Collimating the RASA 11" and "Error" stopped play ...

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mark-vdi
mark-vdi Contributing Member • Posts: 605
Collimating the RASA 11" and "Error" stopped play ...

After taking a large number of hours to finally collimate the RASA 11” - I thought that, well, perhaps describing how I eventually got it sorted, might help somebody else …

I started using a Tri-Bahtinov mask and an artificial star (= taped up LED-torch with a pin hole). After quite a few hours, I eventually switched to a normal Bahtinov mask and simply ensured the "Leg of the Y” that is the basic Bahtinov shape always pointed to the screw that I was manipulating. This produces the “focus bar” of the diffraction pattern perpendicular to this axis and the bar moves towards or away from the screw as it is adjusted. Still I was going round and round in circles (well actually rectangles), placing the star into each corner in turn, I could not bring two axises into focus without completely loosing the third … After finally homing in on what should have been the correct movement to correct this, and many hours of frustration, I finally had to admit defeat. The correct collimation definitely lay outside the adjustment range of the collimating screws of the scope I didn’t expect that …!

I changed and redesigned the camera train to include the Baader tilt adapter, with the three tilt axes matching the original three collimation axes of the scope itself but still ended up going round in “rectangles" and not getting very far ...

Ok - so nothing new in the above that would be of any use to anybody, so what did I now do differently to arrive at a result? I starting playing around with a three legged tripod using a bubble level - I could always adjust one leg to bring the bubble into a parallel axis to that connecting the other two legs, which also ran through the center, and, by adjusting these two legs simultaneously, to center the bubble … i.e. I turned a three axis problem into a two axis problem.

Here the solution as a set of instructions as applied to the RASA:

Part 1)

1) Line up the camera as shown in figure 1

2) Focus in the middle (B) or at point A or C

3) Using only screw 1 (and the focus adjustment of the telescope) bring the stars in the line from A-C into focus. IGNORE the the corners, which may well look terrible …

4) If there is not enough adjustment possible with screw 1, then move both screw 2 and 3 by the same amount, in the same direction, and in the opposite direction to screw 1.

5) Again, during all the steps 1-4 IGNORE the stars (in my case assorted “seagulls", "rabbit ears", "polo mints” etc.) in the corners …

The result is a line of stars through the center of the sensor which are in focus - Turning the camera through 90° should keep the same line of stars (now horizontally across the long axis of the sensor) running through the middle of the sensor (figure 2).

Fig. 1

Fig. 2

Part 2)

6) With the camera in orientation of figure 1 go to one of the corners (I chose D)

7) Adjust BOTH screws 2 and 3 in equal but opposite directions - NB Do not judge the result until both screws have been adjusted and always move them by the same opposite amount.

8) As a check, the in-focus stars at point C (for that matter points A and B as well) should remain in focus - if the best stars have drifted right or left of point C, it means the adjustments to screws 2 and 3 were not identical and a slight correction is necessary.

9) When the adjustment is finished for this one corner (and points A, B and C are in focus) then all the other corners will be (should be) correct as well!

10) If the stars in corner F and G (which should be the of the same quality as each other) are not quite perfect, then potentially a very very minor adjustment to screw 1 may be required (basically means the stars were not identically in focus between points A and C). If they are not absolutely identical, a very slight correction to 2 or 3 should suffice ...

… And that is it … In the end the solution was so “easy" I couldn’t believe it and no chasing round and round the corners … and I can now begin mending the brick wall i’ve been using ...

I hope this will be useful for somebody. I think it should work / could be useful for any scope that has a there point collimation adjustment.

And the fact that I needed the Baader tilt adapter - that’s a bit disappointing … I know the tilt adaptor is a commercial viability because I am not the first person to have needed it, but still, it would have been nice to have not had to …

And so we get near the New Moon in July, I finally have a collimated scope, and … and … the GM1000 stops tracking or more precisely, won’t go from a westerly orientation eg. Arcturus, to a southeasterly orientation eg. Altair without "throwing a wobbly” aaaaaaaaaaaaaah - eventually the tracker went into “test mode” and spat out the error message “00.20.00.00.16” - I think it is never a good sign if you Google an error message and find hits … Baader (in Germany) are being very apologetic and doing their best. 10micron, in its entirety, is on holiday. So no data in July, certainly none in August, and September, well let's see … and, if by chance, anybody has a GM1000 lying around gathering dust and they don’t now what to do with it - LOL - feel free to drop me a line ...

Clear Skies (and a working Tracker …),

Mark

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