Collimation Tests, One Meter in NM

This image of Messier 56 was taken with the one-meter Tzec Maun Foundation telescope.

I am trying to collimate the Tzec Maun Foundation telescope remotely, and this is a result from last night. It’s a better collimation, but I have more to learn. The method I used (average inside and outside focus collimations) has best collimation somewhat off of center.

But this is a good result nonetheless, as it shows the optical quality of the telescope. This is a one-second exposure of the star cluster M56 taken on the one-meter telescope. The camera is very sensitive to light, the Sony a7S Mark II.

The stars are really sharp; we measure that with “full width at half maximum,” a value that is a statistical measure of the distribution of photons on the sensor. FWHM for short. It is 1.2” (arc seconds) in this image.

A weird side effect (should I have used that as a click-bait headline???) of such sharp images is that it makes it harder to get accurate color with this type of camera. Nearly all modern consumer and professional cameras use a Bayer array of four color filters over four pixels (two green, one red, one blue). The values of those pixels are combined to calculate a color. This means that the color data is not as sharp as the brightness data. In the case of such tiny stars, they don’t really cover enough Bayer arrays to generate reliable color. (The sampling is below minimums, and thus the color noise between pixels and arrays is too large to get accurate colors in such small, isolated areas.)

I worked hard in Capture One software to process the above image to get something like correct color.

Another thing we are discovering is that the focal plane is curved—this is expected on an RC (Ritchey-Chretien) telescope without a corrector lens group. The approximate center of that curvature is to the right of the physical center of the image. Stars there are very tiny and sharp; outside of that area, the stars are elongated because of the curvature. The good news is that the optics are of very high quality, and thus they show the curvature so clearly. (At some future date, we’ll design and add a corrector.)

(To avoid the problems of a Bayer array with point sources, professional telescopes use monochrome cameras that do not have any integrated Bayer-array color filters. Color filters are placed in front of the camera, one at a time, so a color exposure requires at least three images which can be combined in software—each color image taken this way is at full resolution.)

Here is another image of the cluster, this time 8 seconds long—the stars are fatter because their statistical diameter was increased by atmospheric turbulence (twinkling). I was not as successful in getting the color right with this one. The FWHM here is 1.5”. But we do see dimmer stars more clearly—that trade-off, more stars means fatter stars—is typical of a night of average seeing conditions (typical twinkling). At the location of the telescope, on some nights the air is so steady that the stars do not twinkle, and the FWHM gets smaller than one arc second.