Ovoids with Mesu and Tak FSQ 85

[Davey-T]

I have the Atik OAG on EFW2 on10" SCT and it works flawlessly, after I refitted the prism that fell out

Dave

[ollypenrice]

Do we know the RA-Dec orientation of the elongation? 

Olly

[steppenwolf]

Do we know the RA-Dec orientation of the elongation?

The image (Pelican Nebula) is pretty much square on the RA/DEC so RA is left-right DEC is up-down with the centre of the image showing a 49 degree slant?

[derrickf]

No problem.  It was a good test for a PixInsight script I'm writing.  It takes the centre, corners and edges of an image, analyses the shape of stars it detects and then overlays the detected stars with a symbol showing the elongation direction.  It then averages the stars in each section of the image and displays a much larger version showing the elongation of the average star, together with statistics:  FHWM, Eccentricity, Angle of elongation.

That sounds like a really useful script  - do you plan to make it available when it is finished?

Derrick

[derrickf]

OK chaps. I'll do some serious reconfiguring tomorrow and then wait for a clear night to test. I am not sure I am quite ready for off-axis guiding - do folks like the Atik OAG? (I have an EFW2.). I would have enough spacing, even if I did eventually get the Tak reducer.

Thanks to everyone who has contributed to this thread - all very helpful.

The Atik OAG works fine for me with EFW-2 and Atik 383.

In response to your earlier question about finding guidestars - I have never failed to find a usable guide star in my Lodestar2 FOV when guiding my 12" RC.

HTH

Derrick

[gnomus]

Do we know the RA-Dec orientation of the elongation? 

Olly

I am not entirely sure if I can answer this.  I set up my camera using that trick you have mentioned on several occasions - that is, take an image whilst slewing the mount for 5 secs and then adjust camera angle to get it squared.  In SGP's plate solve routine it states that my Angle is 0.83 degrees.  At this angle, the full image looks like this (I've only posted portions of the image before) - this is  very basic stack and levels stretch:

Does that help at all?

I've spent all morning getting my ED80 on top of my Tak using the Baader mini dovetail thing, then recabling and rebalancing - I hope this works. 

[ollypenrice]

The image (Pelican Nebula) is pretty much square on the RA/DEC so RA is left-right DEC is up-down with the centre of the image showing a 49 degree slant?

Cheers, Steve, so now I'm wondering whether that argues against flexure on the grounds that most of the correcting applies to RA. On the other hand a flex in Dec would throw up a spurious command. Perhaps it doesn't tell us anything. I might be thinking about tilt again, though.

Olly

[gnomus]

Cheers, Steve, so now I'm wondering whether that argues against flexure on the grounds that most of the correcting applies to RA. On the other hand a flex in Dec would throw up a spurious command. Perhaps it doesn't tell us anything. I might be thinking about tilt again, though.

Olly

If my set up was unbalanced in RA, could that explain the elongation?  I have tried to be careful balancing the Mesu but because there is no way of 'declutching' I do find it a little difficult to tell if I am in balance.  On both axes there is a wide-ish range of settings that will allow the scope to remain stationary when I let go of it.  I have tried two techniques - one where I apply pressure upwards (or downward) on a point approximately equidistant to the centre - when doing this I try to get to a state where it appears that equal amounts of pressure are needed on each side to move the mount.  The other technique I used was to move the weight or the scope to a point where it just started to move if I let go.  Then I'd adjust it in the opposite direction until it did the same - then I would set the scope/weight at the midpoint of these two extremes.  

Are there any other (better) techniques for balancing the Mesu? 

[steppenwolf]

Are there any other (better) techniques for balancing the Mesu?

I use a 'momentum' check on mine. I manually rotate the axis in question (having locked the other axis with the 'hooks') fast enough for there to be sufficient momentum for it to continue moving a little - just a couple of centimetres is fine - then I reverse the rotation and gauge that too. I now have a 'feel' for when the dynamics are the same for both directions and thus the axis is balanced.

[gnomus]

Clear Outside is suggesting that I might be able to get out tonight. If I do I will try out the new set up. If I still get eggs, I will rotate the camera 90 degrees (I think that this will help determine if I have tilt). Are there any other tests that I could usefully run if I still get eggs tonight?

[sharkmelley]

That sounds like a really useful script  - do you plan to make it available when it is finished?

Derrick

That's the plan - yes.  When I found that CCDInspector never gave me the information I needed, I decided to write my own.  PixInsight was an excellent framework for doing so.

