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Doug
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Posts: 28

It's exactly one year since I bought my first serious telescope and mount (Skywatcher MC127 + EQ3-2) and about 9months since I started messing with imaging with a newly acquired Canon EOS450D and William Optics Megrez 72.  When I first started imaging I naively thought that poor polar alignment was going to be my biggest source of rejected images.  I was wrong - with a polar alignment performed using a polarscope and mount which is reasonably well set up other sources of trailing images become increasingly problematical.

 

From my limited experience Periodic Error is the most significant and possibly stiction in the RA axis - I'm not totally sure about the latter but stars that are represented by two blobs with a very faint trail in between signify to me that the movement over the imaging period has not been constant. 

 

After being given help and advice by BLAS members I started having a go at stacking images and that's when I really noticed problems associated with mount imperfections as I was getting only 20-30% usable images with typical 90sec exposure using my EQ3-2 into a FL=432mm Megrez.  Again with local advice I was able to measure with relative ease the errors with my EQ3-2 and seemed to get some curious results in terms of peak to peak PE, depending upon the imaging device chosen (camera, CCTV video, webcam):

  • EOS 450D - taking images at 15sec intervals for 15 minutes and then manually measuring pixel position knowing the arcsec/pixel I was able to calculate that the Peak to Peak PE was about 70ish arcsec.
  • CCTV - took 45mins of video and the grabbed and manually analysed images every 15sec.  Since I did not have the exact details on the sensor I switched off the motor drive and was able to calculate arcsec/pixel by relating the resulting video trace to earth rate at approx 15arcsec/sec.  This gave an average p-p value of PE in the order of 120 arcsec. 
  • Webcam - using metaguide made things so much easier and I calibrated the sensor as above which resulted in p-p PE in excess of 160arcsec.

All I can think is that the above is explained by cyclic maximum of PE over a number of RA slow motion rotations.  It somewhat blows my ideas on modelling the PE and then programming my controller to cancel it out, since the p-p value is not predictable cycle on cycle.

 

I next applied the Metaguide process to the NEQ6, and this was much better uncorrected at about 50max and 20min arcsec pk to pk, however, no two cycles were the same so I fail to see how training for PE is going to have a significant impact on the quality of the resulting images.  I have not yet bitten the bullet and tried the PE training as it is lost between sessions unless the head is parked prior to putting away - something my storage facilities do not support.  One thing that did surprise me was the amount of high frequency noise that was present on this mount.  This could be explained as it was extremely lightly loaded with SW MC127 and associated small counter-weight - not even 20% of its rated load.

 

So the options are, as I see it:

  1. Continue with the current image rejection rates.  I have noted that the best images are taken as the mount reaches its peak or trough in the PE cycle, as you'd expect - this is particulary notable with the EQ3-2.
  2. Try out PE training and see how that affects the rejection rate in practice (NEQ6).  For the EQ3-2 I have modelled the 120arcsec PE based on a four segment straightline model which improves things to an extent.
  3. Opt for autoguiding.  I have been seriously considering this one as it should get over the problems associated with PE and small errors in polar alignment.  However, there are a number of factors that put me off, firstly I could only do it with the NEQ6 and as things stand and it currently takes me about 40mins to set up and a similar time to put away, so adding more cables and a laptop and another scope etc. would make it totally inefficient in the use of time - if I'm not back in the house for midnight I'd be out all night (locked out).  I have looked at the standalone guiders, but even those would take time to set up.  Also if the other mount errors are still prevelant e.g stiction, the guiding is not going to help.

Anybody got any similar experiences with mount problems and any remedies or alleviations that waste less time taking duff images?

 

Doug

 

 

August 28, 2010 at 6:04 PM Flag Quote & Reply

Doug
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Posts: 28

I've thought of, and am considering, a 4th option which just came to me while going through some past Astronomy mags whilst on holiday:

  • 4. Buy a monochrome CCD imager and associated filters, wheel etc.  This addresses two problems, the first being mount imperfections by reducing the exposure time per frame (reduces not eliminates).  The second problem it solves is to enable hitherto un-imaged wavelengths (up towards and into the IR band) to be captured.

