NGC281 (Pacman) - HOO

[strutsinaction]

Hi

I managed to dodge the clouds recently and capture some NGC281 Ha and OIII subs for my first bicolour (HOO) image. This was also an opportunity to test out my QSI690 which had been returned to QSI for the 'Sony Glow' fix.

Details:

Ha 15x600s

OIII 15x600s

Bias

Flats

No darks

FSQ85/QSI690/Avalon Linear

SGPro/PHD2/PI/PS

I'm very pleased with the fixed QSI690 - no more glow so no more darks needed for calibration!

However, this image does show star elongation in the 4 corners, particularly in the bottom right corner. Gnomus noticed something similar with his FSQ85 (http://stargazerslounge.com/topic/255860-ovoids-with-mesu-and-tak-fsq-85/).

I sent a few subs off to Ian King for further analysis to see if this is a collimation issue. I'm wondering though if this is the result of poor focussing. I typically only focus once at the star of my imaging run, manually, using a Bahtinov mask, and assume that the Tak will keep focus for a couple of hours   I apply the focus lock so I susepct the focus is drifting as the temperature changes during the session. I used CCDInspector on the subs and notice that as the subs rolled in the FWHM values increased.

So, my question is this - would stars become elongated in the corners if focus is off?

Regards

John

[MarsG76]

great looking pelican nebula, the only thing I'd suggest is to drop the space blackness level a little bit.

[laudropb]

Very nice image. Thanks for posting.

[strutsinaction]

Thanks, folks!

MarsG76: Here's the revised version - more aggresive curves and an alternative bi-colour palette.

Regards

John

[MarsG76]

Yeah, that's it... Looks great down to the faint nebulosity... It's quite difficult to get the blackness of space without clipping the faint nebulosity... I find that if I have the darkest part of space at a PS level of 15-20, that makes space near black and retains the faintest data.

[sharkmelley]

Lovely image and I much prefer the second version.

However, this image does show star elongation in the 4 corners, particularly in the bottom right corner. Gnomus noticed something similar with his FSQ85 (http://stargazerslounge.com/topic/255860-ovoids-with-mesu-and-tak-fsq-85/).

I sent a few subs off to Ian King for further analysis to see if this is a collimation issue. I'm wondering though if this is the result of poor focussing. I typically only focus once at the star of my imaging run, manually, using a Bahtinov mask, and assume that the Tak will keep focus for a couple of hours   I apply the focus lock so I susepct the focus is drifting as the temperature changes during the session. I used CCDInspector on the subs and notice that as the subs rolled in the FWHM values increased.

So, my question is this - would stars become elongated in the corners if focus is off?

The elongation pattern is not entirely symmetrical so that indicates there might be a tiny bit of tilt somewhere.  Putting aside the issue of tilt, this pattern of elongation, perpendicular to the diagonals, is frequently discussed in relation to the FSQ85.  See for example this discussion https://groups.yahoo.com/neo/groups/UncensoredTakGroup/conversations/topics/59070 on the Yahoo Tak group.

The consensus of many discussions is that if you accurately focus on a star in the centre then you may well see these elongations at the corners.  Instead focus on a star nearer the corner then the stars will look even across the whole field.  This is known as offset focusing because you are focusing on a star offset from the image centre.

Interestingly this idea is supported in the book "Telescopes, Eyepieces, Astrographs" by Smith,Ceragioli, Berry - section 15.5.1 where they discuss the properties of a generic apochromatic quad scope.  Their computer generated ray-fan plots for two different focus positions suggest that you should concentrate the focusing effort on achieving good star shape in the corners rather than the tightest star in the centre.

I should add that I am not an FSQ85 owner but a very happy owner of the Tak Epsilon 180ED.  So discount my comments as you feel appropriate  

Mark

[michael.h.f.wilkinson]

Nice result, especially the second one

[strutsinaction]

The consensus of many discussions is that if you accurately focus on a star in the centre then you may well see these elongations at the corners.  Instead focus on a star nearer the corner then the stars will look even across the whole field.  This is known as offset focusing because you are focusing on a star offset from the image centre.

Thanks, Mark! Very interesting, if not surprised/alarmed by this, but something to try next time I'm out imaging.

So far I've always manually focussed by choosing a bright star, centring with the Frame and Focus bullseye in SGPro, then using a Bahtinov mask and Bahtinov Grabber tool. I'll try positioning the star towards the bottom right hand corner and seeing if that makes a difference.

Once again, thanks for the heads up. It might not make any difference but it seems that this offset focussing approach does work for some FSQ85 owners. Here's what one onwer had to say (from the Yahoo group posting you linked):

That's the normal behaviour of a FSQ85, if you focus at centre you get bad shaped stars at corners in a circular pattern that a lot of people mistake for bad polar alignment. As Merv has said the solution is to focus near the corner, I know what I'm talking about because I have had 2 different FSQ85 (one new, the other second hand), and at the end I sold both of them because even if you focus at the corners, a little variation in temperature leads to elongated stars quickly.

I'll send this to Ian King too for his opinion.

Regards

John

[strutsinaction]

Nice result, especially the second one

Thanks Micahel! The curves tweak in PS has improved the contrast and I also think the this palette is a little easier on the eye 

Regards

John

[Barry-Wilson]

John

Very good Pacman, in both HOO blends (though I prefer the second red one).  You must be pleased.