Mark

[gnomus]

Last night was pretty poor seeing-wise, but I was able to make some progress (I think).  Firstly, the ED80 did find a guide star and did seem able to produce reasonable guiding - here is a screenshot from PHDLog Viewer:

I think this is reasonably good guiding - certainly miles better than I was getting with my CGEM.  There were long periods when the mount seemed to be tracking well with no need for 'correcting impulses' from PHD2.  Is this level of accuracy about what I should be expecting from the MESU, or should it be better?

I took two exposures - one a 10 minute Lum and the other a 20 minute Ha.  I seemed to have cured the issue of star elongation in the centre of the image.  This is a crop from the centre of the 20 min Ha exposure.  I have re-sized this to 400% (how's that for pixel peeping?) and the stars here look bang on to my eye:

In the corners I think things are improved.  I do not see any significant issues in the 10 minute exposure.  In the 20 minute Ha, there is still some elongation, but this is not all in the same direction like before (as evidenced by Mark's PixInsight script in an earlier posting).  Here is a 100% crop (all of these are from the extreme corners):

You might need the 200% sized crop to see what elongation there is - this is from top right:

To me the elongation seen here appears to be radial - that is, the TR corner goes NW to SE whereas the BR corner goes NE to SW, and so on all the way round.  I wondered if this could be field rotation.  The fact that I don't see elongation in the 10 minute exposure suggests that this is not an optical issue.  I did run the PHD Guiding Assistant and it told me that my Polar Alignment error was 1.9 arcminutes.  Previously it had reported an error of 0.1 arcminutes, so I think I must have shunted something a little when taking the scopes on and off during yesterday morning's reconfiguration.  (Of course I have no idea about the reliability or accuracy of the PHD2 Guiding Assistant.)  PHD2 still seemed happy enough with the 1.9 arcminute error, that is it did not ask me to try to improve this.  Would such a relatively small PA error give rise to field rotation?  Am I correct in thinking that the field rotation would look worse when using a wider field of view (my ED80 was 600 mm f/l compared with the 450 mm for the Tak)?  If that is correct would the reducer, by giving me a wider FOV, make things worse?

My analysis, therefore, is that my earlier problems were due to flexure, either caused by my SkyWatcher guide scope bracket not being 'solid' enough, or because of the place where I had attached my cabling.  Flexure appears not to have been an issue last night, as evidenced by my perfectly round central stars.  The elongation I now have appears consistent with field rotation due to the PA not being as accurate as it could be - probably due to my blundering when redoing my set up yesterday.

I'd be very interested to hear what folks think of these new findings and my analysis of them.  If anyone is interested in looking at the FITS files, these can be found here:

https://www.dropbox.com/s/t5u7921md1r4d8e/Test%20ED80%20Guiding_600sec_1x1_L_frame1.fit?dl=0 

https://www.dropbox.com/s/3wy9ehcpnimjn5g/Test%20ED80%20Guiding_1200sec_1x1_HA_frame1.fit?dl=0

Edited November 1, 2015 by gnomus

[ollypenrice]

I would say that the present distortions are, as you suggest, the result of field rotation. And yes, I would expect this to be worse on a wider FOV. The simple test is to take a short exposure through a less restrictive filter (luminance) to see if the distortions are still there. In a short sub rotation won't show. On occasion my PA needs a tweak and a rotation visible in 30 minutes may not be visible in 15 minutes.

Looks like you're on the road to success!

Olly

BTW, below is a typical Mesu guide trace in AstroArt 5, my eccentric choice of guiding software.  The guidescope is the same as yours, so has a 400mm FL and the  Lodestar is working in bin 2 so a hefty 8.46 arcsecs per (virtual) pixel. The graph is in pixels. I run 4 second guide subs. I can refine this level of accuracy by working on it on a night by night basis but at the scales at which I'm currently imaging on the mount it would be a waste of useful time to do so.

Edited November 1, 2015 by ollypenrice

[gnomus]

....... The simple test is to take a short exposure through a less restrictive filter (luminance) to see if the distortions are still there. In a short sub rotation won't show. On occasion my PA needs a tweak and a rotation visible in 30 minutes may not be visible in 15 minutes......

.....

BTW, below is a typical Mesu guide trace in AstroArt 5, my eccentric choice of guiding software. The guidescope is the same as yours, so has a 400mm FL and the Lodestar is working in bin 2 so a hefty 8.46 arcsecs per (virtual) pixel. The graph is in pixels. I run 4 second guide subs. I can refine this level of accuracy by working on it on a night by night basis but at the scales at which I'm currently imaging on the mount it would be a waste of useful time to do so.

Thank you. As I said in my probably too lengthy post I also shot a 10 min Luminance sub and I couldn't make out any significant ovoid-osity in the corners. Field rotation it is then.

