Takahashi FC-100DC: skewed Airy pattern

[pystab]

Thank you all for your many useful comments. Most useful is the strong consensus that what I saw is a collimation error and not normal performance!

I wouldn't try to collimate the scope myself, partly because I don't see any way of doing so (no adjustment screws), but mostly because it is indeed expensive glass and the job should be done by someone who won't wreck anything.

I'm glad to hear that a good retailer will take care of recollimation in the first two years (I got the scope - new - less than 6 months ago) and it's quite likely I will end up pursuing this. However, I owe it to a good retailer to be at least reasonably sure about the problem before inviting them to incur costs on my behalf. OTOH I wouldn't send the scope to an independent expert unless I had no recourse with the supplier, but it's useful to have Steve Collingwood's name if necessary.

I don't have either a Cheshire eyepiece or a laser collimator. I never thought I would need them with my setup!

I suppose what I saw is like the second photo that Ben the Ignorant posted from Wolfgang Rohr's site, in that the rings are more intense on one side than the other. However, my central Airy disc looks nicely round and not like a wee lightbulb, and the inner rings are at least faintly visible on the fainter side. I don't know how much of that difference is my eyeball vs his photo.

I did some tests with a (monochromatic green) artificial star 7 m away. I needed both diagonal and extension tubes to reach focus, and when the star was centred in the field of view I saw the same kind of skewed pattern with both eyepieces as I'd seen with real stars. The pattern didn't rotate when rotating the focuser (by partly unscrewing it), so I don't believe that is the problem.

The pattern became symmetrical when I panned the scope so that the star lay about half-way to the edge of the field to one side. I guess this is where the objective's axis lies. Moving the star a similar distance the other side of centre made the problem worse - then I couldn't see any rings on the fainter side. Curiously, I had to move in opposite directions for the two eyepieces, even though when centred the skew in the pattern was in the same direction for both. I have no explanation for this.

There was something non-concentric about the defocused "star test" patterns - I was paying more attention to the in-focus star - but I will check this again to compare with Ben's images. Likewise I'll try to take photos with my phone, though my previous attempts at this were just hopeless.

I'm not familiar with Ronchi patterns. How are they obtained, and what could they tell me about collimation? (I'm now assuming my objective is aberration-free but misaligned.) I don't have a Ronchi film, but I do have a phone and the means to light up alternating rows of pixels @ ~13 per mm. (I once used an appropriately-scaled image on my phone as a fake Jupiter at the bottom of the garden - it gave me an idea of what I might hope to see of the real one...)

[pystab]

Following up: photos as suggested by Ben went better than I'd expected:

The subject is a green artificial star 7 m away imaged by Takahashi FC-100DC, Baader prism diagonal, 3.2 mm BST Starguider and iPhone. The skew in the focused images (left hand side of picture) is a bit exaggerated compared to what it looked like by eye (there was a bit more visible on the dark side), but it's not far off. The defocused images are more mushy than they looked by eye, where the dark rings were darker and more pronounced, but the uneven illumination from bottom to top and slight non-concentricity are accurate.

[jock1958]

I’ve been following this thread with interest having had collimation issues with my Tal 100rs after it was damaged in transit.

I contacted Steve Collingwood and he was willing to check it over however the cost outweighed the value of my Tal hence one of the reasons I upgraded to the Tak

Your pics resemble what I experienced with my Tal 100 star test and one of the reasons I bought a Cheshire and laser (not that expensive)

The first thing I did when I bought the Tak was star test it against Polaris, the star test was very good compared to your images on the far left, the other pics are a bit more difficult to tell.

Attached some pics I took of my Tal 100 before I attempted adjustment and some after, I fiddled around with it for over a year and still never got it right!