DSLR photometry with camera star tracker

I have done a lot of photometry (hundreds of nights) using a Byers Cam-Trak, a very old version of the current camera trackers.  I used it with a 100 mm focal length camera lens attached to a CCD camera.  It worked very well at this short focal length.  The Byers Cam-Trak has very little periodic error, less than one pixel for my system.  Care is required to get a really accurate polar alignment.  Aiming the camera at the target star is best done with a green laser pointer.  A slow motion on the mount would really simplify adjusting the pointing, but the Cam-Trak is strictly a sidereal speed tracker.  Most of my exposures were 20 seconds, with a maximum of 60 seconds.

John Centala

1. What field within photometry were you using the camera tracker and lens?
2. What f-stop did you set your lens?
3. What CCD camera did you used?

I use star tracking devices exclusively for my DSLR photometry and my nova search program. I find them very reliable and easy to use with standard camera lenses up to short telephoto lengths. I have three currently in service. Each is mounted on its own tripod, and I find that the alignment accurate for months at a time, unless I'm careless and knock one out of position. I use both Vixen and IOptron trackers, with IOptron winning out with the alignment apps available for the IPhone. Realignment is simple and quick. The Vixen is a beautiful, robust and compact device, but requires a sturdy tripod mount that allows fine motion in altitude and azimuth. IOptron devices have such a mount built in to the device.

I only observe brighter subjects, with dome searches going down to approximately the 8th magnitude with an ultrawide lens. I keep photometry to the binocular star program. My longest exposure is 30 seconds. 

Mounting the camera to the device and pointing the camera is challenging. I get the best results using a laser pointer for framing and Manfrotto "gripper" ball heads. The heads are key to avoiding frustration. They allow the camera to be repositioned with only one action, rather than two.

I also use Sigma Quattro mirrorless cameras, which are much lighter than the average DSLR and lens combinations.

John Pickett

I have been using Sigma Quattro cameras for several years of nova searching. They make beautiful images, and are lightweight, robust and reliable. The optics are exceptional for astronomy, at all focal lengths in the dp camera series. I have no experience with the new model that uses interchangeable lenses. I have all four fixed lens cameras, with lenses from ultra wide to short telephoto. I use the ultra wide dp0 for patrol images, the dp1 and dp3 for photometry of the binocular program stars. 

I’m new to photometry, but had the kinks worked out within a month. Foveon images are a proprietary format, and you need to use Sigma Photo Pro software to view the x3f images, make adjustments and export the images as 16 bit tif files to allow processing and analysis using AIP4Win. The latest camera firmware updates allow the images to be recorded as raw files in DNG format, but I haven’t been able to get AIP4Win to open them. Foveon images have all the same color information as any Bayer image, and once in AIP4Win, can be separated by color for analysis. The key difference is in space. Each photosite has information for all three colors, because color data is computed vertically in terms of depth instead of horizontally using a bayer filter array. Red light penetrates the deepest, green light somewhat less, and the blue is read on the top layer. This is how color film used to work, and this is the key to the high quality Foveon imagers provide. My sampling size is much more in keeping with a monochromatic CCD. I avoid the broad defocusing required for adequate sampling with bayer imaging. My photometric images are sharp, and I can get workable images of brighter stars with a 30 second exposure at apertures from 2.8 to 4.

Once converted to TIF, Foveon images are like any other color digital image, provided the image is uncompressed and converted with settings suitable for photometry. I have addressed this problem to Willmann-Bell, and hopefully AIP4win will soon allow processing DNG files. I’m always open to feedback. I was able to shoot M67 this month and will be setting up for transformations soon.  I’m looking forward to seeing how these images stack up. I can post samples if you would like to see them.

John Pickett 

Hi All,

I've been following this conversation keenly as I too was wondering how foveon sensors would perform for photometry. The version of AIP4Win I have is 2.4.8 (beta) and it already does open conventional DSLR RAW images that have been converted to DNG format files. These DNG images still have the RGGB pattern of the bayer array.

It appears that the DNG images saved by Sigma Quattro cameras have a different format from the normal DNG files.

Is there a function in the Sigma Photo Pro software that allows separate R, G and B images to be saved?

Cheers,

Mark