Hi,
I'm returning to imaging after a 7 year hiatus with a focus on science observations.....with variable star photometry being at the top of the list. I'm putting together an imaging rig now, and have a capable mount on loan from a generous friend but I'm working on a decision and an order for a permanent mount setup. The Planewave L-350 direct drive mount is on the short list. I'm intrigued at the prospect of an altazimuth mount with field rotator on the focuser. I know it can be mounted on a wedge equatorially, and that's a consideration too....but the point of this query is specific to its use for photometry and other high scientific precision operations in altazimuth mode. Here's the query:
How does one effective flat frames with a field rotator? This is of course assuming that the telescope is a reflector with diffraction spikes that would not rotate along with the focuser and camera at the other end. There are other potential pitfalls as well, such as dust motes on a field corrector on a CDK or RC that wouldn't rotate along with the focuser and camera. The question is mostly academic for my own knowledge - how does one get accurate flat frames for precise photometry in an altazimuth mounted setup? Most modern professional telescopes of any aperture are using altazimuth mountings these days.....how are they doing it?
Thanks,
Mike
Hello! I had thought of this as I considered a new telescope set up. I could not think of a way to overcome field rotation with flats - everything continues to change with the derotator as you noted.
The only way to handle an altazimuth scope, I believe, would with with an equatorial platform. However, I am not aware of any systems that combine an equatorial platform with a go-to system on the scopes "altazimuth" axes. I think it is doable; however, the math would get funky as the "altazimuth" coordinate system would always be rotating.
The lack of a flat field would not prevent such a derotator system from being used, I would think, but the error/uncertainty goes up. Darks can still be applied. And depending on the f-ratio and the chip size, perhaps there could be only minimal vignetting contributing to the error/uncertainty of measurements. With the vignetting of my system, I know that the chip is not centered perfectly in the light cone. With a field derotator, the area of vignetting would rotate around the frame during an exposure. Flat frames would likely just increase the error/uncertainty of my measurements in that case.
Another approach would be forgo a derotator in order to keep the exposure time short enough so that star trails are minimized. Exposure times would vary with the coordinates of the target and the focal length of the system. Flats could be applied in that case.
I'll be curious to hear what others say. Perhaps there is a way to use flats with a derotator with which I am not familiar. Best regards.
Mike
As Ed and Mike note, there is field rotation in an altaz system. Normally, this is compensated for by a derotator at the eyepiece/camera position. Once that happens, you have several issues:
- the spyders rotate in the camera field of view, so flatfields are compromised at some level
- any defect in the primary-secondary mirror system will also rotate, such as earthquake clips or large dust
- if you do not have the optical path exactly coaligned with the mechanical rotation, and you have any vignetting in the system, then the vignetting pattern will rotate.
The spyder rotation and mirror dust are not major issues, as these are out of focus. If you look at the area subtended by the spyders, it typically is a few percent at most. Spread the light out by defocus, and the effect is much smaller. You can convince yourself by taking flats at 45-degree increments and then ratioing the flats. When I was using the WIYN 3.5m telescope, I took flats at 45 degree increments and could not see enough difference to warrant their use. I found it far more important to guide accurately and keep the target and comp star on the same pixel.
Dust is primarily an issue when it is on an optical surface much nearer the camera, such as an entrance window or a filter.
The misaligned vignetting pattern can be a bigger problem if your system has substantial vignetting, such as the early Celestron Schmidt-Cass systems. The C14 had about 20% vignetting over a 10arcmin radius, for example.
Arne