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Use of a rotator on a Dobson to eliminate field rotation.

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#1 Mikehuerto

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Posted 17 October 2021 - 05:26 AM

I bought a Pegasus Falcon rotator a couple of weeks ago and I’ve been doing a few tests on it. https://pegasusastro...falcon-rotator/.

My main reason for purchasing the rotator is to increase the useable FOV. For longer integrations of 20-40 minutes I lose a lot of the useable field of view due field rotation. See Fig 6 as an example of the problem. Secondary reasons include the possibility of increasing frame exposure time and being able to precisely frame a target remotely.

 

Details of my Dobson and associated devices are in my signature 

 

This is a bit of a learning experience, so I wanted to start this thread and will update as I learn more about the rotators features and limitations. Bottom-line, it does function very well to eliminate field rotation on my 14” GOTO Dobson.  I welcome others to post on their experience with this and/or other makes of field rotators.

I am posting this thread because several folks on the EAA image gallery forum requested it, and prior to buying the deviceI was unable to find any decent road tests on the web for its use on a Dobson to counter field rotation. Though, I am aware of Filippos use on a 23” truss Dobson. https://www.astrobin...sers/filiscoop/

Fig 1 and 2 show the scope pointed  at the pole star with 10 minutes of SC live stacking, without derotation and with rotation.  You can see it does a good job of compensating for field rotation.

Fig 3 shows a screen shot of the rotator control panel in front of the SC screen. The Falcon control panel features some smart software. Its ASCOM driver links to the ASCOM platform via the Synscan App ASCOM driver and picks up the Az/Alt coordinates of the mount and derotates in the appropriate direction and speed. So, towards the North, its deretotating the camera anticlockwise, and towards the south, its derotating the camera clockwise. This is all automatic, there is not need to make adjustments as you slew the scope from North to South.

The Falcon software control panel shows you the mount coordinates and the rate and direction of derotation (+ = clockwise – minus + anticlockwise). In this way, you can see that the speed of field rotation increases with altitude when pointed towards the North or South, and decreases when pointed towards the East or West. A well known phenomenon.

The frame artifacts I still see in Fig 2 with derotation, are probably due to the inexact tracking of the SW Dobson Synscan mount.  However, the Synnscan App allows for adjustment of sidereal rate and Speed compensation. I am now experimenting with this. In addition, I am experimenting with the SharpCap4 sequencer to platesolve and recenter the image every 60 secs, this appears to work quiet well. As expected, the SC livestack rejects the image taken during recentering. This is not a big issue (1 of 60 images or 1 of 15 images depending on the exposure time of 1 or 4 secs).  More on both of these later!

As regards weight, the rotator weighs 670 g which together with the ZWO-ASI 294MCpro, coma corrector and adaptors makes a total of 1280g, this compares to the 350g for the wide angle eyepiece and adaptors that the Dobson is designed for, so some rebalancing will be necessary, and this will probably improve the tracking.

As regards live stacking in Sharpcap, there will probably be an issue for dark and flat subtraction, since these are done for each frame captured during live stacking. So,I expect that I will need to stack post-capture with DSS or similar software. Unless of course Dr. Robin Glover can figure out how to rotate the dark and flat frames at each capture in SharpCap. 

For EAA with bright objects such as M42, live stacking and derotation without darks and flats is not an issue since the dark level can be increased to hide vignetting (Fig 4), but for feinter objects such as M33 flatssubtraction is needed to compensate for vignetting (Fig 5).

Besides de-rotation, it is also very useful for framing a large image in a FOV. It seems that several EQ mount owners on CN use this device soley for this purpose. For example, I was able to use the – and + rotate buttons to position M42 in my FOV to fully capture the wings of the orion nebula.

 

I also plan to test it for planetary imaging. Thus, although Autsotackert can deal with some rotation, use of a derotator might improve stack alignment and final image quality. 

 

I welcome any feedback and contributions by others on this thread who have experience with this or other models of rotators.

I will update this thread with more results in the next few weeks.

Attached Thumbnails

  • FIG 1 Stack_40frames_600s_WithDisplayStretch.jpg
  • FIG 2 Stack_42frames_630s_WithDisplayStretch.jpg
  • FIG 3 2021-10-13  23-21  M57 SC and Falcon scree n.jpg
  • Fig 4 M42 derotation Stack_302frames_302s_WithDisplayStretch.jpeg
  • FIG 5 M33 Stack_156frames_624s_WithDisplayStretch.jpg
  • FIG 6.  Stack_601frames_2404s_WithDisplayStretch.jpg

Edited by Mikehuerto, 17 October 2021 - 01:26 PM.

