Bhatinov Mask issue

andrea tasselli:
Most likely a thermal plume within the tube. Stars look good. Don't fret over it.

Come to think about it, it was very warm and humid.  But it makes it really hard to focus!   It wasn't until I realized I could use the geometry of the outer extent of the spikes that I had any success.  

Thanks--good news is hard to come by these days

Bruce Donzanti:
"The question is--can a very slight miscollimation cause the B-mask image to look as it does?".......

I do not think so.  At least on my C11" EdgeHD, when it badly needed collimation, I do not recall the mask image changing.  So, I am not sure what is causing this in your case but given your nice image and star shapes, I would not lose any sleep over this.

Thanks---now, finally, off to bed!

Rodd,
I, too, have a C-11, and use the Bahtinov mask and focus using a Starizona electric MicroTouch focuser.  You did not say in your post what the magnitude of the star was or what your exposure time was. If you were focusing on a bright star (say 2nd mag) and are exposing for longer than 2 to 3 seconds, the star will saturate and "bleed" into the base of the diffraction spikes. Your image of 5907 certainly looks like it is well focused.
Let me know regarding the brightness of the star and the exposure time.  Next time out, I'd recommend trying a shorter exposure and see what this does to the spikes.
Stargazer66207

Thank, Stargazer.   I typically use stars that do not saturate for focusing.  I have tried all approaches--bright stars with very short exposures, very faint stars with 10 sec exposures, and middle brightness stars that don't show up well in the B-mask with 1 sec exposres, but do yield a decent pattern with 3-4 sec exposures.   I don't know if it's tube currents or seeing, but lately, nothing seems to produce the textbook, sharp B-mask pattern.   I do not know what the magnitude of this star is--the star is located close to NGC 5907--if you look on your planetarium screen, you'll see two stars southeast of NGC 5907.  they are much brighter than any star in my FOV for this image.  I tried using stars in the FOV, but even with 10 sec exposures they just looked like blurry patches.  I used the one closest to the galaxy When everything is working properly, I find that stars that do saturate bleed, as you say, into the surrounding spikes--but they don't cause cocked spikes like these--they just get large and bleed together.

Rodd,
Some more questions:
1) What type of B mask are you using & what brand?
2)Do you re-focus with the mask again later in the evening, or just at the start of an imaging session?

This is an intriguing problem!
Stargazer66207

I forgot the brand.  I got it at Opt I think.  It’s plastic.  I dont remember having this issue before.  I focus pretty much every time my fwhm creeps above a certain number.  With  the C11Edge On bad nights it’s frustrating because no matter how hard I try I can’t beat 3 and mid 3s makes me want to quit.  On good nights I might achieve 1.6 but it will typically hover around 2.3. When it climbs past 2.8 I refocus.  It can bounce in .5 arcsec humps because subs are short.  So I make sure focus is drifting and it’s not seeing related.  So I refocus many times in a session. Except those rare occasions when my fwhm stays put for hours ( I love those nights)

Rodd,
I can sympathize with your frustration with FWHMs that creep upward. I fight the problem constantly. I also forgot to ask what focal length you are shooting at (native, or with a reducer, and if so, which one).  I fight FWHM problems mainly due to the fact that all of my imaging is done using a Starizona HyperStar III system, which is f/2 (560mm).  The brutally short light cone of f/2 makes the critical focus area extremely small!!  Using HyperStar, I consider myself lucky if I can keep the FWHM under 4!  But, the stars look good in my images, so I can't complain, I guess.
Stargazer66207

Well, as the pixel scale increases (usually focal length goes down) larger fwhm are fine, otherwise you end up with single pixel stars.  With the C11Edge edge at present I am shooting native (2800 mm). I have the celestron 0.7x reducer.  I figured I would try 2800 as I hadn’t imaged at that focal length before.   I don’t want the larger FOV for galaxies. Full frame would be nice though. Or even aps-c

After reading through all the responses, I can add a couple of things here.

