astro-tech rc Astro-Tech RC 14"

[starhoo]

I purchased an Astro-Tech 14" RC a few months ago.  Suffice to say I am still trying to figure out how to collimate this scope.  There are three collimation options: the primary mirror, the secondary mirror, and the focuser.  When I first got this scope I looked thru it visually at the moon and it was very ice.  I defocused to see the collimation rings that they looked like evenly concentric circles.  Being used to an SCT (this replaced a Meade LX850 which I regret selling now).  With an SCT you've got the coarse focus knob that makes it pretty easy to fit any imaging train onto the OTA.  With the RC, since it is a fixed mirror (and that's one reason I wanted one), you have to attach the correct extensions to the focuser housing in order to then have your Optec TCF or Startlight Instruments temperature compensating focuser have the ability to reach focus...  This took several trials but I got it working. 

The next thing was to take some images and see how the whole rig performs.  So far - not very good.  The stars were ovals, revealing some collimation issue.  But where?  Primary mirror, secondary, focuser?  I decided that since the primary and secondary seemed ok before I attached 9 lbs. of imaging and focusing hardware that I should start with the focuser.  The focuser has three 3mm hex screws and two smaller hex screws beside the 3mm ones.  I have found ZERO documentation about guidelines for adjusting these screws despite sending requests to Astro-Tech twice.  Anyone know?  The only choice with no reference is trial and error.  So I was able thru much unintuitive adjusting to get the stars to appear circular, but this caused a good deal of coma in the image (see attached).  Also, although it is a brand new mirror and I was imaging on a clear moonless night, there are many odd circular reflections in the image that I'm not even sure flats could remove.  I've been trying to figure out what they are - are they surface defects on the mirror(s)?  Are they reflections off the baffles?  (Oh, why did I sell that Meade LX840 14"!!!)... 

There's much more to this saga, and I am not hopeful that anyone out there will even read this post, but if you do, please, I welcome your thoughts, insights, help!

[Ikonnikov]

For my 10 inch rc I use a takahashi collimating scope then fine tune using the DSI method (google DSI collimation). In essence this uses your camera to monitor star shapes on and off axis to guide adjustments of primary and secondary mirror collimation. It works very well. You could probably  just use the Dsi method from your current collimation state without needing the Tak scope.  

Regarding your focuser i suspect that the large bolt is a pull screw and the two smaller ones either side are push screws. In which case before dsi collimation procedure I would reset the focuser tilt by fully tightening the three pull screws making sure you've sufficiently loosened the push screws to allow this. You may still have some tilt in the system but at F8 this is unlikely to be a major contributor to the misshapen stars. Once the collimation is as good as you can get it by mirror adjustments, if there is still a focus gradient you can then try to alter focuser tilt and monitor its effect with a programme like pixinsight or ccd inspector.

The circular defects you describe are residual dust bunnies not fully removed by your flats. This could be due to a differing focus position between flats and lights or by a difference in calibration between the two e.g. no dark/bias subtraction from on or the other or different readout speed etc.

 

Hope this helps.

 

Paul

[ollypenrice]

It's a great shame that Astro Tech can't be bothered to help you. Frankly that's disgraceful. I can't help with the collimation but your experiences define exactly why I stay away from this kind of scope and stick to simple designs.

Is there an optical shop within reach of your home? It would be great if you could employ them not just to sort it out but to take you through the procedure.

Olly

[sloz1664]

I feel your pain, but before we go of on the imaging aspects of this RC scope let's get back to basics. What is the scope like visually? Are you getting the correct concentric circles in and out of focus, if so, we have a good collimated scope. If not, there are some very good RC collimation videos on youtube. Now if we have a good collimated scope let's have a look at imaging...

Although you haven't included any additional equipment; mount, guiding equipment, camera etc. These can have a bearing on the quality of your images. Any additional information will aid and help to point you in the right direction.

Steve 

[Fellside]

I`m almost sure this scope is fitted with a built-in focuser collimating ring. This is used to align the image train with the primary center marked by a round donut sticker. Because of this ring you should not need to touch and collimation screw on the focuser.

What additional equipment, software are you using?

Graham

[vlaiv]

Collimating RC is not that hard.

First thing is to identify what needs collimating. I can help you with some basics, but there are a few videos on youtube that will explain it a bit better.

First thing is to see if star elongation is due to wrong collimation or there is something else at play. Inspect your subs, rotate camera then inspect subs again. If in each sub star elongation is pointing in the same direction across the field and it rotates when you rotate your camera - then you might be having a mount / guiding problem. 14" RC has a lot of FL, so you might be imaging at very high resolution - any sort of guiding / PA error will show, and show a lot.

So if you happen to have above condition, examine which direction aligns with star elongation - if it is DEC, then you need better PA, if it is RA, you might be having problem with guiding (this would mean that your periodic error is not guided out completely).

If you have round stars in one corner, and elongation in other corners it can be collimation related, and to do collimation do following:

Step 1:

Put star in center of the field and defocus it. Make sure doughnut is concentric - this you will achieve by collimating secondary.

Step 2: focus the star in center of the field and then using some sort of aid check how much out of focus it is in corners (don't change focus, just slew scope and take frame and measure FWHM of star, or put Bahtinov mask on and look at defocus). At this point you can figure out if you need to collimate primary or focuser, depending how much defocus is there in corner stars. This is a bit tricky to get right, but software like CCD Inspector can help. If there is linear gradient in defocus (for example two top corners have same amount of defocus, and two bottom corners have same amount of defocus but different one to top corners) - you need to fix the tilt and that is done by collimating focuser. If on the other hand you have "bowl" like distribution that is not centered on frame center - then you need to collimate primary.

After collimating primary you need to go to step 1 and repeat. After collimating focuser / tilt you don't need to do it. So it is best to leave tilt collimation as last, and only if you do indeed have tilt.

Here is a good guide that will help you out:

https://deepspaceplace.com/gso8rccollimate.php

[starhoo]

2019.09.26 - Collimating 14" AstroTech RC Update

Some months ago after some success using the Howie Glatter laser collimator, I still wasn't seeing good collimation.  I decided to focus only on the secondary mirror, and making collimation adjustments only there.

What I found was that the collimation screws (which I had replaced with Bob's Knobs) were not long enough to reach the back surface of the secondary mirror housing.  Both the original screws and Bob's Knobs version were the same length (about 1.75").  The spacing between where the screws pass thru the collimation holes on the housing to where they contact the backside of the housing is about 1.5" - so there is little extra length in the screws for adjustments requiring more than I guess it was designed for.

I ended up taking some washers and placing them between on the backside of the secondary housing and where the screws make contact. These shortened the length needed for the screws to make contact.  It worked.  Since this change I have not had signficant collimation issues.

Edited September 26, 2019 by starhoo