RC Collimation - Advice desperately needed

[ribuck]

Hi All,

I'm looking for some help with RC collimation and i posted on a forum in the uk and someone recommended posting my question on here as they believed there are some RC experts who know these scope inside an out.

So i'm glad i came as it looks like a great forum, which i'll look forward to using in the future.

Well onto my issues...

I bought a second hand 8" RC and i'm trying to collimate the scope with a Takahashi collimating scope, but now matter how i try i cannot get the the secondary donut reflection to align using the secondary screws. The only way i can seem to get it to align is by adjusting the primary, which is no good as it then throws the primary way out.

I start by using a laser and the tilt adapter plate to ensure that my focuser is properly aligned.......check.....

Now i move onto the secondary. So i'm looking into the Tak Collimation scope and i see the black dot and i can get it central adjusting the secondary, but within the black dot, there is a smaller faint white ring, which i'm assuming is the secondary donut reflection, and now matter how hard i try i cannot get the faint white circle concentric inside the black spot using the secondary adjustment screws.

If i adjust the primary screws, i am able to get the faint white circle in the centre of the small black dot, but this then throws out primary out of whack.

Could this be a mechanical issue with the scope itself or am i just misunderstanding the collimation process ?

i'll try and take some photos later today, which might help explain a bit better.

Many thanks,
Rich.

[Hans Tucker]

Hi Rick,

This topic comes up for discussion from time to time so there are some old threads you might want to check out.

A couple:

http://www.iceinspace.com.au/forum/showthread.php?t=120591

http://www.iceinspace.com.au/forum/showthread.php?p=1203517

[ribuck]

Thanks for the links Hans,

After going back and double checking things, it looks like i've been chasing a ghost that didn't exist.

The problem of the donut shadow in the black central spot being off centre was actually due to the slop in the Tak collimating scope - The focusing / sliding component of the Tak scope has a fair bit of slop in it.

Takahashi - shame on you.......you are supposed to uber precision brands......they should have made the slider bit as some sort of twist / screw mechanisim which would not result in these types of issues.

Rich.

[glend]

In my experience the TAK scope only has 'slop' at the very top of its slide range, as any sliding tube in tube arrangement would as well.. I would not think you would need to slide it that far out. There are many links and threads on this subject, and doing more research and reading might help you. I'd suggest you start with the basic tools, like the laser and get the focuser aligned first and go from there. It can become an iterative nightmare when you don't have a solid basepoint to start from, then you chase corrections back and forth. It's worth removing the internal baffle tube for alignment because it is much easier to see mirror tilt and the vane alignment when your not looking down the baffle tube. Baffle tube removal is dscussed in my RC collimation thread and in Wade's on the Cloudy Nights forum. You will need a final collimation adjustment with your camera in place, e a extra focs star test.
I am sure many of the experts on this subject are suffering from 'Collimation Fatigue' from all the help they have provided to so many, including to me. I can only add that the information is widely availabe now, it's the newbies application that often creates problems and gets them into further trouble - hence the reading suggestion.

[Paul Haese]

Hi Richard,

please see the second link Hans put up for my comments and instructions.

From your description it sounds like you don't have the camera centring ring. See here (http://www.bintel.com.au/Accessories/Collimation/Bintel-GSO-RC-Focuser-Collimation-Ring--RC8-/1771/productview.aspx)

If you don't have that you will need to purchase it to get all the components totally centred.

Feel free to ask any further questions you might have regarding collimation.

[ribuck]

Hi Paul,

Thanks for the replies. I have the focuser tilt ring. my 1st step is always to align the focuser with the optical axis of the scope with a laser using the focuser collimation ring, followed by alignment of the secondary and final tweaks of the Primary, and then subsequent iterations to fine tune.

Glen,

I consider myself suitably reprimanded and i do understand your frustration about newbies, that said I did read through many tutorials and watched many video's on the topic. The reason i posted and asked for advice was because none of the tutorials covered the issues that i was seeing and none of it made sense, rather than not understanding the process.

Unfortunately to achieve the correct focus with my tak collimating scope i need to extend it around 2" inches and it appears to have an awful lot of slop in the tube, which i have found is common after researching the issue. This slop resulted in the optical anomaly where the secondary donut reflection appeared offset and before i understood about the slop issue, it made it appear that the optics weren't properly collimated so i was trying to compensate for it by tweaking the collimation.

Now that i know that the issue is down to the tak collimating scope, i should be fine with the collimation process, and now just waiting for a bit of clear sky to test the collimation and do final tweaks on the primary.

Many thanks for all the help.

Rich.

[glend]

My apologies, was not trying to lecture or repremand. One can never know about the level of knowledge of people that just pop up with questions. Good luck sorting it out.

[ribuck]

Hi Glen,

no need to apologise at all, as i do appreciate any help that i can get and like you say it's hard to judge anyone level of skills or competence from a few posts.

Rich.

[Ausrock]

Being from N'cle (Aust) it's nice to see someone from N'cle UK posting here :thumbsup:.

[ribuck]

Well, i can say your Newcastle will be a lot warmer than mine, as it's around 4 degrees celsius today, but at least i don't need to worry about everything that creeps, crawls or swims killing me lol

hopefully if the weather ever clears, i might get to test my RC and do some imaging, but the forecast for the next 3-4 weeks is pretty much solid cloud.

