Rifast collimation

The Rila is a new scope for me, mine’s a 400mm f3.75. I’ve previously collimated an SCT and of course my refractors haven’t require any changes since I’ve brought them.

I’ve got to say the manual from OS looked like it was designed for an RC but the guys in their support are really helpful and know what they are doing. Cris at Astronomy Alive was a God send, not only putting me in touch with the OS guys but also other amateurs who have had to work through similar configurations.

The Rila has a number of possible adjustments.

1. The adjustment of the spider that holds the secondary mirror
2. The adjustment of the collimation screws on the secondary
3. The adjustment of the collimation screws on the primary
4. The adjustment of the tip/tilt adaptor that attaches the imaging train to the back of the scope.

So all up its 13 degrees of freedom. Be prepared to spend some time.

First of all if your collimation is out the cause is most likely your tip/tilt adaptor. Play with this first. Check out your corners.

The equipment setup:

photo 4

 

 

I had a replacement collar (the red bit connecting the focuser to the scope) sent out from Italy so I could attach the collimator and the tip/tilt was very different.  I should have marked where everything was first. Next time.

Laser attached:

photo 2

Don’t bother to remove the primary baffle, it makes no difference. Don’t bother using the laser with circles because the rings cannot be seen because of the size of the secondary. See photo  below.

photo 1

 

But the rings are clearly visible on the secondary.

 

photo 3

OK, the steps:

1. Measure the distance from the secondary mirror holder to the external octagon. It should be possible to get the four equally spaced measurements from any section to the secondary within 0.5 mm. I used a vernier calliper all measured up to 140.72mm.

2. Loosen the locking screws on the primary mirror. Screw down the collimation screws on the primary and secondary mirrors so they are fully done up, but not super tight.

3. Put the laser in the back of the scope with the provided collimator bracket and adjust the secondary until the laser hits the center of the circles in the secondary.

4. Use the Tak collimation scope and put the dot in the center of the secondary mirror circles. The Tak doco is pretty good on this. However you cannot see the spiders and you cannot see the reflection of the edge of the primary mirror. You can see the shadow of the secondary but it should like up pretty well because of step 1. The tak causes an additional shadow which you can see around the outside of the secondary mirror shadow. Ignore it. Mine was not quite a circle and I discovered it move when I rotated the Tak anyway. When you have the Tak at the right focus you can see the central dot and what looks like diffraction rings around it. using these you can get a very accurate positioning of the dot inside the secondary circles, adjust the secondary’s collimation screws to ensure the dot is in the center. Just beware that this is still mechanical alignment.

5. Pull off the collimation adaptor and put your imaging rig back on, defocus so you can see the diffraction rings clearly, bigger stars say >20 pixels across is fine, we can make them smaller later, start taking say 2 second frames with your imaging rig and put the star smack in the center (ok plus or minus 20 pixels is fine) and make sure your tracking is working well. Point it at a not to bright a star say mag 5 and check out the star shape. This is typically what you do with SCTs but here you are playing with the primary mirror collimation screws only. You will need to back each of them off say 1/2 a turn to start with so you have some room to adjust. The basic idea is to move the star, using the screws. The star should move in the direction of the squished diffraction rings. The idea is to get a nice circle. Equally spaced diffraction rings on each side. You will need to play to determine which screw moves which angle and if tightening or loosening moves the star so that the diffraction rings spread out or move closer together. This is standard collimation stuff and there are lots of examples on the net about how to do this. You will need to do this with a star that is well out of focus, and then closer to focus. The closer you get the smaller the movements need to be. This star has to be kept in the center of the image all the time so be prepared to move your scope after each adjustment. This is in my opinion the longest and most frustrating step. Ignore the star shapes on the edges of the frame. Take longer shots up to about 30 seconds to see how you alignment goes as you get closer to focus.  This means you end up with something like this, one sub 1200 seconds, Ha, stretch only, nothing else:

M16(en)-003Ha E11p4 E1p0 E1p0

 

The central stars are pretty close but check out the corners!  The top left and top right are miles out, with the bottom left OK – but not brilliant – and the bottom right almost as good as the top right.

6. The final stage is to make sure the stars are the right shape across the entire image. You do this by making very small adjustments to the tip/tilt plate that attaches your imaging train to the scope. Mine is a red collar with small hex grub screws. undo the larger grub screws a very small amount and try tightening the smaller grub screws fractions of a turn. Check out the edges of you image while you make these adjustments so you can see if the star shapes in the corners are headed in the right direction.

Good luck its taken me days.