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#2 Maxchess

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Posted 17 October 2021 - 07:05 AM

It’s clear that you can get good results with this kit and surprisingly good images. However I am a great believer in using the right tools for the job. A big Dob is a great tool for observing and by association for EAA. You great a real bang for your buck, where aperture is king, but do either of these applications really need derotation. Imaging is an interesting add on.

However for high quality imaging the derotation and tracking challenges need to be met at source, with an EQ mount. In imaging aperture plays a part but very precise tracking allowing longer exposure times and very accurate frame alignment are the key.

I have considered a rotator for my EQ kit and I am interested in experiences, however so far I have resisted as I tend to image for 4 hours per target and I can live with manual adjustment, especially with the “manual rotator” feature in NINA. Clearly where a rotator really scores is remote imaging where it would be essential

Edited by Maxchess, 17 October 2021 - 07:10 AM.


#3 steveincolo

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Posted 17 October 2021 - 07:51 AM

 

For EAA with bright objects such as M42, live stacking and derotation without darks and flats is not an issue since the dark level can be increased to hide vignetting (Fig 4), but for feinter objects such as M33 flatssubtraction is needed to compensate for vignetting (Fig 5).

Since the camera rotates with the image, I don't see why darks would be an issue.  But the camera is rotating with respect to the optical train, so I can see that flats would be a problem. 


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#4 steveincolo

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Posted 17 October 2021 - 07:55 AM

 

The frame artifacts I still see in Fig 2 with derotation, are probably due to the inexact tracking of the SW Dobson Synscan mount.  However, the Synnscan App allows for adjustment of sidereal rate and Speed compensation. I am now experimenting with this. In addition, I am experimenting with the SharpCap4 sequencer to platesolve and recenter the image every 60 secs, this appears to work quiet well. As expected, the SC livestack rejects the image taken during recentering. This is not a big issue (1 of 60 images or 1 of 15 images depending on the exposure time of 1 or 4 secs).  More on both of these later!

With such short exposures, it seems like one of the advantages of a derotator, taking care of star trailing, isn't being used.  It would be interesting to see longer exposures, if you can figure out how to work them with recentering.



#5 steveincolo

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Posted 17 October 2021 - 07:55 AM

How is the rotator powered?



#6 randcpoll

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Posted 17 October 2021 - 08:42 AM

With such short exposures, it seems like one of the advantages of a derotator, taking care of star trailing, isn't being used.  It would be interesting to see longer exposures, if you can figure out how to work them with recentering.

Field rotation comes into play even with short exposures if many are taken and stacked over a long period of time like they would be for these deep sky photos.


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#7 Mikehuerto

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Posted 17 October 2021 - 08:52 AM

With such short exposures, it seems like one of the advantages of a derotator, taking care of star trailing, isn't being used.  It would be interesting to see longer exposures, if you can figure out how to work them with recentering.

Agree - testing different exposure times with and without deroation is on my to do list. Since star trailing and/elongated stars has both a tracking and rotation component, it will be interesting to see the results. The 1 - 4 sec exposures used in this test are simply the times i previously used for EAA, as a benchmark and comparison.

 

That said i did try a few 15 sec stacks and got nice sharp stars - will need to test further.


Edited by Mikehuerto, 17 October 2021 - 09:26 AM.


#8 Mikehuerto

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Posted 17 October 2021 - 08:55 AM

How is the rotator powered?

12 V ac-dc mains adaptor,- not included in purchase. Though they do provide a battery (cigar lighter type plug) to 12V cable . Check the pegasus link i provided for more details


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#9 Mikehuerto

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Posted 17 October 2021 - 09:37 AM

Since the camera rotates with the image, I don't see why darks would be an issue.  But the camera is rotating with respect to the optical train, so I can see that flats would be a problem. 

Agree - with darks - and since most dust issues will be on the sensor and filters it will still be interesting to test the efficacy of flats in removing vignetting while derotating - i guess it depends on how symetrical the vignetting is across the FOV with my light train. Will test it.


Edited by Mikehuerto, 17 October 2021 - 09:46 AM.

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#10 steveincolo

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Posted 17 October 2021 - 09:53 AM

Field rotation comes into play even with short exposures if many are taken and stacked over a long period of time like they would be for these deep sky photos.

I'm talking about star trailing in an individual image, aside from field rotation cropping effects from stacking. 



#11 Mikehuerto

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Posted 17 October 2021 - 09:56 AM

Field rotation comes into play even with short exposures if many are taken and stacked over a long period of time like they would be for these deep sky photos.

Apologies, I think I misread  this when I first replied. Here is my edited response. I agree, Sharp Cap stacking takes account of rotation as it stacks, so each star is correctly aligned with the same star in the previous frame - the result is that stacking non.derotated short exposures does not result in star trailing, but you do see a rotation in the usable stacked FOV aka field rotation artifact. This is the main reason I purchased the rotator was to eliminate this type of artifact. . See Fig 1 and 6 for examples.


