RC collimation

[martin_h]

So, I have here an image I snapped a while ago of an out of focus star and as you can see something is out of whack.What now?

 

[tony210]

Hi - I assume you have done rough collimation in daylight and this is your first star test of the scope?- if so, I would gently tweak what you think is the opposite to the eccentricity of the defocused star shape,  primary adjustment screw set and see if the in and out of focus star images are more symmetrical or less - then you know which way to go - Tony

[nightster]

I used a standard Cheshire to aid in collimation. Actually, I attached a webcam to the site hole and was able to collimate it while watching a live view on a computer screen using Sharpcap and Mire De Collimation. Made it easy. This will get you close and a Star test is final fine tuning.  Image of my diy video collimation attempt.

[davies07]

Hi Martin,

i think your primary mirror needs a tweak. Slacken the push screw (the little black one) around 8o'clock on the primary mirror and give the pull screw (the larger silver one) a little tighten. You must ensure you know which way is up in relation to the image and the mirror or you will be tweaking the wrong screw. You'll then have to recentre the image and repeat the process - pull the secondary shadow towards the pull screw.

A good test is to look in the front of the scope and inspect the "hall of mirrors" effect. Look just above the secondary mirror into the primary and you should see multiple reflections, repeating into infinity. They should be perfectly straight. The reflection of your eye must be either vertically above the secondary centre screw to see if reflections go off to the right or left, 6o'clock or 12 o'clock positions, or, your eye reflection should be perfectly horizontal relative to the centre screw (9 and 3 o'clock) to see if reflections go up and downwards. A curved hall of mirrors indicates the primary needs adjusting. Slacken the primary push screws half a turn each, then tweak the pull screws to straighten the reflections.  Then gently retighten the push screws  

The free guiding software Metaguide has a very good facility for assisting collimation. It presents a red dot on a star image on the side of the star having coma. it is very interactive in use and works well.   The only issue with Metaguide is that you need to use a guide camera that works as a video camera..

Good luck. Collimating a RC is not a good past time for someone with OCD  I've spent months tweaking mine and I think I'm nearly there.. :-)

David

 

 

 

[alan potts]

I don't know if all RC are the same but I have had two and the main mirror is fixed, colimation is done solely from the secondary, judging by your picture I would say you are not far away. If what I said is true for your scope follow a list of how to colimate a SC scope. There one was one I wrote on the beginners section, I always increase the mag of the eyepiece until I am happy, once you have done it a few times it takes no more than 2 mins to do. Getting it spot on is important and it being so makes all the difference.

Alan

[vlaiv]

I found it real easy to collimate RC with use of imaging software and bahtinov mask.

You don't really need bahtinov mask if you have a way of judging focus - CMOS cam and SharpCap can do the trick if you use focusing aid (star FWHM or something) - other imaging software probably has something similar.

Here is the procedure:

1. Center star on screen, defocus (out focus - move sensor away from OTA) to get doughnut. If it is not concentric - you need to align secondary. Put hand over front of the OTA until you find where it shows on fat side of defocus star. Tighten screw that is closest to your hand (you should loosen just a bit other two to compensate). Repeat until concentric defocus star.

2. Now again put star in center of field and focus sharpest you can. Now slew ota to each corner and look at defocus "numbers" on focusing aid (or judge by eye where there is the most defocus). Choose corner that has the worst defocus and using push pull screws of primary move star in direction of center of frame. If defocus difference is small move it just one quarter way, if defocus is much larger than in other corners move it half way (this is just approximate, but you should not move it more in one go than this). After you are done go to step 1 because your secondary will be misaligned again.

Repeat until at step 2 you find that all for corners are roughly same defocus compared to perfect focus in center of the frame.

I found this at following site:

https://deepspaceplace.com/gso8rccollimate.php

There are couple of videos to help you out, and a bit more explanation for each step.

[Northernlight]

Hi, just my 2 cents. Rc collimation is often seen as being a nightmare, but with the right tools it's very simple.

1. Use a good quality laser to adjust tilt on focuser so laser dot appears centered in dot of seconday reflection.

2. Use a Tak collimation scope, to adjust secondary, and then fine tune primary

3. Repeat steps 1 & 2 and you're done........simples..