1) A misaligned secondary on a SCT produces comatic imaging and as long as the misalignment is small, I agree that you won't see much effect on focusing with a B-mask.  However, you can detect astigmatic errors pretty easily in any telescope with a B-mask. If you rotate the mask in the pupil and you see a change in focus with angle, that's an astigmatic error.  That kind of error can come from a few things but it's typically due to a manufacturing error in the optics somewhere.  It can also come from a mechanically distorted fold mirror.

2) When the diffraction pattern is split or dancing around between exposures, that's an indication of poor seeing coming from within the tube or the atmosphere.  You are simply catching the changes in the wavefront as it dances around.  Obviously that will translating into a larger blur diameter when using a long exposure.  So in that case, it's not only is it harder to find the best focus but finding and achieving the best focus isn't as important.

3) Your image looks very good but I agree that your measured FWHM value probably isn't right.  There are a number of things that can go wrong that aren't always immediately obvious.  If you've double checked your pixel scale and you are absolutely sure it is right, here are some things to try.

a). Look at how many stars are being fitted.  If it's over a couple hundred, something is wrong and you may be picking up noise.  It's a good idea to set the threshold for star detection to be fairly large and to set the max intensity no higher than 90% to avoid picking up over-exposed stars.  Shoot for fitting about 200 stars.  In rare cases, it helps to restrict the measurement area to avoid the edges.  Setting a boarder of 100-500 pixels can sometimes help get a more accurate reading of FWHM in the middle of the field.  This is usually most helpful when the FWHM is reading too large; not too small.

b). Look at the Moffet order for fitted stars.  2.5 is typically a good value but for a very sharp image 1.5 is better.  If the order is set too low, the fit won't be good and your FWHM values will be too large.  If you really want to get accurate use the dynamic PSF tool to measure the actual shape of your stars before measuring the FWHM.

I'm going to try to post a sub of the same object with a measured FWHM in the range of about 1.6" so you can compare it with yours to see how close you got.  I had some trouble attaching it to this post so I'll try in a subsequent post.

John

Rodd Dryfoos:
I focus pretty much every time my fwhm creeps above a certain number.  With  the C11Edge On bad nights it’s frustrating because no matter how hard I try I can’t beat 3 and mid 3s makes me want to quit.  On good nights I might achieve 1.6 but it will typically hover around 2.3. When it climbs past 2.8 I refocus.  It can bounce in .5 arcsec humps because subs are short.  So I make sure focus is drifting and it’s not seeing related.  So I refocus many times in a session. Except those rare occasions when my fwhm stays put for hours ( I love those nights)

This is why you want an auto-focusing system.  Without it, you are wasting valuable imaging conditions!  I use real-time astigmatic focusing on my systems, which measures and sets focus every 1-5 seconds all night long.

John

John,
what auto focus system do you use that automatically adjusts focus every 3 to 4 seconds as you are imaging?
Stargazer66207

John,
Thanks for your feedback re:focusing, and for the fascinating links you included. The text and excellent photos you included were very informative. Unfortunately, I am guiding via a co-ax mounted 80mm refractor, so I would not be able to use an auto focus program that
uses the guiding image to adjust.
What you have done is amazing. Wish I could do something to make my continuing search for perfect focus come to fruition, but I guess I'll just have to keep muddling along!  Again, thanks for the feedback and for the informative links!!
Stargazer66207

Rodd Dryfoos:
But is it a collimation issue?

Most likely you are having an issue with the chromatic aberration. Or your object is too close to the horizon. Try test focusing with a single color filter instead of L. If the problem goes, it's definitely chromatism.

Another check: if your RGB filters are parafocal, you should not notice sufficient focus shift when changing between R - G - B, once you have focused, say in Red. But if the scope has chromatic aberrations you will clearly see the focus shift when changing the filters. Especially with the Bahtinov mask.