My Soul is black (and white)

[poogle]

Sooo, finally got around to processing my first target of the season. It's ~6 hours of HA data captured in central Stockholm. I like the capturing process and end result the most, so no heavy processing apart from DBE and a stretch. The Soul nebula doesn't fit in my FOV, so I framed the part I like the most  I'll have to decide if I should do a mosaic or add OIII and SII data as well.

I'm quite pleased with the result, except with the somewhat bloated stars and the star shapes bottom left. I guess the bloated stars might be due to poor guiding with my AVX-mount, my RMS is around 0.8, I'm not sure if this is good or bad with a pixel scale of 1.65? Any inputs on this?

I blame my Coma Corrector, refractor collimation or CCD spacing for the elongated stars bottom left. Not sure how to fix this before my next imaging run...maybe by buying a refractor, lol.

Any suggestions or pointers on how to improve are always welcomed!

Thanks for looking! 

[vlaiv]

What is your gear that you used for this image?

You say you are at 1.65"/px - but this image looks like it is taken at much higher sampling rate. Did you drizzle by any change?

Stars in left part of the image (both top and bottom) - are out of focus, that indicates tilt.

In your signature, I'm seeing SkyWatcher 150P newtonian - that scope is really not suited for imaging due to focuser. Did you change focuser on it? Did you move primary mirror up the tube?

I'm also seeing Nikon D5200 as camera - so you have DSLR and above is Ha image.

When shooting Ha with DSLR - you are actually only using every second pixel so resolution is halved and you are effectively imaging at 3.3"/px - which is fine, but you should not be having this large or zoomed in image. My guess is that you drizzled (which is really pointless thing to do in this case).

 

[poogle]

Thanks for your input vlaiv, much appreciated!

I image using my SXVR H-18 CCD with Baader NB filters. You are correct about drizzle, I followed the tutorials on Light Vortex, and they drizzle, therefore I drizzle 😁 When is it suitable to use drizzle?

Regarding my 150p I replaced the stock focuser with one from Moonlite, so that should be fine. What about focus in the rest of the image? I run the autofocus routine with my motorized focuser in SGP before each session.

How would I go about fixing, testing and measuring the tilt? I know my camera has collimation screws to adjust tilt, but which one should I adjust, and by how much? Do I need starry nights, or will flat frames suffice?

Again, thank you for sharing your wisdom! 😃

Edited December 5, 2021 by poogle
Autocorrect

[ONIKKINEN]

8 minutes ago, poogle said:

How would I go about fixing, testing and measuring the tilt? I know my camera has collimation screws to adjust tilt, but which one should I adjust, and by how much? Do I need starry nights, or will flat frames suffice?

I would say its far more likely that you have tilt between the secondary mirror and comacorrector instead of between the corrector and camera sensor since you're imaging with a newtonian. I recently also thought that i had sensor tilt but the more i looked at it the more i realized that i have no way of knowing for sure whether its actually sensor tilt or not. Therefore its probably a good idea to check secondary mirror collimation first as it will also produce effects that look like tilt. Your focuser could have tilt aswell if its not perfectly in the middle of the tube or if there is focuser slop. Focuser slop is probably not an issue since you have a good focuser. Some focusers have adjustment screws to adjust tilt and with those and secondary mirror adjustments you can be sure to rule optical tilt out.

What tool are you using to collimate your secondary mirror to be exactly under the focuser drawtube? Laser collimators or eyeballing will not work for this. You will need some kind of tool that can better show centering of the secondary. This is a cheap and effective tool for the purpose: https://www.teleskop-express.de/shop/product_info.php/info/p5506_TS-Optics-Concenter-2--Collimation-Eyepiece-for-Newtonian-Telescopes.html . There are also the expensive collimation tools like howie glatter lasers but i would try with the cheaper alternatives first.

[vlaiv]

4 minutes ago, poogle said:

When is it suitable to use drizzle?

If you operate HST and have camera with very large pixels that can't resolve airy disk at said scope

(but almost never in conditions amateurs experience).

16 minutes ago, poogle said:

How would I go about fixing, testing and measuring the tilt? I know my camera has collimation screws to adjust tilt, but which one should I adjust, and by how much? Do I need starry nights, or will flat frames suffice?