Edited by Mikehuerto, 17 October 2021 - 01:37 PM.

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#12 Mikehuerto

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Posted 23 October 2021 - 06:04 PM

Tried flats and darks tonight - looks like it they work fine, but need to do further tests and comparisons

Clouds and moon stopped play!

 

Posted a 5 minute stack in the EAA gallery  https://www.cloudyni...ery/?p=11449187


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#13 Mikehuerto

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Posted 24 October 2021 - 06:04 PM

Ok, so here is a  Sharpcap live-stack of M33 from last night, captured over a period of 45 minutes (21:42 to 22:23) from the same session as the 5 minute one above - that is it was saved 40 minutes after the 5 minute PNG image was saved (1240 x 1 sec at 490 gain with darks and lights subtracted by Sharpcap - I paused stacking a couple of time so thats why the time between start and finish is 45 minutes.)   You can see that although there is, frame shift, there is no frame rotation after 45 minutes. I was re-centering by plate-solving in Sharpcap every couple of minutes to keep the image centered. This is to make up for the poor tracking of the big 14¨ GOTO Dobson mount. The image is pretty faint as I was battling the rising moon rising just below it -  plus the understandable limitation of Sharpcaps stretching and processing on the fly. To demonstrate that, I´ve also included a cropped and processed image after processing the 16 bit FITS file of this 1240 x 1 sec  stack in Startools.  I call this EAA Plus. That is, I get the instant buzz of live stacking, and always have the option to beautify and process afterwards from the FITS file. I routinely process my live stacks in this way. So definitely a reasonable strategy for imaging feint DSOs with a big clunky Dobson.

 

One thing I´m a bit puzzled with, is that although I was recentering the image every 2 minutes, I still see some frame shift (not rotation). This is strange because, I could see that after recentering the stars in the bottom right of the image were always in the same position.  Any thoughts? Perhaps I need to recenter more frequently? I´ve set up a sharpcap sequencer file to automate this in future. 

Attached Thumbnails

  • Stack_1214frames_1214s_WithDisplayStretch40percent.jpg
  • Stack_16bits_1207frames_1207s v240percent.jpg

Edited by Mikehuerto, 25 October 2021 - 06:22 AM.


#14 Mikehuerto

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Posted 25 October 2021 - 06:52 AM

One thing I´m a bit puzzled with, is that although I was recentering the image every 2 minutes, I still see some frame shift (not rotation). This is strange because, I could see that after recentering the stars in the bottom right of the image were always in the same position.  Any thoughts? Perhaps I need to recenter more frequently? I´ve set up a sharpcap sequencer file to automate this in future. 

I think I can partly answer my own question here. If you look at saved stacks I did at 314, 1083, and 1214  frames, you can  the same stars are present in each corner of the frame (including the black area). It looks like Sharpcap is not able to align and stack the entire field as the stack grows, even though the star field is the same.  It gradually excluding incrementally the right edge as stacking continues  - the bottom edge stays the same. I´m wondering if there is adjustment that can be made to the stacking to accommodate this. I´ll try a longer stack next time to see if this plateaus.

Attached Thumbnails

  • Stack_314frames_314s_WithDisplayStretch - Copy.jpg
  • Stack_625frames_625s_WithDisplayStretch.jpg
  • Stack_1083frames_1083s.jpg
  • Stack_1214frames_1214s_WithDisplayStretch.jpg

Edited by Mikehuerto, 25 October 2021 - 01:41 PM.


#15 Mikehuerto

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Posted 05 November 2021 - 03:28 PM

Here´s a few more 15-20 minutes tests with the rotator. 

 

https://www.cloudyni...ery/?p=11477558

 

Basically working well. Although it eliminates rotation, it does not eliminate frames shifts due to poor tracking of the Dobson so nudging or recentering by plate solving is needed to capture the entire FOV.  In addition, its quite useful for correctly framing large images - such as the horsehead and flame nebula.



#16 Tiago Ferreira

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Posted 08 November 2021 - 08:18 AM

 I call this EAA Plus.

I like the "EAA Plus" concept and your expression to define it.

I may adopt it waytogo.gif

Saludos!



#17 Mikehuerto

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Posted 08 November 2021 - 11:10 AM

I like the "EAA Plus" concept and your expression to define it.

I may adopt it waytogo.gif

Saludos!

Glad you like it Tiago!

 

Here is a second attempt  using the rotator for 40 mins on M33 (4 sec subs).  This time captured during the new moon.  A considerable improvement over my previous post.

