Rings on my images

[almcl]

Saturday night was the first chance to try out my newly acquired ASI 2600 mc.

Below is a stretched stack of the first target I pointed it at after getting the OAG and main camera para-focalised.

I was a little concerned to see the 'rings', some of which are indicated in the image below.  At first I though they were dust bunnies, but I can't see them in the flats and they look more like an out-of-focus star as seen before focusing.

The imaging scope is the SW 190 MN, so the light path consists of corrector plate, main mirror, secondary mirror, OAG (which should be out of the light path)  and then camera.  No filters (at the moment).

While I can get rid  of these in post processing, does anyone have a suggestion on the cause or how to reduce them?

 

[vlaiv]

Each of those looks like embossed - which means those are dust bunnies but with flats that moved a bit - which is rather strange as you mention no filters at the moment - these usually happen with filter wheel that is not perfectly repositioning itself.

Did you by any chance move anything in your optical train between lights and flats.

Also, I'm worried about poor flat fielding - there is rather distinct background in this image. How did you calibrate and what is your master flat like?

[almcl]

Thanks for responding, Vlaiv.

Don't think anything moved in the imaging train - I moved the scope to horizontal to take the flats, but that was all.

But, now you mention it, I've looked again at an individual stretched + binned flat and yes, there are rings (see below).  The off-centre vignetting (and the pretty awful tilt) are down to the stock focuser on the MN 190 not being up to the job, particularly with the single screw clamping arrangements.  That's another thing to address!

 

 

Edited December 14, 2020 by almcl

[almcl]

Just measured the dust doughnut size on a full sized image and it's ~540 pixels.  Taking that with an f5 scope and 3.76 um camera pixel size suggests a distance-to-sensor of ~ 10.1 mm which is pretty close to the distance from the front of the protective screen to sensor distance for the ASI 2600.  

Need to improve my flat panel arrangement, I think!

[vlaiv]

2 hours ago, almcl said:

The off-centre vignetting (and the pretty awful tilt) are down to the stock focuser on the MN 190 not being up to the job, particularly with the single screw clamping arrangements.  That's another thing to address!

This could be the cause of slight movement of dust between lights and flats.

Well actually dust stays put but fact that there is some play in connection means whole camera assembly shifted.

[tomato]

I used to see these embossed dust bunnies when I had a 2” eyepiece type clamp arrangement to hold my camera in place. The embossing effect disappeared when I moved to an all screwed together optical train, so I’m sure Vlaiv is spot on with his explanation.

[almcl]

5 hours ago, tomato said:

 Vlaiv is spot on with his explanation.

See what you did there 🙂

It turned out to be dust on the camera glass, which yielded to a few good sharp puffs with the Hurricane blower.  Probably the act of lowering the OTA to the horizontal to take flats just dislodged it a bit.  No spots last night!

[wimvb]

On 14/12/2020 at 13:32, almcl said:

The off-centre vignetting (and the pretty awful tilt) are down to the stock focuser on the MN 190 not being up to the job, particularly with the single screw clamping arrangements.

There's a simple way to improve the focuser. You just need to put a narrow strip of tape (I used aluminium tape) between the extension tube and the focuser tube, since it is the estension tube that can wobble. This wedges the extension firmly in place.

[almcl]

Thanks, Wim.  I'll try that.

I have noticed that when tightening the individual single screw on the extension or the focuser,  there is a massive amount of movement before things get tight enough to hold the ASI2600 and OAG in place.  And tightening the focus lock moves things about quite a bit, too. 

Ideally I would like to use my Baader 2" click lock but can't see a way to screw it on.  

[wimvb]

3 hours ago, almcl said:

Ideally I would like to use my Baader 2" click lock but can't see a way to screw it on.  

I don't think there is a way. Some people have replaced the stock focuser on the MN190, but collimation afterwards is tricky, since you're not supposed to move the secondary up or down . In stead you have to move the focuser in order to centre it over the secondary.

[wimvb]

On 14/12/2020 at 13:32, almcl said:

 

 

I think that the vignetting is a bit excessive. But it's hard to assess this from the stretched image. How good is your collimation, especially the secondary?

[almcl]

Last time I checked it was OK - the marked secondary 'circle' is rather irregular which makes an accurate assessment a bit difficult.

but the initial collimation looked OK:

Trouble is with nearly 1 kg of imaging kit hanging off the side, it may not be quite that good now.

[wimvb]

I think this should be of interest as well. According to the formula used, and some quick measurements of the bunnies on your flats, the dust should be only 2.5 - 3 mm from the sensor, ie on the window where you found it.

On 23/03/2018 at 11:23, Oddsocks said:

Whenever you see ‘Bas Relief’ effects (3D) in subtracted or divided images this is almost always due to mechanical movement between the two source images, in this case the images are the flat master, or flat stack and the target image stack.

DBE has detected and enhanced the image artefacts resulting from mechanical differences beween the master flat and image stack. The dark areas around the galaxies is most likely due to poor placement of the sample points in DBE/ABE

Most likely there was some physical movement of the L filter, camera, flattener, focuser etc, either during the flats acquisition or the lights acquisition and this may have affected the entire series of flats, or lights, or just a few individual subs in either stack.

Since you report that RGB were ok but only L was affected I would suspect the filter wheel maybe was knocked during the sequence, possibly the L filter is loose in its holder or the wheel detent is a bit loose in the L postion allowing some movement as the telescope assumes different positions for flats and target imaging, or possibly the flattener to filter wheel coupling is a little loose, even slight focuser movement can cause these effects.

The artefacts are quite large making me think these are dust particles further up the image chain than the filters. You can measure the donuts in the image and determine how far from the sensor the dust particles are and that will give you a better idea of what moved, filter or coupling between filter wheel and flattener, focuser, OTA etc using the formula:

D=Pdf

Where D= the distance from the sensor to the dust casting the shadow, P=The width of the dust donut in pixels, d=The width of a single pixel and f=The focal ratio of the telescope. If you use the pixel size in millimetres then the distance will be in millimetres too, if you use pixel size in microns the distance will be in microns so you just need to be able to convert accordingly. Once you have the distance calculated you can work out what moved.

HTH.

 

[almcl]

Thanks Wim.

I should have mentioned (!) that the image posted above had been binned to 25% so the distance needs to be upscaled by x4.  Applying that to the Oddsocks formula gives 10mm, which I think is almost exactly the distance to the front of the window...