The Cave Nebula

[Stefan B]

Since August had lots of clear skies I managed to get enough data for another target: the Cave Nebula. Therefore I combined broadband and Ha data:

This region in Cepheus proved to be a very rewarding target as it contains lots of different objects like reflection and emission nebulae, dark nebulae and lots of dust.

See https://www.astrobin.com/oqhiqu/ for technical details.

Actually I am not even sad that the clear sky period is temporarily over and the moon gets brighter...I need some sleep

Before imaging I tried to check collimation with a Tri Bahtinov mask. It looked okay to me but maybe someone here has more experience with it and can share his thoughts:

Maybe the positions 2 and 8 o'clock are a bit off?

Regards
Stefan

[decay]

Simply another excellent and perfect image, Stefan. (Just to be nitpicky : There are some ringing (SV Decon) artefacts on _some_ smaller stars. But they are visible at high zoom levels only.)

Before imaging I tried to check collimation with a Tri Bahtinov mask.

I don't think that there are problems with collimation. AFAIK collimation problems will result in triangular sharped stars or stars looking like small crescent moons. Please forget of what I said about coloured halos of your stars! Please don't change anything with your setup, your images are awesome. Regarding the 'halos' (if any): You could try to compare to images taken with other 6" f/5 newtonians. Maybe that's just the way they look? What about obstruction? They say, it would lower image contrast and I maybe this causes blurring effects and/or such halos? Another thought: What about dust or dirt on optical surfaces? Regarding the images of Daniel Nimmervoll which you lately mentioned: AFAIK he uses a 10" newtonian having less obstruction and better resolution simply due to greater aperture. And his site may be located at an altitude a few 100 m higher than yours?

Best regards, Dietmar.

[Mike in Rancho]

I've never used a tri-mask.

But yes, it appears that 2, 8, and 10 o'clock may be slightly off. Very slightly, and I'm not sure it is enough to make a visible difference, as long as the focus point stayed put throughout the session.

With my regular B-mask, I have found that sometimes the brighter stars actually make it more difficult to get a perfect center of the lines. So I will go to the fainter stars. Those end up forming into little blobs which are easier to center.

Your tri image actually breaks into some multi-colored blobs close in towards the bright star, so those could conceivably be used in the same way. But they do look quite close to being correct.

Another thing I've noticed is that the spikes/blobs can differ across the field, possibly due to tilt and/or backspacing of the CC, which I haven't bothered trying to perfect (and may not be possible anyway due to deflection of OTA or focuser). So I check a number of stars all over, and pick the fine focus that seems best from an overall perspective, even if some spots are not perfect.

[Stefan B]

Hi Dietmar and Mike,

thanks for your feedback!

Please forget of what I said about coloured halos of your stars! Please don't change anything with your setup, your images are awesome.

Don't worry, I am not doing that because of your comment I always wanted to see if I could improve collimation by checking it with something other than a laser collimator. I find the electronic solutions interesting but a bit too expensive. So I went for the TriBaht mask.

Maybe that's just the way they look? What about obstruction? They say, it would lower image contrast and I maybe this causes blurring effects and/or such halos?

I already discussed the matter with another person who uses basically the same imaging train and sees also the halos. We also reasoned that it might be due to the primary/secondary ratio causing the bigger halos. And I guess that's at least part of the answer.

What about dust or dirt on optical surfaces?

The camera and the coma corrector are clean but indeed the primary is very dusty. Full of pollen still from spring. But I don't think that this makes a huge difference. I got the same look of stars right after cleaning it all

Just to be nitpicky : There are some ringing (SV Decon) artefacts on _some_ smaller stars. But they are visible at high zoom levels only.

You are absolutely right. No combination of parameters cured that. Maybe I could have masked those stars in Shrink and dering them. But well, I am a vey lazy processor and hate manual masks. So I just went with it.

My laziness is one of the main reasons for using ST. It has a straight forward workflow and I like the "do it once and do it right" attitude

But yes, it appears that 2, 8, and 10 o'clock may be slightly off. Very slightly, and I'm not sure it is enough to make a visible difference, as long as the focus point stayed put throughout the session.

Yes, I agree with that, Mike. Would be interesting to know how much the spikes can be off until it makes a difference...

Another thing I've noticed is that the spikes/blobs can differ across the field, possibly due to tilt and/or backspacing of the CC, which I haven't bothered trying to perfect (and may not be possible anyway due to deflection of OTA or focuser). So I check a number of stars all over, and pick the fine focus that seems best from an overall perspective, even if some spots are not perfect.

I always use the center of the FOV (I even plate solve a bright star, at the moment usually Deneb). I hope that this approximates what you call the "fine focus that seems best from an overall perspective, even if some spots are not perfect".

Regards
Stefan