May have some problems with the finance committee though.  I would be potentially looking at the Atik 320E with filterwheel - may have to content myself with greyscale initially with possibly just one or two filters - not the complete LRGB set just yet.  On that basis would it be possible to integrate the colour information from the DSLR into the final image in the interim?

 

Doug

August 30, 2010 at 4:38 AM Flag Quote & Reply

Doug
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Posts: 28

Update:

 

Going for option 4., hopefully in October/November, i.e. the Atik 320E with filter wheel deal, but will have to leave filters until later.  The smaller pixels will be better for planetary imaging as a bonus - I nnly hope the 6 year old laptop holds out.

 

Having noted that there was a constant drift when I ran the PE test on the NEQ6, I have re-aligned the polar scope which was about 10arcmin (described a circle of diameter of approx 20arcmin) off centre and two days against the index mark.  Whether this is significant or not I am not sure, but I will be able to judge through use.

September 11, 2010 at 12:50 PM Flag Quote & Reply

Doug
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Posts: 28

Update on EQ3-2:

 

Now that I've had some chance to trial the modifications to the motor drive I can report that it has been very successful.  From 30-40% hit rate on images its now 90% with exposures up to 2 minutes, and that's with the Equinox 80 ED Pro (FL=500mm).  It does rely on remembering to start the correction function at the right point in the PE cycle - which is complicated by the fact that I use setting circles to obtain objects so cannot fast forward.  Still angling after the 320E though.

October 29, 2010 at 10:26 AM Flag Quote & Reply

Doug
Moderator
Posts: 28

Update on EQ3-2:

 

Had another go at measuring the PE to confirm that it varies with position of the worm on the crown wheel.  Manually this time as the outside temperature was -7C at the time.  Graph (in DIY album) illustrates the problem which in this case is a 150 arcsec pk-pk error.  Have measured between 70 and 150 arcsec error.  May have another think about the PE correction algorithm in my RA drive.  Got the 320E and filters but the weather has not been suitable for a proper trial yet, and whether I can reduce significantly the exposure.

December 21, 2010 at 6:10 PM Flag Quote & Reply

Doug
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Posts: 28

The use of the Atik 320e has brought its own particular problems with regard to accurate tracking:

  • The camera, filterwheel and associated cabling adds significantly to the weight over the plastic bodied EOS, which has the result of moving the C of G further back so that the scope is way off balance when mounted on the standard (attached) mounting arm of both the Megrez 72 and the Equinox 80.  This means that there is always a quite significant moment being applied in declination - not good.
  • The pixel size is significantly smaller than the EOS so is less tolerant to tracking errors.
  • The cables have a habit of snagging, moving in the breeze which may be contributing to the problem.

I did try the PEC training on the NEQ6 but, I think I actually made things worse since I used the camera view on the laptop which was not giving sufficient effective magnification.  Also I've since read that un-eveness in the drive train means that PEC training is only good for about 3 turns of the worm (i.e. 20-30mins) anyway.  Something I suspected may be the case from the limited measurements I have taken.

 

The result is that I am getting about 30-40% usable images with the Equinox/Atik for example.  In simple terms I can get about 30mins from a total of 1.5 hrs of imaging.

 

Having seen what Allan has done with his stand alone guider, I have come to the conclusion that guiding is the way to go so am considering buying either the Baader LVI Smart Guide, or the Synguider from SkyWatcher.  Clearly the SW is most attractive in terms of price and probably ease of setting up, apart from the fact that the display is on the back the unit which is attached to the guide scope.  So if a guider adds say 30mins to the setup/breakdown time and my good images increase say 70% then I break even, time-wise, after 90mins.

 

I will also need to get a side-by-side mounting bar to enable me to mount both imaging and guide scopes, and hope that there is in the field of view of the object I am imaging that is bright enough to image on.

