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