applied RC collimation

Doomsayer · #1 08-04-2009, 08:35 PM

I would concur with comments around this and similar threads on RCs about the critical importance of optical alignment and collimation. I've been getting some personal experience with such RC issues having built two mostly DIY 12.5" ritchey chretien CF truss 'scopes. Not an expert yet...

I've found that in practise there are some significant differences when collimating and aligning RC optics as opposed to newtonians, cassegrains or SCTs. In particular, having had long experience with newtonian/sct/cassegrain collimation, you tend to have visual assumptions. With a newtonian for example, coma can be an aid to diagnosing misalignment, giving a visual clue that you need to re-align components in the ota. With an RC, coma and spherical aberration are pretty much absent. So apart from the concentricity of the out of focus star image, the visual clues are more subtle. As said in an earlier post, decentering or improper spacing gives a general blurring effect. The other thing is that these blurring effects due to minor centering and spacing are pretty much even across the field (ignoring inherent astigmatism/edge curvature)'

There are some interesting academic papers on some early professional US Ritcheys which describe these empirical challenges (more recent pro RCs have had the benefit of CAD opto mechanical design and manufacture). Add seeing effects and optimum collimation can be elusive for amateur level RCs.

So apart from accuracy & smoothness of the RC mirror figure, optical placement and mechanical design are significant challenges. Lately though I'm finding the coincidence between my free time and clear sky to be the biggest challenge to optical alignment perfection.

As another comparison, I have recently rebuilt a Vixen VC200L using a carbon fibre truss configuration and existing Vixen parts - this seemed easier to get properly collimated than the RCs I have built. Commercial RCs such as RCOS and OGS would likely not have so many challenges to colllimation/alignment for the owner as I have had with an ATM project.

cheers
guy

KenGee (Kenith Gee) · #2 09-04-2009, 01:02 AM

It's nothing that a Tak scope and CCD inspector can't sort out.

Doomsayer · #3 09-04-2009, 02:18 PM

Yes - in most cases, as per the RCOS website. I guess I was talking more generally (or vaguely) about how the Ritchey optics behave during alignment and within the frame of a telescope - not so much the solution.

Do you use the tak scope Ken?. Not suitable for reflectors with a relay lens according to Takahashi. I have a Tak scope on backorder however (and use CCDAP/stack/inspector) The tak scope requires a precise centre mark for the secondary mirror - not sure if the new GSO RCs have such a mark?

Gama · #4 09-04-2009, 09:24 PM

The main RC providers like OGS, RCOS etc, make their scope to very high standards with machining etc. Not because of competition or asthetics, but actual rigidity, and strength. Like Guy said, its a must, not an option.
These scopes have so many adjustments and sequences needed to do a complete alignment, that you need to make sure it holds night after night.
It can be a frustrating event, or a simple one, depending on experience of the user.
We are talking tolerances of a mm or less in some areas, and pictures are garbage. Delicate instruments require precision to maintain precision.

Theo.

KenGee (Kenith Gee) · #5 10-04-2009, 12:58 AM

Doomsayer, as noted the RC don't cope well with missalignment the Tak scope will get me to within 20 secs of alignment and CCD inspector gets me the rest of the way. As Gama says these instruments tend to be made very well, even the "low" cost ones from Deepsky instruments. So they will hold their alignment very well. I think the Tak scope is the one of the best atro items I've ever got. The cost is repaid the first time you use one very easy. To give you an idea a 1/8 of a turn on any screw is very easy to see. Field curve can change a large amount even with a flattener if not alligned correctly.

Doomsayer · #6 10-04-2009, 08:32 AM

thanks Ken & theo.
the tak scope gets a lot of good press. Sounds like your DeepSky RC is delivering on quality. I've been able to hold collimation & alignment well so far - plus a good 2" laser has been very useful. I also found the Catseye 2" autocollimator to be very helpful with secondary mirror adjustment. The 2 RCs I built are relatively massive and very rigid. Same goes for my Vixen VC200L CF truss rebuild - stiff with zero flexure - the original vixen steel tube had quite a bit of flex. Camera flexure was another issue I had to fix in the RC. The many connections between CCD camera, filter wheel and rotator gave some minor challenges.

Recently I've had a few different camera rigs on and off the back. I accidentally mixed up the AP 2.7" drawtubes slightly and discovered that my images had degraded noticably - I had put the mirror spacing out when I refocused with the wrong camera position (the Van Slyke secondary focusers have about 3" of travel - so they can compensate quite a bit). I have fixed the issue now. I am still using an St8me as my main camera - a large CCD chip camera is a way off yet for me. However, I am converting an EOS 5D and now have a serial cable for it plus a CCDsoft dslr driver and plan to use it to test out/tweak the image performance of the RC riding on the Paramount.

guy