I recently switched ccd to the QSI683 and Ian King supplies the correct Tak adapter for spacing using the reducer.  What I have found is that the stars in the corners of the chip are distorted when out of focus.  It is only as the focus point reaches the CFZ at the bottom of the AF curve that the corner stars 'snap' into shape.  I have one corner in my images where the stars are still ever-so-slightly elongated and I think I have the reducer adjustment screws to tweak further.  I don't think you use the reducer though, do you?  I also have been wondering whether the CAA locking screw has the potential to push out a corner marginally if too firmly locked onto the CAA.  I have adjusted the pressure on mine but not had any luck to image for 5 weeks now due to weather, so I don't know if this has made a difference.  Having a motor focus I don't tighten the focuser locking screw either, is this something you do?

When I had my Atik 383L, I felt that the two grub screws that clamp the ccd to the EFW2 had the potential to induce some tilt and it might be worth examining yours to see if this is a possibility.  With mine, I found that the angle the grub screws made contact with wheel was different and one in particular was making contact with 'rim' of the adapter before fully seated.  This difference I found may not have been inducing a tiny tilt but it prayed on my mind.

I refocus every 1 deg C change with my Tak and do notice that the focus point moves over an evening's sequence.

HTH

Barry

[strutsinaction]

Thanks for your thoughts, Barry!

In answer to your specific questions - I don't use the Tak reducer and I do tighten the CAA locking clamp quite a bit (and also the focus locking clamp). There's no obvious play in the focuser when drawn all the way but I guess that doesn't mean there no tilt in this part of the imaging train.

There are a couple of things for me to try the next time I'm out imaging - offset focusing and refocus after a temperature change. This latter point my prove to be significant. My current practise is to manually focus on a central bright star (using a Bahtinov mask) before my imaging run but so far I've not refocused. It makes sense to re-check focus as the temperature drops rather than assume the FSQ will keep focus 

Regards

John

[gnomus]

Thanks, Mark! Very interesting, if not surprised/alarmed by this, but something to try next time I'm out imaging.

So far I've always manually focussed by choosing a bright star, centring with the Frame and Focus bullseye in SGPro, then using a Bahtinov mask and Bahtinov Grabber tool. I'll try positioning the star towards the bottom right hand corner and seeing if that makes a difference.

Once again, thanks for the heads up. It might not make any difference but it seems that this offset focussing approach does work for some FSQ85 owners. Here's what one onwer had to say (from the Yahoo group posting you linked):

That's the normal behaviour of a FSQ85, if you focus at centre you get bad shaped stars at corners in a circular pattern that a lot of people mistake for bad polar alignment. As Merv has said the solution is to focus near the corner, I know what I'm talking about because I have had 2 different FSQ85 (one new, the other second hand), and at the end I sold both of them because even if you focus at the corners, a little variation in temperature leads to elongated stars quickly.

I'll send this to Ian King too for his opinion.

Regards

John

This is certainly interesting, but raises a couple of questions.  It is not clear how offset focussing could be done if you choose to have the software focus for you (with an electric stepper motor focusser).  Secondly, if you have to offset focus in this way, does that mean that the central stars and object(s) will not be as sharply focussed as they could be?

If "That's the normal behaviour of a FSQ85, if you focus at centre you get bad shaped stars at corners ..." is an accurate statement, then I don't think it is widely known.

[gnomus]

John

Very good Pacman, in both HOO blends (though I prefer the second red one).  You must be pleased.

I recently switched ccd to the QSI683 and Ian King supplies the correct Tak adapter for spacing using the reducer.  What I have found is that the stars in the corners of the chip are distorted when out of focus.  It is only as the focus point reaches the CFZ at the bottom of the AF curve that the corner stars 'snap' into shape.  I have one corner in my images where the stars are still ever-so-slightly elongated and I think I have the reducer adjustment screws to tweak further.  I don't think you use the reducer though, do you?  I also have been wondering whether the CAA locking screw has the potential to push out a corner marginally if too firmly locked onto the CAA.  I have adjusted the pressure on mine but not had any luck to image for 5 weeks now due to weather, so I don't know if this has made a difference.  Having a motor focus I don't tighten the focuser locking screw either, is this something you do?

When I had my Atik 383L, I felt that the two grub screws that clamp the ccd to the EFW2 had the potential to induce some tilt and it might be worth examining yours to see if this is a possibility.  With mine, I found that the angle the grub screws made contact with wheel was different and one in particular was making contact with 'rim' of the adapter before fully seated.  This difference I found may not have been inducing a tiny tilt but it prayed on my mind.

I refocus every 1 deg C change with my Tak and do notice that the focus point moves over an evening's sequence.

HTH

Barry

I agree about the concern Barry - these seem to be pretty small screws.  However, when I use the camera with my ED80 x 0.85 the field was fine.  As a result of my issues I downloaded CCD-Inspector Evaluation and the ED80/EFW2/383L tested A-OK.  I know I am at only f/6.3 (or thereabouts) compared with f/5.3 for the Tak.  

[sharkmelley]

This is certainly interesting, but raises a couple of questions.  It is not clear how offset focussing could be done if you choose to have the software focus for you (with an electric stepper motor focusser).  Secondly, if you have to offset focus in this way, does that mean that the central stars and object(s) will not be as sharply focussed as they could be?

That's right, with offset focusing, the stars in the image centre will not be so sharply focused.  The idea is to achieve the best compromise between stars in the image corners and stars in the image centre.

Mark