 

Anyone got views on this subject?

February 20, 2011 at 12:40 PM Flag Quote & Reply

Doug
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Posts: 28

Having seen Allan's recent images taken using a standalone guider, I have decided to follow suit and have ordered the SBIG SG-4 from Ian King Imaging at a somewhat knock down price of £500.  So hopefully the days of 70% rejected images should be over.

February 22, 2011 at 5:17 PM Flag Quote & Reply

Doug
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Posts: 28

25-02-2011 - Tried the SBIG SG-4 standalone guider out on M42 and M78, after focussing and calibrating etc using a PC.  Worked a treat.  No dropped frames apart from when cloud obscurred the target.  Does not take long to set up either now that I have locked the focus on the guide scope - Skywatcher Skytravel 80 (fl=400mm f/5).  Very pleased.  I did note during setup in the house that the CCD has some blotches over its surface and quite a few hot pixels but they did not seem to affect its performance; the hot pixels are mapped out by internally generated dark frames. Only tried with the EOS attached to the Megrez 72FD, though.

 

 

February 25, 2011 at 8:56 AM Flag Quote & Reply

Doug
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Posts: 28

Update on Autoguiding: Tried the SG-4 out in purely standalone mode taking the settings from the 24th Feb.  Struggled a bit as cloud was a bit of a problem between 21:00 and 22:15.  Targetted M97 and M108 initially with mixed results at 2'30 subs, although I was still getting 60% usable images - probably due to the target's proximity to the celestial pole (angular rate translated into pixel rate?), the lack of bright stars in the FoV of the guidescope, and a gusty wind?  Anyway when I targetted M51 later on, the wind had died down and every sub-frame was spot on.

Setting up is a bit of pain and I am optimising the order that I set up the, now quite complicated, configuration of two scopes, two cameras, mounting saddle, mount etc..  Things will get much more tricky when I introduce the Atik 320e into the equation.

February 27, 2011 at 7:08 AM Flag Quote & Reply

Doug
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Posts: 28

08/03/2011 - Now had two more sessions with the SG-4 autoguiding into the NEQ6 through the ST80, using the Megrez 72FD and the Equinox 80ED for imaging (on separate occassions).  Pretty impressed with its performance.  I had a total of 2+hrs imaging around Orion's belt where there are a multitude of reasonably bright stars and I did not have a single dropped frame.  I did notice when it came to combining the LRGB stacked frames that there was a gradual drift of a few pixels - equivalent to the disk diameter of the fainter stars, between the colour sets (sequencing was LLLLL,RRRRR,GGGGG,BBBBB).  I suppose this may well be due to slight relative movement between the two scopes as gravity has its effect during rotation in RA, as the ST80 is not the most rigid looking item.

The SG-4 failed to find a bright enough guide star when I had a go at M1 and it struggled with M109.  It could be that on the dimmer stars the mount needs tuning with the guider - I have used the factory presets for rates etc.. I am considering an off-axis adjuster for the guidescope so that I can locate a guide star within a small cone of the imaging scope pointing angle, if all else fails.

Anyway very pleased so far.  It may take longer to set up, but the ability to take multi-minute images coupled with a near 100% pass rate far outweighs the shorter setting up time and 30-50% pass rate at 60-90sec exposures - in my view.

March 8, 2011 at 4:53 PM Flag Quote & Reply

Flamencopaul
Member
Posts: 5

Finding a suitable guide star can be a problem...we have tried such remedies as: using a focal reducer on the guide scope (less accurate guiding, but better selection of guide stars), mounting either the guidescope or imaging scope in adjustable O - rings to give some freedon of movement (more messing about until you get the hang of it - but cheaper than a tandem mount with fine adjusters). It helps if you have the FOV oriented to an equatorial grid and can find a guide star directly above or below the target object (i.e. you only need offest in declination, not RA and dec both)

Paul.

