The width of a single pixel in mm.
f = The focal ratio of the telescope.

Gathering the information for your system from the .fit's header of your posted image:

P = Width of the dust donut in pixels = 1050
d = width of a single pixel for your camera = 4.54um = 0.00454mm
f = focal length (solved) = 824.5mm and diameter of OTA = 130mm gives a f/r of f6.34.

Solving for D, 1050 x 0.00454 x 6.34 = 30.22mm

The object casting the shadow in your M81 image was ~30mm away from the surface of camera sensor.

William.