Finally clear skies with a half Moon yesterday! so perfect for fine-collimating the C14.
Last week, astro-friend Erik (Hemiksem) could lasercut two tri-Bahtinovs, one for F11 native and one for the F7.5 configuration using the Starizona reducer.
This was the intial image before collimation:
As can be seen on the image above, the collimation is pretty good, actually near-perfect. The principle of a Bahtinov is that the middle line is perfect in between the two outer lines. A Bahtinov does that with one axis (on set of lines resembling a star), and a tri-Bahtinov does that on three axis. By aligning the Bob’s knobs with the axis of the mask, you can easily adjust each knob untill the coïnciding middle line is perfectly centered.
Closely looking at the starting image, it’s clear that the 10 o’clock-4 o’clock axis line is a bit off. After some small tweaks with the corresponding knob it looks as follows:
A possible pitfall is that the collimation already needs to be near-perfect for this procedure to work. A SCT can be misaligned but still show an aligned tri-bahtinov. So take care to collimate as perfect as possible visually on a star first.
With the constant bad weather, it was probably a good idea to collimate the 180mm F/15 Maksutov properly, as it might be off slightly.
This involved a quite afternoon, a long inside range in the house free of obstacles, and a Geoptik artificial star (fixed brightness). The star diameter is 50 µm.
The collimation took place in bright daylight, however in the house, with the curtains closed. I positioned the star on a photo tripod, at a distance of about 25m, not sure however if that was far enough in relation to the 2700mm focal length. Normally 20x the focal length is adviced (Suiter) that would be closer to 60 meters.
The Maksutov sat on a HEQ5, allowing good positioning, and also re-positioning of the scope after tweaking the mirror.
The first image that came out looked as follows:
It may look like the collimation is already ver good. However, on closer inspection (using Sharpcap and a ASI2600MC camera) with a bullseye ring overlay, it was clear that the Poisson point (the little dot in the center) was not dead center in the middle of the diffraction rings.
Quality assesment of the diffraction pattern: smooth mirror surface, no pinched or deformed optics.
The de-centering is also visible in the in-focus diffraction pattern. It’s obvious that the rings on top of the star are closer together then the rings on the lower side. The upper side almost touches, where there is a clear gap on the lower side!
So the next step was to tackle this de-centering by using the push/pull bolts on the OTA backside. I used visually with a 4mm (675x) and 7mm (385x) eyepiece the in-focus pattern to slightly move the point towards the center. Ofcourse, everytime I touched the bolts, the star would move out of the field, so there it came in handy that I used the HEQ5 which allowed me to correct electronically using the mounts stepper motors.
As can be seen in the picture above, tightening the bolts too much results in pinched optics: a three-lobe star pattern becomes visible.
Loosening th escrews again, and centering the poisson point gave this image
More final stage pictures:
Measuring the image by the number of pixels on the camera (for a ASI183MM, each pixel is 2,4 µm), the diameter of the central point is 6 pixels or about 14 µm.The theoretical radius of the Airy disc can be calculated using: R = 1.22 x F x lambda / D which for this telescope comes to 10 µm or a 20 µm diameter. It would be strange that the actual result is below the theorethical value?
Referring to Suiter’s “Star Testing Astronomical telescopes, second edition”, a commercial maksutov of the Gregory Type (aluminized spot, no seperate second mirror or Rumak) would have a theorethical central spot radius of 8,2 µm. So in this 180mm f/15 the measurement the radius would be about 7 µm.
The second diffraction ring is clearly visible, but the picture underneath also showes evidence of the 3, 4 and 5th diffraction ring. There is no sign of other light scattering or a diffuse halo. Maybe the sensor could not detect those low light levels.
Central obstruction for the scope is 23%, the concave corrector is made of Schott BK7 borosilicate glass. It’s focal lenght is 2700mm
It’s been a while that the C11 was put to use, today I could clean the corrector, and also I found the very very small engraved numbers close to the edge, that line up with the line (and the same number next written next to it) found on the back of the primary. With the tri-bahtinov I could focus and collimate the optics. See also the links here and here.
make sure the three area’s of the mask line up with the three collimations screws of the secondary. The procedure is to focus like a normal bahtinov (ate least one set of lines), and then to look for the area (one of three) where the central line is not nicely central between the spikes. That becomes easily visible when covering an area: when that dims or obscures that is the guilty area, and the corresponding collimating screw needs to the turned (very very slightly). You see the effect on the central line. When centralized, repeat for the third area when necessary.
This is really easy, choose a star high in the sky. On the accuracy of this method I have little data or calculations unfortunately. What I do see is that (with a star in-focus) a very slight turn on the collimation screw (Bob Knob’s in my case), like 1/8 of a turn, gives an immediate effect on the Bahtinov image. The lines across the three sections will not be symmetric anymore. The area that corresponds to the collimation screw that was changed, will display a Bahtinov cross that is not centralised. From this observation I do believe that this method is accurate enough for collimating a SCT.