Newtonian Collimation question

[Astroscot2]

Im finally getting round to mounting my Newtonian in the Observatory and have been tinkering

with the collimation the last few nights - Im using the Cats Eye collimation kit which are fantastic and easy to use.

I have one part of the collimation process that is annoying me and I wonder what is the root cause and does it have any bearing on the end result.

For the life of me I can not get the Secondary to appear circular when viewing through the Telecat. I have tried the following

Squared my focuser to the tube.

Centered the Secondary In the tube via spider as accurate as possible.

No matter what position I try the Secondary I can not get it to appear circular when viewing through the Telecat.. What could be causing this skew?

Moving on from positioning the Secondary as best as I can the rest of the collimation process is fine, The Primary appears circular using the Telecat and the reflections of the centre spot line up nicely.

Are there any consequences on the secondary appearing non circular other than field illumination

Thanks

Mark

[Singlin]

Hi Mark,

You have to loosen the secondary center allen bolt and rotate the secondary with your hand as you are looking through the telecat -

Your aim is to see the secondary completely round and not oval and also centered fore and aft and up and down.

Make sure that the telecat is at the correct focal legnth when centering the primary mirror.

What I mean by this is that you have to slide the telecat in or out of your focuser draw tube until the center spot reflection fits precisely on the inside of the circle of your telecat-

This also applies to the Infinity XLK-

For the infinity I had to put an extension in order to get the infity at the correct focal length-

Here is a link to Astro Baby´s collimation procedure to better understand the process.

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

[Astroscot2]

Hello Singlin, I ve tried all of the above you mention, Its not miles out but in unable to view the Secondary as circular no mater what position I try to move it. Its an odd skew that I can not fathom what the cause is, I wonder how necessary and what the end effect will be not having a truly circular Secondary but the alignment looking OK

Mark

[Jason D]

It is hard to tell what is going on. Can you do your best to achieve the best collimation then take a photo though the empty focuser? Make sure the camera's lens is centered and square to the empty focuser tube. The photo should provide enough hints to root cause the problem.

Jason

[Moonshane]

Could the secondary not be quite elliptical?

[Astroscot2]

Hello Jason, I will attempt an image at the weekend and post.

Moonshane, yes that crossed my mind too.

I will get back with a photo

Thanks

Mark

[opticalpath]

It's possible that a big error in tilt of the secondary can be in a way 'compensated' by a rotational error.  I wonder if that could give the appearance of good collimation in all other respects, whilst presenting a non-circular outline?

Is it possible to check mechanically that the top of the secondary mounting is parallel to the top of the OTA and that the 3 adjustment screws have been extended by approximately the same amount?  That would give a clue to any gross tilt error.

Adrian

[Astroscot2]

Hello, ive attempted an image of the effect im seeing. not great but shows what I mean,  This is as close as I can get to the secondary appearing circular.  if the net effect is uneven illumination across the field I may just leave it alone, if you know different and pointers welcome. 

Thanks

[Astro_Baby]

Jason would be the real expert here - I have no experience with the telecat but its possible the secondary is not quite true. If you cannot make it round no matter how much tilt you apply that would be my best bet.  Jason may have other ideas.

I have my own headaches with collimation at the moment - honest - but rather than risk a thread jack I'll start a new thread.

[Jason D]

Here is my suggestion to the OP:

- I assume your focuser is located at the right-hand-side of the OTA when standing in front of the scope looking down the OTA opening

- Loosen the 3 set screws by a tiny amount -- just enough to be able to rotate the secondary mirror freely

- Rotate the secondary mirror slightly (just a tiny amount) clock-wise

- Recollimate 

BEWARE: If your 3 set screws gauged the top of your secondary mirror stalk then the above will not work. In this case, I recommend adding 1 or 2 discs cut from a soft plastic Milk bottle and place them between the set screws and the top of the stalk.

Having said all the above, the position of the secondary mirror is not too bad. The difference in illumination between what you have now and the optimal secondary mirror position can't be detected by the human eye. You can ignore all the above and enjoy your scope. 

[Astroscot2]

Thanks for the reply Jason.

Yes , focuser is on the right as you say. I have already added a stainles steel ring to the back of the secondary holder which helps conciderably. I will give the scope a whirl and see how it performs as is and try the camera also.

Will report back.

Thanks again

Mark