Collimating a RC telescope with the GoldFocus mask?

[buzz]

I'm trying to collimate my 10" Truss RCT from Altair using the GoldFocus mask.  I'm not having much success and suspect I am doing something wrong. I can get 'good' collimation with classical outside focus star testing but when I use the GoldFocus mask, to try and improve upon it, for some reason the adjustments make things worse.

Originally I was using the mask in the same way as the DeepSky Instruments use star testing - adjusting the primary mirror to set a central star and the secondary to balance the appearance of peripheral stars. After getting some advice, I switched over, with little effect.

I would be interesting to see if any SGL members have used this clever device to set both mirrors, and how they went about it.

regards

Chris

[ChrisLX200]

Not seen that one before, but there's always someone with a bright idea - and if American, trying to make money out of it. A Howie Glatter holographic laser seems to do a pretty good job for me.

ChrisH

[buzz]

Yes Chris - I have one of those too - part of a big evaluation of all the major techniques for collimating a RCT. It is running to 25 pages already. I made a discovery early on with my Altair Astro Truss RC and that was to do all primary adjustments with the OTA vertical. If you don't the mirror sometimes shifts when you loosen an adjuster.

Currently a simple HG laser to line up the focuser/secondary and the hall of mirrors on the primary get me very close. I found an excellent reference by Deep Sky Instruments on a particular form of star testing that is very sensitive too. According to the website, the focus mask technique should be able to improve on that, especially in medium seeing conditions. The science is compelling but so far, ahead of my practice.

 Collimation Procedure

[r3i]

Interesting, this kit is also a new one on me. Does it give any clues as to whether it's the primary or secondary mirror that needs the adjustment?

[buzz]

Mike - It is sort of like three Bahtinov masks looking at the variation of focus by angle. When all the light focuses at one point, the three focus positions agree, i.e. collimated. For any coma, astigmatism or other abberation, the focus is smeared and the readings for the different orientations of the mask differ. It is virtually impossible to distinguish between small primary or secondary errors with one reading, just like star testing.  The focuser part is really useful, since it can directly tell you (after calibration) just how far to move the focuser with one reading. The collimation is intriguing but more elusive at the moment, hence the post.

[NeilBuc]

I have one of these for my GSO 8RC. I emailed the guys at gold astro direct and they sent me the instructions to collimated a RC with the mask. Was a bit complicated and I already had the Hotech Advanced CT collimator which had made a really good job anyway. For that reason I have left it alone and not tried since.

Not at my pc but if you don't get the instructions let me know.

Cheers

Neil

[tony210]

Although not cheap the Tak collimator telescope is my choice for pre star test collimation-best wishes Tony.

[buzz]

Well, after quite a few frustrating sessions trying to use the GoldFocus mask readout as an alternative assessor in an out-of-focus star test, I'm pleased to announce I was able to get the GoldFocus mask to work very well indeed.

Jeff worked on an updated instruction manual for dual mirror adjustments. It was not something you would have figured out by random intuition but looking at the procedure, it does make sense. In an hour I was able to collimate both mirrors on a 10"RC with high accuracy.  That was also helped with a night of good seeing and changing my locking grub-screws to pointy tipped ones. The original ones were cork-screwing the main mirror by a few microns (laterally) and causing the air to go blue.

The great thing is that it achieved this using numerical analysis and one iteration between secondary and primary movements. I could have gone for a second iteration but clouds stopped play. Out of focus stars were even symmetrical  donuts but I'm sure that a second iteration would have nailed the last few percent.

[NeilBuc]

Well, after quite a few frustrating sessions trying to use the GoldFocus mask readout as an alternative assessor in an out-of-focus star test, I'm pleased to announce I was able to get the GoldFocus mask to work very well indeed.

Jeff worked on an updated instruction manual for dual mirror adjustments. It was not something you would have figured out by random intuition but looking at the procedure, it does make sense. In an hour I was able to collimate both mirrors on a 10"RC with high accuracy.  That was also helped with a night of good seeing and changing my locking grub-screws to pointy tipped ones. The original ones were cork-screwing the main mirror by a few microns (laterally) and causing the air to go blue.

The great thing is that it achieved this using numerical analysis and one iteration between secondary and primary movements. I could have gone for a second iteration but clouds stopped play. Out of focus stars were even symmetrical  donuts but I'm sure that a second iteration would have nailed the last few percent.

Any chance you can share the instructions with me? I would be interested to see if they have changed at all. I will PM you the ones I got.

Cheers

Neil

[buzz]

These are the draft instructions. They worked for me and I cannot think of anything particular I would change. I used TSX's jog command to move the star about and their feature of being able to download and save subframes, which speeded up things a lot.

I stacked 15 images and with reasonable seeing, the readings were quite stable. 

I only managed one iteration before the clouds came over. I'm hoping for another clear night to complete a second iteration and then image a star cluster as the acid test.

GoldFocus Plus Off-Axis Cassegrain Collimation Analysis.pdf

[HenryNZ]

Are there more updates on this technique?

[buzz]

For info - there is a new version of the software on the GoldFocus website. It does not appear to be radically different but I notice it has fixed the ASCOM focus disconnect bug.

 

It also now has an updated instruction supplement that takes you the intricate process of collimating a dual mirror system. 

Edited February 21, 2017 by buzz
update

[REphotog]

On 10/06/2016 at 07:11, buzz said:

For info - there is a new version of the software on the GoldFocus website. It does not appear to be radically different but I notice it has fixed the ASCOM focus disconnect bug.

 

It also now has an updated instruction supplement that takes you the intricate process of collimating a dual mirror system. 

Wow, that’s some serious instructions,,, I think I follow, but just to clarify, when they say “slew’ they don’t mean move the scope , they mean move the reticule measuring window to another star in the photo right?

[inFINNity Deck]

The collimation of a RC8 has recently been discussed on this forum. Based in that discussion I made a step-by-step procedure for it, which I tested on a RC8 of a friend and gave very good results. The procedure is on a Dutch forum, but it will translate well when opened in Chrome.

Nicolàs

[buzz]

When I wrote my book The Astrophotography Manual, I wanted to explain how to collimate an RCT. I gave myself a 5 page budget.  There are many, many alternative possible ways of collimating and in the end, I tried most of them, if only to discount them. The chapter took 25 pages.

Those techniques that rely on anything other than mirrors are flawed - there is no guarantee that mirrors are centered in housings or center spots are centered. In essence, it takes three passes. The first checks the secondary spacing to ensure the focal length is right (big difference to field curvature), second does basic alignment with tools such a centered laser and hall-of mirrors and lastly, using star test to do the last little bit. There are some other variations that work too but you have to distinguish between folks' suggestions, that just get lucky with one technique compared to a robust method that works in all cases.  I spoke to the astronomer who wrote a positive review for a US collimating tool. Turns out he only used it to adjust the secondary. Rather misses the point as it is the primary that is the PITA.

Edited January 30, 2021 by buzz