6 replies to this topic

[TicoWiko]

Why has the astrophotography industry not borrowed a simple trick from professional astronomy : camera rotators for AZ mounts ? Yes ZWO just came out with the CAA, but that's a camera rotator that's just meant to frame your shot. What I'm talking about is a precision rotator that rotates with the night sky as you track, which would turn all AZ mounts with tracking into suitable instruments for long exposure astrophotography. All of a sudden all those giant goto dobs people have turn into very interesting astrographs. A well collimated newtonian with a Paracorr in front can pump out some clean images with massive aperture behind it. The reason this is the norm in professional astronomy is simple : try building an EQ mount that can handle an 8 meter scope. It just makes much more sense technically and economically to rotate the camera rather than the whole scope.

So what am I missing here ? Why is this not a commercially available option ? Is it really hard to make a precision rotator, and is therefore only cost effective on multi meter class scopes ? Is there some other essential advantage to EQ mounts I'm not seeing ? Are amateur AZ mounts generally bad at tracking, so despite the derotation stars get smeared in long exposures ? My best guess for all of these questions is "no" so I'm really at a loss here. Then again I have no idea what it would take to make a precision 2" camera rotator, so very keen to hear from those who might.

Clear skies

Edited by TicoWiko, 10 January 2025 - 05:17 PM.

[BlueMoon]

 

What I'm talking about is a precision rotator that rotates with the night sky as you track, which would turn all AZ mounts with tracking into suitable instruments for long exposure astrophotography.

Put an AZ mount on a dobsonian equatorial platform and you've accomplished it, mostly. It works fairly well for short exposure sequences for up to an hour, maybe a bit more. A camera derotator can go hours and one can do longer exposures. Cheers.

 

http://www.equatorialplatforms.com/

Edited by BlueMoon, 10 January 2025 - 05:26 PM.

[idclimber]

They are commercially available, they just out of most budgets. I have a NiteCrawler rotator/focuser on my refractors on my refractors that can do it. Optec and Planewave make rotators that are used by some imagers in their home observatories. These range from 3 to something like $8,000. 

[afd33]

Falcon’s rotator has that option. I would think ZWO would eventually have that capability if it doesn’t at launch. I would guess other brands have that option as well.

I’m not sure what brand they have, but a local astronomy club has a 17” Planewave telescope and I’m pretty sure they use a rotator to get DSO photos.

Edited by afd33, 10 January 2025 - 05:30 PM.

[TicoWiko]

Oh ****. I guess the price point answers my question. I saw that the CAA was going for about 400 euros, and never imagined a precision derotator would mean an extra 0 at least. My googling couldn't even find any because only simple rotators would show up. I guess at that price point they just show up much less in search results.

[psandelle]

I played with the Falcon rotator and it worked pretty well up to 3 minutes (and that was probably the mount's fault), and that's a shade under $700, so not that expensive. My Optec Hercules can do it even more accurately, but I haven't tried. So, not such a hug thing - it's just getting a good enough alt/az mount for your dob. I mean, the mount's still gotta be accurate.

 

Paul

Edited by psandelle, 10 January 2025 - 06:56 PM.

[555aaa]

The cheaper rotators don't have the resolution. Suppose you want one pixel error (say 3 microns) over a 20mm radius (corner of a full frame camera). That's 42,000 steps per revolution or 30 arc seconds per step (120 steps per degree) and it needs to hold that accurately during an exposure (as opposed to holding an angle between micro-steps). The optec gemini rotator is 600 steps per degree but the cheaper Pegasus Falcon 2 is only 32 steps per degree, and those are microsteps which can't be held very accurately. The Primaluce Arco is 3600 steps per degree which is plenty.

 

The low cost solution is do do this all in software, using short exposures. The image below is a stack of 54 30 second exposures taken with a high quality alt az telescope with no rotator. (SharpStar SCA260) Also this is unguided.

 

The limitation with un-rotated is that you can only go as long as 30 seconds in a few parts of the sky, in some areas you can only expose for maybe 2 seconds. Also the smaller the FOV the longer you can expose for.

 

Edited by 555aaa, 10 January 2025 - 07:00 PM.