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Maksutov Focus Failure - Help!


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I have been following this thread, with interest, first of all i hope sam gets to the bottom of this and gets his mak working as it should.

I have been having issues with my 140 omc, i came to the conclusion the collimation could well be out so tonight i set it it up indoors and using the daytime collimation guide it appeared to be of, so what the heck out with an allen key, twiddle twiddle and it now looks right, concentric rings at the 2000mm focal length, just need to star test now, then try a lunar image

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Had a look , it's a Gregorian (?), be wary since they are notoriously difficult since the there's "no way " to collimate the 2ndry- tho Sam was aware of this already so prepared to take the risk. Primary only.

Sure you knew that already :)

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Hi guys,

Have watching this thread with great interest, I get as big a kick out of finding out how the hardware works as I do using it. Plus I have a Skymax 150 pro that I have just collimated so I'm comparing notes.

One observation I'd like to make, on collimation4 the largest degree of error appears to be somewhere in the 5 o'clock direction. Does the dent that you fixed earlier line up with 5 o'clock? or even 180 degrees around from that if there is an image reversal I have accounted for.

That dent suggests the scope, or the mirror/centre tube assembly at least, has been dropped/clouted at some time bending the centre tube out of true.

So it could be you need to effect a repair rather THEN a realigment.

That said, the Saturn image you posted is identical to those I'm getting so you must be damn close with the alignment.

Amazing work so far, well done.

Regards

Barry

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Had a look , it's a Gregorian (?), be wary since they are notoriously difficult since the there's "no way " to collimate the 2ndry- tho Sam was aware of this already so prepared to take the risk. Primary only.

Sure you knew that already :)

by secondary i assume you mean the silver spot on the front, that is fixed and can never be altered, so that just leaves main mirror with 3 collimation hex bolts, proof will be in the pudding so to speak

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http://stargazerslounge.com/equipment-help/136500-mak-help.html

This image is an example of why i thought collimation was of, see the top left hand side where it sort of fades........the very same imaging train does not do this with the tal 100 rs but very confident now that its sorted

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Ok, minor update on my continued efforts... I've got my laser collimator back, and I have mixed results. The reflection from the secondary does appear to be coming back pretty straight, though not perfectly... But what's really worrying me is that the reflection is a really big blurry lump, not a nice sharp point. Is that to be expected from the convex secondary?

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ok sam this is a right stab in the dark, just mulled this over, i guess you are firing the laser down the visual back where the ep sits, could it be like reverse focal point? to explain, light entering the front gets focused to a narrow beam at the ep, so this must be reversed when light enters the wrong way round

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I think you would get a blob back from the secondary for the reason you say. It's defocused by the convex secondary.

It would only be a dot if the original beam was infinitely narrow.

Does these collimators have a spec for beam width?

Barry

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This is the laser collimator I am using: Antares Laser Collimator. New & Improved Version | Telescope Accessories | Rother Valley Optics

Not sure of any details on the beam width, but I'm pretty happy that the blurred reflection is due to the convex secondary. It makes sense in my mind anyhoo :)

So, another update... I've found that perhaps some of my issues are caused by the Revelation Dual Speed Crayford focuser. It has a lock screw that allows the whole thing to be rotated around, and apparently it's easy to re-lock it in a not-quite-lined-up manner. I found this out quite by chance, but it's a good thing to know. I now press the focuser firmly towards the scope before re-locking, and that has given me a secondary reflection that's very near the centre spot. Which is nice :eek:

I really want to get it out for a star test, but after 3 solid days of slogging my guts out painting half the house I'm on my knees :S I'll pop out now and see how clear it is. If there's a hint of cloud I'm totally bailing with that as an excuse! :)

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Right then, so it was clear... Therefore I sucked it up and lugged all my gear outside for a testing session. Once I got going it wasn't so bad, it really cheered me up after days of miserable DIY slavery :eek:

So I think I'm getting somewhere! Visually Saturn looked great, and a star test gave me some much more pleasing results, see the pics below... Any ideas why the one side of focus gives me good concentric rings and the other is a nasty mush? Purely down to mirror shift? And if that is the case does that mean I should always ensure that I focus in the "right" direction to get the best optical alignment before imaging? Also, what's with the two diffraction spikes on the focused star?

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I was hoping for more from my Saturn capture, but it's turned out pretty pants once again :) I'm really starting to think it's just me. It looked by far the best on screen this time, I could actually make out banding and the rings passing in front, but it's not really come out very well. Hey ho, definite progress. I feel like I'm just a few small tweaks away from getting this fella firing on all cylinders now :)

post-16299-13387759581_thumb.png

post-16299-133877595763_thumb.png

post-16299-13387759578_thumb.png

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Hi

Just for info, the diffraction patterns you have posted are exactly the same as I see in my mak, an omc200. I was concerned when I first saw this, expecting that they should be identical either side of focus. Apparently though this is characteristic of Maks so I would just concentrate on getting everything concentric

Stu

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The first thing I thought when I saw your star test pics was its not yet thermally equalised. That would cause the mush. How long did you let it cool for? Looking past that, looks central and concentric, just right. Maybe a suggestion the primary is clamped too tight? slight pulling into a triangular shape?

As for the Saturn image, looks overexposed, try with the exposure knocked back a couple of notches.

Barry

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