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Refractor Collimation with Artificial Star


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Not sure if I'm going about this the right way, but although I have done a fair bit of collimation on the newt, collimating a Frac is a bit of an unknown art to me and I'm not altogether sure if it actually needs doing...

So 1st up given the poor seeing of late - I decided to setup a quick artificial star and do this indoors (conservatory/garage). The test star is a white LED source, conditioned with a 2m 9/125 single mode fibre patch lead. Distance between the scope and star is roughly 7m. I've used the QHY5L-II to image this at focus and either side, taking 100 frames. The results were then stacked in R6 and cropped, but no further processing. here

So here's the star at focus, infocus and outfocus:

gallery_26731_2575_107312.pnggallery_26731_2575_107314.pnggallery_26731_2575_107316.png

Not sure if this is done optimally, but to my inexperienced eye this all looks nice and concentric - Anything to be concerned about here, or should I leave this one as is?

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Looks concentric enough, maybe a slither off, but i've had a drink or two...

More worrying though is that artifact that moves from lower left to upper right when out of focus...

But then, i dont know much about frac collimation, much less with a star test, only things i've seen are using a cheshire.

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Looks concentric enough, maybe a slither off, but i've had a drink or two...

More worrying though is that artifact that moves from lower left to upper right when out of focus...

But then, i dont know much about frac collimation, much less with a star test, only things i've seen are using a cheshire.

Thanks Jimmyjamjoe - had a glass myself, but hoping that the artifact is dust or drying mark on the lens. The previous owner was a heavy smoker and despite wiping all the externals down I cans still smell the old ashtray every time I get close to this - quite possible there are some smoke residues on the optics.

It looks OK to me. I use a simple cheshire eyepiece to do my refractors.

Thanks John - can you recommend a good guide for cheshire method on a frac - I've got loads of good stuff on newt collimation, but resource on collimating refractors seem rare as hens teeth

Thanks for you responses on this - Jake.

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That's a pretty strong artifact. Does it's position correlate to the foil spacers between the front doublet?

Watching this thread with interest. :)

Russell

Cheers Russ - not sure if I could tell yet - nothing visible in normal use so far. The guide star was pretty much centred in the optical path, but I shall try and take some more images tomorrow to try and work out what is going on and double check for obvious dust/marks. My focuser also racks out at full extension and needs a hand/lift to get it to feed back in so I also want to try and find any end track adjustment on this (nothing in any of the manuals though so may have to find my way on this).

Bedtime now as seeing is very poor despite good transparency tonight - gave up imaging after two runs at Saturn :(

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The artificial star may be too close for correct startesting - there would likely be an appearance of some spherical aberrations as a result. I used a Hubble artificial star at 20m distance for collimation of my 70mm and 80mm refractors. For me, the rotating focusers were a cause of problems (appearance of astigmatism) until I locked them down, and I got concentric and round images as a result.

For your images, are there any marks or sleeks on the front window of the chip? The out of focus patterns should only have the diffraction effects (concentric patterns) but there are inhomogeneities visible going top left to bottom right in both of the out of focus images. Only careful examination of the in-focus airy disk will tell you about collimation, with the symmetry of the Airy disk and the appearance and distance of the diffraction rings telling you want aberrations if any are present. It looks as there is a spherical aberration present in the images you present above given the differences between the intra- and extra-focal images - probably due to the artificial star being too close.

Rotate the camera in the focuser if you can and the change in orientation will tell you immediately if the aberrations are dust/sleeks on the chip or are present on the objective. Given that the orientation is the same through focus I'd be inclined to believe that the diagonal marks are local to the camera.

Edited by cathalferris
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Thanks Cathalferris - very useful post and I shall experiment with the camera to see if I can re-orientate to find out in the diagonal stripe/smear is indeed local to this rather than the scopes optics. Distance wise I'm at about the maximum I can do inside the house, though I will try and see if I can use a mirror to reflect/extend the distance between source and scope. Using the single mode fibre my star diameter is just 9µm, which should be okay at 6m (though I do need some extension to achieve focus at this distance),

I had to google inhomogeneities - though it makes perfect sense (now)!