I don't think flats can be used - at least I never heard of it nor can imagine how it could be used, but I don't think you need to waste clear night on that. It can be done with artificial star or maybe some sort of high contrast edge somewhere.

First method will be precise, second approximate.

With star / artificial star - you simply focus in center and move scope so that star ends up in each corner and measure FWHM - you adjust tilt until FWHM is equal in each corner once you focus star in center and that should be it.

With high contrast edge - you do the same - except you rely on your vision to make edge the sharpest in center and equally sharp in each corner.

If you think that your focuser is square on the tube and that camera has tilt - there has been recent thread about DIY piece of kit that you can use to square tilt plate on sensor without using telescope. It requires laser and something to rotate camera about its axis. Let me see if I can find that for you

 

[poogle]

1 hour ago, ONIKKINEN said:

I would say its far more likely that you have tilt between the secondary mirror and comacorrector instead of between the corrector and camera sensor since you're imaging with a newtonian. I recently also thought that i had sensor tilt but the more i looked at it the more i realized that i have no way of knowing for sure whether its actually sensor tilt or not. Therefore its probably a good idea to check secondary mirror collimation first as it will also produce effects that look like tilt. Your focuser could have tilt aswell...

Good point, might as well be secondary or focuser tilt...I should have bought a refractor from the very beginning ☹️

Anyway, I have a Cheshire/sight tube, maybe that's the first step, but I find it really difficult to verify that the secondary is perfectly round in the sight tube. I can't even imagine how to make sure that the focuser is square, I'll have to resort to Google I guess 🙂

1 hour ago, vlaiv said:

...but I don't think you need to waste clear night on that. It can be done with artificial star...

 

Great explanation, I'll give it a try after trying to make sure that the focuser is square and that the secondary is well aligned. I now remember that I made an artificial star some years ago, just need to find it 😃 And I'll also stop drizzling until I start my new career as a Hubble image processer at NASA 😃

[ONIKKINEN]

3 hours ago, poogle said:

Good point, might as well be secondary or focuser tilt...I should have bought a refractor from the very beginning ☹️

Anyway, I have a Cheshire/sight tube, maybe that's the first step, but I find it really difficult to verify that the secondary is perfectly round in the sight tube. I can't even imagine how to make sure that the focuser is square, I'll have to resort to Google I guess 🙂

This is what the view from a concenter eyepiece (the one i linked) looks like. I was going to show you my perfect collimation but then realized that it is not in fact perfect 🧐. Well, as an example this is great since its very easy to see where the adjustments are off.

You can see that there is just a tiny bit more black background which is the inside of the telescope tube visible between the rings and the bright mirror on the right side. This means i need to push the secondary just a little bit towards the primary mirror. The top part is also perhaps not perfect. To do this you need to block the primary mirror so that you dont get reflections, i do this just by placing a piece of paper between the secondary and primary. Also, bright light makes it easier so there is a powerful LED headlamp in the tube. I have also blackened the non-coated edge of my secondary mirror to further make this easier.

I alternate between using this and using a laser to check primary to secondary alignment (paper off for this of course) until its good enough. Honestly i find the whole procedure a bit annoying and i probably wont try to adjust this more, unless stars are truly horrible on one side but i doubt they are for this kind of error.

[poogle]

Thank you for your detailed explanation ONIKKINEN! I'll put it up on my christmas wishlist 🙂

I downloaded CCD Inspector and it clearly shows my left-right tilt. I was gonna fix this issue tonight, but on my first image one half of the mechanical shutter got stuck and wouldn't operate, making half the image black 😭 I did however rotate the camera 180 degrees and took another picture and by eyeballing the focus of the stars it would seem that it is still the left side of the image that is out of focus. This would indicate sensor (or maybe focuser?) tilt and not telescope collimation issues if I'm not mistaken? 

Well, pleased with the fact that I now knew which screws to turn (due to the position of the malfunctioning shutter in the image) I gladly removed the camera and brought it indoors, just to realize that both shutters of course had closed upon disconnection of the camera. What a night... I did consider both incineration and trashcan for the whole setup, lol.

I haven't tried taking a new image after taking the camera indoors, but the shutter probably just didn't like the cold, -14 degrees here tonight. Back to step one, either fixing the shutter or just putting it back on the scope and redo the rotation to verify the origin of the tilt, sigh