I will never post any smug 'look-at-how-flat-my-guide-trace-is' images again after your AstroArt screenshot! How are you doing that? BTW, I am now using a 600mm guide scope (the ED80) and it does not seem that it is possible to set my ZWO guide camera to bin - there is an option under the PHD2 brain, but it is greyed out. So I think that gives me 1.29 arcseconds per pixel. I intend improving my PA by looking at the video coming from my guide scope - that will work won't it? I don't need to be looking through the imaging scope do I? (I really don't fancy unscrewing all those Tak adapter rings again.)

Edited November 1, 2015 by gnomus

[Freddie]

The two guide graphs are actually pretty similar. They are not showing the same scale. You are arc sec and Olly pixels. If you take Olly's average being 0.05 pix and convert to arc sec it is ~0.5 which is pretty much the same as yours.

[ollypenrice]

The two guide graphs are actually pretty similar. They are not showing the same scale. You are arc sec and Olly pixels. If you take Olly's average being 0.05 pix and convert to arc sec it is ~0.5 which is pretty much the same as yours.

Indeed so. The Mesu is a good mount!

Olly

[sharkmelley]

Here's the result of running those two frames through my PixInsight script.  Note that an eccentricity figure of 0.4 or below is usually considered round, so the 1200sec image shows good guiding (round stars at the centre of the image)

The analysis seems to confirm what is obvious to the eye.  But I can't really explain what we are seeing.  I don't think field rotation is the explanation.

Mark

Edited November 1, 2015 by sharkmelley

[Davey-T]

Don't stars in this circular pattern indicate incorrect flattener spacing ? if you're using one of course. there was a thread on it a while back.

Dave

Post #25 in this thread

http://stargazerslounge.com/topic/225831-flattener-spacing/page-2?hl=flattener#entry2442998

Edited November 1, 2015 by Davey-T

[gnomus]

I am not using the reducer.  I believe that the Tak has an inbuilt flattener.  I am using the correct Tak rings as specified in the manual. 

[Davey-T]

Sounds like this could descend into one of those imaging train ? tilted sensor ? ??????????? threads, best sell it all and go back to EQ6 ED80, much less stressful 

Dave

[gnomus]

Here's the result of running those two frames through my PixInsight script.  Note that an eccentricity figure of 0.4 or below is usually considered round, so the 1200sec image shows good guiding (round stars at the centre of the image)

StarShape1.JPG

StarShape2.JPG

The analysis seems to confirm what is obvious to the eye.  But I can't really explain what we are seeing.  I don't think field rotation is the explanation.

Mark

Thanks again for looking at these Mark.  I can imagine a circle drawn around the yellow lines in your analysis of my 1200 second exposure (except for perhaps the middle left panel).  The 600 second analysis appears to show random errors.  I'm not sure that your figures for the 600 second exposure marry up with what I am seeing on screen.  If I understand your data correctly, your script is suggesting that my roundest stars are in the middle left panel.  This is not what I see in the processed image.  

I should add that last night was quite horrible.  Visibility was only 4 miles according to 'Cloudy Outside'.  Not only that but temperature and dew point were pretty close.  The equipment and observatory walls were dripping wet again after only an hour or so.  When I looked into the objectives at the end of the session, I wondered if I had my dew heaters turned up high enough.  Certainly the PHD2 image seemed to be out of focus by the end of the exposures.   The neighbours decided to set off a number of fireworks and from time to time firework smoke drifted across the field of view.   I do recall also that PHD2 reported one of those "guide camera unresponsive" messages during the 5 min exposure (although I reconnected everything pretty quickly and the guide trace seemed unaffected).  Finally, one disadvantage of having an observatory is that you can get set up and going too quickly.  I knew I only had limited time last night so whipped the roof off and got going immediately without leaving any time for the gear to cool down.  The 10 min exposure was taken first and started before astronomical darkness. 

Thanks again

Steve

[gnomus]

Sounds like this could descend into one of those imaging train ? tilted sensor ? ??????????? threads, best sell it all and go back to EQ6 ED80, much less stressful 

Dave

No need to sell it Dave - if it ain't right it's going back.  (You weren't hoping to pick up a bargain were you?)  

[Davey-T]

No need to sell it Dave - if it ain't right it's going back.  (You weren't hoping to pick up a bargain were you?)  

Damn, saw right through me 

Dave

[Zakalwe]

No need to sell it Dave - if it ain't right it's going back.  (You weren't hoping to pick up a bargain were you?)  

Nah, it's knackered Steve. I'll take it off your hands for £50 and that's me cutting me own froat.....

[gnomus]

Nah, it's knackered Steve. I'll take it off your hands for £50 and that's me cutting me own froat.....

Was that your attempt at a Cockney accent Stephen?