 

The rotator behaved nicely the whole time. 

 

EDIT - The first image is the Sharpcap Dislplay Stretch Output. 

 

The second image is the enhancement of the FITS file in Startools (what Im calling EAA Plus). As you can see it really helped bring out some of the OIII and  Ha

 

Best

 

Mike

Attached Thumbnails

  • Stack_609frames_2406s_WithDisplayStretchRESIZE.jpg
  • Stack_32bits_610frames_2410s.jpg

Edited by Mikehuerto, 08 November 2021 - 03:19 PM.

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#18 asquecco

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Posted 08 November 2021 - 02:07 PM

I bought a Pegasus Falcon rotator a couple of weeks ago and I’ve been doing a few tests on it. https://pegasusastro...falcon-rotator/.

My main reason for purchasing the rotator is to increase the useable FOV. For longer integrations of 20-40 minutes I lose a lot of the useable field of view due field rotation. See Fig 6 as an example of the problem. Secondary reasons include the possibility of increasing frame exposure time and being able to precisely frame a target remotely.

I'm thinking about buying a derotator for my Dobson 10" for the same reasons.
But my additional requirement is to be able to start new sessions of the same object (eventually a week later) with the same initial angle.
Is there any simple solution for this requirement? Otherwise at the end I'll have the same field rotation issues when stacking all the images together.



#19 Mikehuerto

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Posted 08 November 2021 - 02:37 PM

Havnt tried it , but you should be able to align the rotator to match the angle of the previous session. The device and software allows for manual adjustment


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#20 Mikehuerto

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Posted 09 November 2021 - 05:06 AM

Pasted from EAA Gallery

"mklosterman1, on 08 Nov 2021 - 8:21 PM, said:
I have been looking to get a rotator for my 18" dob but 1) cost! and 2)probably not enough in-focus travel right now to add another gadget in the optical train... unless I cut the truss tubes (ouch!)  Can you PM me a photo of your rig sometime? Especially a closeup of how you have the rotator connected? Thanks"

 

 

Mike 

Here are a couple photos of my rig. As you can see its fully loaded with an electronic focuser, the ASI 294 MC pro, and the rotator. All gadgets plus the mount are connected via a USB-ethernet extender through 30 meters of Cat7  to my PC inside the house (running ASCOM 6.5) .

 

The rotator comes with two 48-54 adaptors, one for the 2" focuser tube and one for the camera side. They are both quite long (2-3 cms)  However, to fit the rotator onto the SW coma corrector that I have in the focus tube, I needed to buy a thin (2mm) 48 F-54 M adaptor to replace the 2cm tube side adaptor. I could probably shorten the light train by another 2 cm, by doing the same on the camera side, creating total available back focus of 4 cm. More than enough for a filter wheel. 

 

Add to that I still have an additional 9 cm of backfocus available by using the  binoview holes on the the three trusses.. If you look carefully at the top truss you can see one of the binoview holes on the right close to the shroud (which is pulled back).  Perhaps you could just drill similar holes on your trusses to push the focal point further out of the tube?

 

Hope this helps!

 

Mike

Attached Thumbnails

  • DOBSON full viewLR.jpg
  • Rotator close upLR.jpeg

Edited by Mikehuerto, 09 November 2021 - 05:06 AM.


#21 Eclipsed

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Posted 09 November 2021 - 12:08 PM

Glad you like it Tiago!

 

Here is a second attempt  using the rotator for 40 mins on M33 (4 sec subs).  This time captured during the new moon.  A considerable improvement over my previous post.

 

The rotator behaved nicely the whole time. 

 

EDIT - The first image is the Sharpcap Dislplay Stretch Output. 

 

The second image is the enhancement of the FITS file in Startools (what Im calling EAA Plus). As you can see it really helped bring out some of the OIII and  Ha

 

Best

 

Mike

That's a great image after processing Mike.  Well done!!

Martyn


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#22 Mikehuerto

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Posted 17 November 2021 - 04:45 AM

I´ve just posted a new thread on use of SC sequencer to automate the whole process:

 

https://www.cloudyni...son/?p=11503249

 

Mike



#23 Mikehuerto

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Posted 19 November 2021 - 04:33 PM

Here´s another before and after processing using the field rotator

 

377 X 4 secs (25 mins)

ASI 294 MC -10 C

ZWO UV-IR Cut filter.

 

No cropping in the original and minimal cropping in the Startools processed version. 

 

For higher res go to  https://www.astrobin.com/liqejm/

 

 

Attached Thumbnails

  • Stack_377frames_1508s_WithDisplayStretchRESIZE.jpg
  • Stack_32bits_376frames_1504sV6-150sat-ASTROBIN-4.jpg



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