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I'll second what Cathalferris mentions re distance. I've checked collimation with a bodged up artificial star indoors before and if it's too close you'll see some nasty spherical abberation. When I've done it on a real star the spherical aberration is much better corrected. You can check if things are lined up ok though.

Just don't go over analysing about optics correction as you need your point source at infinity focus (or as close as you can get it) to make reliable tests of qulaity of the optics.

Adding other stuff like mirrors to increase probably isn't the best tactic unless they are 'perfect' as they could affect your results

**edit** thinking about it you probably want it far enough away that you at least don't need to use that extension tube to achieve focus

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By way of update:

I stripped and fixed my focuser yesterday - the issue with the focuser clicking out at the full extension was due to the focus shaft retaining plate having worked loose. When I pulled the draw tube out a small strip of plastic from the focus lock fell out and this may well have been loose within the focuser as I was getting some striation along the top of the draw tube. There was also some metal swarf from the rack/pinion evident at the bottom of the tube - Initially my stress levels rose a fair bit, but on close examination of the rack and pinion under my desk magnifier there appears to be no serious damage here - Phew! Everything was cleaned up and I carefully relubed the rack with a small amount of TF2 (lithium grease).

With the focuser out I was able to have a good look through and at the optics under bright light, revealing some quite nasty marks and dust to the surface of the objective - after removing the loose dust and fibres with a lens blower I very carefully cleaned the exterior surface with Baader Wonderfluid and a microfibre cloth. There are a couple of small/minor bits of dust on the inside surface of the primary, though (being an inate coward), I decided it was probably most sensible to ignore these for the time being.

All reassembled and the focuser now feeling much happier, a lot more positive/solid and stopping at full travel - it is worth noting that the draw tube in held in place with 6 longitudanal bearing strips (poss teflon), after you extend out beyond about 70% travel there is a little play - but the focuser is a long way out at this stage and not sure if I'll need this much extension in normal use.

Anyhow this took me well past my bedtime and my youngest woke up and demanded attention at this point - so I shall have to re do the artificial star tests tonight.

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On the subject of distances for the Artificial star - I've done a little light reading and messing around in Excel this morning (beats working for a living). I found quite a good site with explanation and equations for caculating resolving power (minimum mag and lots of other good stuff @ http://www.rocketmime.com/astronomy/Telescope/telescope_eqn.html

A nice simple equation for resolving power (Dawes Limit) in Arcseconds PR = 116/Obj Diameter (mm)

For my BM AR152S, PR = 116/152 = 0.76 arcseconds

and for my SW Exp 200P, PR = 116/203 = 0.57arcseconds

I've calculated the angular size of my source/artifical star using f10f03c9836c36537d2539196058bfa2.png = 206265 * (d / D) arcseconds. d= optical core diameter of the fibre optic cable in mm and D= distance in mm.

Using a single mode (9/125um) fibre I get an angular size of 0.309 arcseconds @ 6m, which is less than half of the resolving power so I reckon this distance should be ok. If I was using multimode fibre (OM2/3/4 at 50/125um), a distance of 20m would give me an angular size of 0.52 arcseconds, which should be sufficient for testing either scope.

Star angular diameter:

Singlemode fibre @ 6m = 206265 x (0.009/6000) = 0.309 arcseconds.

Multimode fibre @ 6m = 206265 x (0.05/6000) = 1.72 arcseconds.

Multimode fibre @ 20m = 206265 x (0.05/20000) = 0.516 arcseconds.

These distances ofcourse assume that I can achieve focus - 6m is just achievable with the AR152S, with a 1.25" diagonal (though I would obviously like to test without this as it's cheap and nasty). I tried with the 200P, but think I need a lot more distance or extension (an empty barlow lens may assist).

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