[ribuck]

Well i got literally a 4-5 min hole in the cloud, just enough time to power up and grab a quick 60 sec unguided sub.

the collimation is far from perfect or a level i would be happy with, but it's a step in the right direction.

When i get a clear night with at least an hour of clear sky, i'll continue tweaking the collimation until i get it right, but looking on the bright side, the stars dont look like comets any more.

http://cdn.astrobin.com/images/thumbs/c4995f6a37ec51a920065ed6e16fca4b.18 24x0_q100_watermark.jpg

Rich.

[ribuck]

So all, i used al's collimation aid and as best i can see with my eyes, everything appears to be perfectly concentric and looks great. This seems to match what i'm seeing in my Takahashi collimating scope.


Checking the shadow of the primary, that also seems pretty central as far as i can see.

Looking at the image i attached, i can see elongated stars in all 4 corners, which all seem to be in opposite directions.

So in the top corners of the image, the stars are elongated in opposite directions (Top Left = NE to SW) and (Top Right = NW to SE )

So how do you adjust for elongated stars in opposite directions ??

Many thanks in advance,
Rich.

[JohnH]

As that was an unguided sub and, I assume, only roughly aligned it could just be you are seeing field rotation? AFAIK a correctly collimated RC scope will still show field curvature (depending on ccd/fov) and thus you should see stars shapes in the corners with a slight oval shape oriented radially ie:

\ /
+
/ \


John

[Paul Haese]

Agreed that it looks like field rotation at hand there. All the stars in the corner are rotating around the centre. The only other possibility is that your focus might be slightly inward from focus and that can cause that look too.

[ribuck]

thanks for the replies, one thought dawned on me today and that is that i haven't done a fresh polar alignment since installing the 8" RC.

it's pier mounted, but might be worth a try. The subs themselves were only 30 seconds in duration, so i wouldn't have thought that rotation would come into play here.

Someone on another forum suggested it could be as a result of tilt in the optical axis, so it might be worth me investing in a decent laser collimator such as the howie glatter, as my current hotech sca collimator might be giving me a poor reading when aligning the focuser on the optical axis

Rich.

[glend]

Any properly collimated pin point laser will suffice for aligning the focuser to the centre spot. You can test your laser collimation by rotating it on a V block to see it the laser stays on a fixed target point. The TAK Scope can take it from there.

[ribuck]

hi glen, i have the hotech sca, which has a self centering mechanism, which i'm not entirely convinced about. you twist a compression ring which in turn compresses and expands some rubber rings in the adapter.

i literally just checked, and when i checked the laser position, then loosened and rotated 90 degree's and performed the check again the laser dot was not in the same place. so this suggests that it's either not well collimated or it's the self centering mechanism is reliable / repeatable.

Rich.

[glend]

What does the TAK Scope show? Is the little back dot centred and is the secondary sticker centred around it?

[marc4darkskies]

It's almost certain to be a combination of collimation and misalignment / tilt of the mechanical axis with respect to the optical axis. I wouldn't rule out guiding issues either so when collimating it's best to take very short exposures (max 10 sec) on a dense star field to eliminate guiding as a source of misshapen stars. Moreover, it's very difficult to diagnose a specific single cause for collimation issues so my approach is always to start from scratch and eliminate things one by one. First, make sure the optical axis is coincident with the mechanical axis and this is coincident with the spot on the secondary - using a laser.

I use a Howie glatter quite effectively (1.25" / 2" version). http://www.bintel.com.au/Accessories/Collimation/Glatter-1-25-/2--Holographic-Collimator/430/productview.aspx




Yep, a compression ring eyepiece holder won't work reliably / repeatably. You need metal on metal contact.

I gave up trying to fix the laser to the scope and adopted a low tech approach. This only works for me because I have the 1.25" / 2" Glatter and a Tak collimation scope adaptor (screw fitted to the imaging train) that has an opening somewhat larger than a 1.25" eyepiece holder and a flat surface surrounding that (ie fortuitous):

hold the laser flat up against the adaptor for the tak collimation scope with the 1.25" tube of the laser sticking through the threaded opening of the adaptor and the 2" surface flat against the adaptor. I watch the dot on the secondary and physically move the laser in a circular motion. This assumes you have machined surfaces on the laser and adaptor that are flat and large enough to make good contact. This movement traces out a circle on the secondary
by eye, I note where the center of the laser circle is with respect to the spot on the secondary. (I did say this was low tech.)
I repeat this a number of times and rotate the laser between trials
I also determine what adjustment on the tip/tilt collimation ring on the scope is required and then make that adjustment
I repeat the steps above until the laser circle is centered on the secondary spot. If your laser is a bit skew then the above method will show a laser circle whose center orbits the dot as you rotate the laser between tracings

Regardless whether you can do exactly the same as I do above or not, being able to trace out a laser circle on the secondary is probably the most reliable way of using a laser to do the initial collimation.

After this then use a Tak collimation scope - an essential piece of kit IMO!



See here (https://www.dropbox.com/s/ktamnbbb7wgzrko/Collimation%20of%20a%20RC%20telesco pe%20using%20the%20Takahashi%20Coll imation%20Microscope.pdf?dl=0).

[ribuck]

Thanks Marcus,

appreciate the help.

I've ordered a howie glatter laser as i think it's important to have the right tools when owning a rc, at at least if i know the focuser is correctly aligned it gives me a fighting chance of success.

Rich