Fuzzy halos round stars???

[RikM]

First night out with the DSLR since 23rd March!

What is going on with the fuzzy halos around the stars? Is this seeing, focus, collimation, haze?

[ollypenrice]

I see that was with the reflector so it might be seeing. I'm not good at this but the diff spikes don't scream 'collimation' to me.

Olly

[RikM]

Thank Olly. I was hoping it was atmospheric, rather than something I had got wrong.

Details...yes, this was with the 150P. Only an hour and half of 3 min subs at ISO800 but with a full compliment of calibration frames.

Collimation was checked before starting using a Cheshire and looked good. It's an f/5 scope so quite easy to get true.

Focus was using a mask and Bahtinov Grabber and the individual subs look the same from the start to the end. FWHM average for the subs was about 6, so not good but at least there was no slippage mid run. It's normally about 4 for this combo but I am not proficient in evaluating the numbers you can get out of DSS.

[ollypenrice]

OK well if the FWHM was six expect soft stars. I don't know what you usually get but in my case I sometimes squeeze under one and give up at about 2.7 so I think that must be it.

Olly

[Mike Hawtin]

Rik it might be worth just trying again, I use the same focus method of Bhatinov mask and grabber and trusted it implicitly until a few nights ago when it reported within critical focus but my subs were clearly out of focus. I used two different capture applications for the grabber but with the same result. I gave up stumped and tried again with exactly the same set up last night and everything worked perfectly! Perhaps the order in which the applications are opened has an effect, it's the only variable I can think of in my case.

Mike

[ollypenrice]

Is B Grabber actually necessary? I get a very clear image of the cat's whiskers pattern and that does get me very close to a careful FWHM. Like all software it might be prone to misbehaviour. I've actually started to trust the visual pattern now, certainly for RGB.

Olly

[RikM]

Thanks for the help

In the past I used the spike pattern and lined up by eye, but since using B. Grabber I have been getting noticeably better focus. A bit like with star size vs. FWHM, you can make a reasonable tweak to focus and the numbers change but you can't really see it on screen.

I will try again tonight and take a couple focussed using the pattern, a couple using B. grabber, and try using the FWHM tool in APT again and see which gives me the best result (or even if there is any difference between the methods)

I don't know how to compare FWHM values across different set ups and software. DSS gives you a value for the whole sub (I've never got a score less than 3), with APT you get different numbers depending on screen zoom and the star you choose. I can get a value of 1.something if I use a short exposure on a not too bright star and view the image at 20% size.

Is there a correct way to do it? I take a short sub (so the star is not saturated - I normally use 6 sec subs with 10 sec pauses on a loop) View it at 100% and tweak until the number is as small as possible???

[ollypenrice]

FWHM seems to be a moveable feast and I can't answer your question.

Your subs for focus, certanly in FWHM, should not be too short because you need to smooth out the seeing. Three seconds? I speak for my own skies. I'm afraid they are the only ones I know from an imaging POV.

Olly

[rfdesigner]

I've been looking into star fuzzyness, specifically edge of star fuzzyness.

Using the usual gaussian model for atmospheric distortion I came up with a list of various star sizes for different relative brightness

brightness, diamater / FWHM

0.5_________1

0.1_________1.8

0.01________2.7

0.001_______3.2

0.0001______3.7

so a star with a FWHM of 4 arc seconds would be down to 1/10th of peak at a diameter of about 7 arc seconds and at 1/10000th of peak it would be about 15 arc seconds diameter.

I've compared this theroy to actual stars in some of my images and it seems to be a very close approximation.

If your stars are substantially brighter in their 'halo' compared to their core than this then there is something to be done (probably focussing, tracking or dew management)

Derek

[Shibby]

I've been plagued with halos like this. Just as I had convinced myself the corrector was to blame (the 2-element SW & Baader correctors do introduce aberrations) my next image showed no sign of the halos... I concluded I must have had dew problems almost every session for months! I'm still a bit unsure, though.