March 9, 2011 at 7:46 PM Flag Quote & Reply

Doug
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Posts: 28

Thanks for the advice Paul.  I have looked at a number of options, and before reading your views I was seriously thinking of getting a £30 0.5x focal reducer.  The problem with the SG-4 is that there is no display apart from a flashing LED to indicate the lock-on status for a guide star unless you have a PC connected, which defeats the object of buying a somewhat expensive piece of standalone hardware.  Although the flashing green LED is supposed to indicate 'lock', it does not guarantee that guiding is working, as I found out with M109, so, that's why I was thinking of whether the set-up parameters needed changing... but to what?

I have contacted a good friend who has access to maching facilities with a view to making mechanism for allowing say 5deg in X and Y off the axis of the imaging scope.  In the mean time I'll have to stick to objects with bright stars in the vicinity or go back to open loop tracking.

EQ3-2 PE Measurements again

I did some more measurements last night with a view to modifying the software in my EQ3-2 RA controller to provide a range of PE correction parameters (60, 90 and 120arcsec) - see the DIY photo album.  The most recent measurements show that the mount error currently is round about 85arcsec pk-pk which is reduced from where it was before Christmas using the same measurement set-up - again see DIY album.  The trick with this approach to reducing tracking errors is engaging the correction algorithm at the right point in the worm cycle, and my measurements seem to indicate that this too changes slightly, although I can't think why it should.  Theoretically if I can engage the PECs at the right time it should provide short term errors down to a few pixels on a short FL scope.

I would also be interested in solving another conundrum: with the combination of scope and camera the theoretical resolution is 0.76 arcsec per pixel.  Now when I measured it by disengaging the RA motor and taking a number of readings as the star traversed the FOV I got 0.66 arcsec per pixel.  This is consistent with previous measurements, any ideas.  MC127 (FL=1500mm) into a Canon EOS450D.

March 11, 2011 at 4:00 PM Flag Quote & Reply

Doug
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Posts: 28

Addendum to the above (11/03/2011).  The run also illustrated that PEs are by far the biggest contributor to trailing images for a reasonably aligned mount.  I performed a typical polar alignment using the polar scope with the mount unloaded and the resulting drift due to polar alignment was in the order of 1arcsec per minute which to all intents and purposes is un-noticable on a 2 minute exposure with a short FL scope.

March 11, 2011 at 4:16 PM Flag Quote & Reply

Doug
Moderator
Posts: 28

01/05/2011 - Final experiments with a modified motor drive, giving me the ability to select 60,90 or 120 arcsec PE compensation worked pretty well.  Getting approx 50% good images at 2'30sec exposure with the Equinox 80ED Pro into a canon EOS.  After all that work I have decided to follow the trend and go to auto-guiding, in my case using a SBIG SG-4 (+ 0.6x Anteres focal reducer) into Sky-Watcher ST80 which works a treat providing that you have a few degrees of adjustment in one axis, and with it being stand-alone it takes very little time to set up.  So, I'm afraid the EQ3-2 won't get used much now, still it provided me with an insight into the errors of a mount and a challenge to correct them - Photos in the DIY section.  Main lesson learnt - for a mount aligned using a calibrated polar scope, when it comes to astrophotography, Polar Alignment Errors are insignificant in comparison with Periodic Errors for your average Sky-Watcher type cheap-ish mounts.  

May 1, 2011 at 4:39 AM Flag Quote & Reply

Doug
Moderator
Posts: 28

01/05/2011 - Addendum (ran out of time on the edit of the last post).

The arcsec per pixel question.  Although it does no explain why I got a higher resolution, the arc-sec per pixel will change as you move the centre of the FoV from the celestial equator to the celestial pole, since the rotational component takes over from what is effectively a linear component on the equator?

May 1, 2011 at 4:55 AM Flag Quote & Reply

Richard
Member
Posts: 5

Great report...............thanks for sharing

Rich

May 5, 2011 at 7:15 AM Flag Quote & Reply

Doug
Moderator
Posts: 28

Thanks for that Rich.

This is just a quick update on the use of a standalone autoguider, which in my case is the SBIG SG-4.  In summary:

  • It works a treat with both the NEQ6 and the EQ5 Pro, although I found that I had to up the guide speed from 0.5x to 0.75x when using a heavy scope. 
  • The calibration seems to be equally applicable for both mounts which saves several minutes re-calibrating each time I change mount.
  • It probably adds no more than 5mins to set up and put away, i.e. negligible, unlike setting up an imaging session with the CCD and laptop etc.
  • For the first time I have been able to use my old Maksutov Cassegrain (SW 127) on objects other than planets, albeit with quite long exposure times.  Still no go for very faint DSOs.
  • To get the guider to work as advertised keep the focal length of the guide scope down to 200mm or there abouts.  I do have a rather Heath Robinson contraption which allows some movement in azimuth, which is sometimes needed - the trick is to adjust a degree-ish at a time and re-acquire; when the camera acquisition time is in the order of a second to 2 secs it works a treat, any longer and not good (probably because I don't know what to adjust in terms of camera guide settings).  I use what a lot of people have adopted for the OTA for guiding, namely the Sky-Watcher 80mm 400mm f/5 achormatic, but with a 0.6x focal reducer - brilliant as you can lock the focus and don't have to faff about re-focussing - I haven't touched it for months.  NOTE: to get the configuration to focus with the 0.6x reducer, I had to take about 5mm off the end of the guide camera nose-piece.
  • Don't use light pollution filters with the guide camera as they just reduce the amount of light reach the sensor - tried with and without - much better performance without.

In short, I won't be going back to unguided imaging, apart from the odd occassion when I use the EQ3-2 with my built in PEC function, and then only with short focal length scopes.

May 23, 2011 at 5:20 PM Flag Quote & Reply

Doug
Moderator
Posts: 28

Finally tried the EQ5 Pro unguided.  Target area was in the constellation Lyra so the results are indicative of what can be achieved, but await a more scientific investigation with a target on the celestial equator - next full moon when DSOs are out of the question.  Anyway, over 18 mintues there was a total drift of 15 pixels (WO Megrez 72ED, EOS 450D with a 0.85x focal reducer => 2.9 arcsec/pixel) which was due to poor polar alignment.  This drift was modulated with the periodic error with a maximum deflection of aroun 6 pixels.  Three of the 7 images I got with 3minute exposures using this set up would have been good enough for stacking.  The benefit of running unguided being a reduction in set-up and dismantle time, at the expense of rejected frames.  I should have deliberately misaligned in Declination then taken a long exposure which would provide a trace of the error on top of the RA drift error.

October 25, 2011 at 6:19 PM Flag Quote & Reply

Doug
Moderator
Posts: 28

26/10/2011 - As promised I've taken a set of measurements (mandraulically) using my Mak Cass 127 (FL=1500 with a 0.85 Focal Reducer), into an EOS 450D.  Exposure repeat was 13sec. Object which was used as the target was 8deg above the celestial equator - near enough for the purposes of computation to the equator for my calcs.  The graph of error against time (83 frames analysed manually) is posted in the Astro DIY album.  In short the error peak to peak is between 45 and 50 arc sec, which is actually pretty good for a Sky Watcher mount I think.

October 26, 2011 at 5:55 PM Flag Quote & Reply

Doug
Moderator
Posts: 28

29/10/2011 - Measured EQ5 PE over a number of cycles using Metaguide, webcam and MC127.  Results show a high frequency noise typical (1 arcsec pk to pk) of a stepper driven mount (same effect with the EQ6), superimposed on a PE of approx 60 arcsec max to min.  Note that the target was Altair at a declination of 8 degrees so the PE in the graph in the Astro DIY album are slightly worse than depicted in the chart.  It does illustrate the difficulty in removing PE using a pattern derived from a single run during PEC training.

October 29, 2011 at 11:29 AM Flag Quote & Reply

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