Maksutov Focus Failure - Help!

[samtheeagle]

I picked up a Mak from the forums here about 6 months ago, and having used it a few times now I'm getting concerned that there may be something wrong with it :S

I've tried capturing Jupiter, the Moon, and last night Saturn, and I've never been very pleased with the results. I have until now assumed that it was down to my poor focusing. But last night I spent some time pointing at a bright star, and it seems that there was never a sweet spot of tight focus. At best the star was still quite bloated, with a kind of triangular diffraction blurriness about it.

So my thoughts of possible issues...

(1) The age old collimation joy. I focused in and out, and got a nice circular pattern, and the black dot in it seemed pretty well centred. The only thing is that it was a pretty solid disk that I saw, whereas all the info suggests I should be seeing a set of concentric rings, so I'm not sure what's going on here.

(2) By the looks of the screws it appears that the front of the scope has been removed (the corrector?). So does the orientation of this matter? What if it was put back on in a different position? Would that affect the optics significantly?

(3) Cooling. The scope had plenty of time to cool, with the aid of an extractor fan, and there were no heat plumes like I've seen before, so I'm pretty happy that this isn't a cooling issue, but maybe I'm wrong?

Any thoughts or suggestions very gratefully received!

[Cornelius Varley]

What make/model telescope do you have?

Peter

[SiriusB]

A triangular diffraction pattern sounds like a classic case of pinched optics to me.

My money would be on the corrector, if it shows evidence of tinkering as you seem to imply.

If your feeling brave, i'd be tempted to VERY carefully loosen the corrector retaining ring a small fraction of a turn & see if this helps, as it may have been over tightened by the previous owner?

No expert on Maks, only had mine a few months, but that's what it sounds like to me.

Maybe wait & see what other replies you get first.

Ps, you won't get as well defined diffraction patterns as in your 80ed even with everything spot on.

[samtheeagle]

What make/model telescope do you have?

Peter

There's nothing to accurately identify it on the scope. It looks almost identical to this one: 152mm (6-inch) Maksutov Cassegrain (OTA only)

[nightfisher]

sam as you already know, when you star test you should get a nice ring both sides of defocus, my suggestion would be to see if you can get another mak owner to have a look if some one fairly local to you

[NGC 1502]

Hi. Sounds like you can rule out cooldown as the problem. From your post,

the clues are the triangular diffraction pattern, and youv'e said it looks like

the front corrector has been off. Any signs of further dismantling ?

Worth a look here http://stargazerslounge.com/equipment-discussion/81189-astronomica-152mm-maksutov-cassegrain.html

Good luck, Ed. (Edit, scroll to post #4)

[samtheeagle]

I found this page, which seems like a great reference: Diffraction pattern and aberrations See Figure 59 at the bottom. I think the one that is closest to what I saw is #7, which is pinched optics, as suggested by SiriusB.

So now the question is how does one go about fixing that? The corrector retaining ring... That's the small plate right on the very front of the corrector right?

[samtheeagle]

Ok, so the retaining ring is slackened right off now.

Another thought... Would over tightening the collimation bolts at the rear also cause such a condition? Maybe I've over cooked it? Speaking of these, is there a general rule as to which is the ajusting bolt and which is the locking one? They're paired up, the one on the very outside is set deeper than the one closer to the centre. Can anyone confirm which is which?

[GazOC]

Sounds to me like the primary is overtightened. I don't think theres a realistic way to cause pinched optics involving the corrector/meniscus on a Gregorian Mak/Cass

[beamish]

Sam can you post a piccie of the rear end of the scope with the paired screws ?

I'd be tempted to agree with GazOC, likely to be overtightened primary.

[LMC]

The front corrector (meniscus) should be fairly tolerant of tighteming. I've had the one on my 715D off and on a couple of times (bugs!) with no trouble.

It does indeed sound like an overtightend primary. I am not familiar with this brand, but don't worry - its not even close to fatal.

Merely annoying and a bit time consuming.

Here's a link that may help. The 'scope is different but the same principles should apply.

Telescope Reviews: M809 - Complete makeover: a photo journal

Good Luck!

Pete

[beamish]

another 715D owner !!

Just read the link- crikey, wish I'd seen that prior to having to do the exact same thing to mine

[samtheeagle]

Sam can you post a piccie of the rear end of the scope with the paired screws?

The screw pairs are actually under the rubber caps. I probably should have removed them for the picture! DOH! >< Another question, what are the silver screws for?

I've very kindly been contacted by the chap from whom I purchased the scope from with some advice on this matter... He also thinks it's the primary being over tightened too. The two bolts are in fact a push-pull arrangement, rather than one being for locking and one for adjustment, so I have to give and take between the two. I have of course tried not to over tighten, but apparently failed, so now they're only just at the point of meeting some resistance.

[beamish]

Is it a Rumak mak or Gregorian ? ie Sam does it have a silvered spot on the meniscus or is there a secondary holder with a mirror.

The reason I ask is that the latter type of mak is reasonably simple to collimate the former isn't.

firstly I am not familiar with this scope at all, so be wary. take the covers off the screws and determine which are push and which are pull, always a help.

Do you know if the back of the mak ever came off or primary removed even?

[samtheeagle]

There's a silvered spot on the inside of the corrector plate, inside a black plastic cone, so is that a Gregorian Karlo? Not 100% sure how many bit's it has been in but I think it has had some work done on the focus mechanism which wasn't connected up correctly originally.

I'm fairly happy to try out a re-collimation, being much more gentle on the bolts... All I need is for the skies to clear. I've never been happy trying to do it but looking at it from the front like the intertubes suggest. It doesn't feel very accurate to me!

[beamish]

If the silvered spot is actually on the meniscus ie you can see a dull silver finished circle on the front and no 2ndry collimation screws it's a gregorian , if there are collimating screws then a Rumak.

Clearly you're not having any luck using the scope so the decision is really upto you as to whether you wish to have a go at sorting it out.

It's very difficult knowing what to suggest-is it down to overtightened colly screws? or perhaps overtightened retaining collar in front of the mirror ?

Having taken the strain off the rear plate screws you might want to wait to take another looksie thru it before making a decision about removing the backplate and mirror etc

If you do decide to go down that route, you'll need to be very careful about mirror position.

Lets hope it has improved !!

[samtheeagle]

Right then, I got lucky with some clear skies last night so I had a chance to work on "Mak". Took me a while to get the hang of things, but I can report that things are greatly improved! Still not perfect, but soooooo much better than before. Here are a couple of frames taken to show the current collimation state of affairs:

So clearly it's not 100%, there's still a bit of a heat plume, and I still have a slight flare to the top of the star... To be honest I found this a real problem to try and resolve, as the horrific mirror shift associated the the telescope's own focusing mechanism seems to be throwing the collimation out on each use. Now I do have a Revelation crayford on there to mitigate this somewhat, but switching between different EPs / cameras requires a bigger change in focus than the 30mm afforded by that, so I'm forced to use the coarse focuser. It seems that I can't get it to be collimated on both sides of the focus point :S

Anyway, after a few hours of backbreaking collimation I swung around to Saturn to see what I could achieve. Nothing to write home about, as it was still pretty low in the sky, right in the orange soup from the town centre, but it's showing signs of life at least...

[beamish]

Actually collimation doesn't look too far off. Pleased you've had some luck with it anyways

Have a looksie on Rik's (riklaunim) website it offers a DIY cooling method for Mak's/SCT's

[samtheeagle]

Thanks Karlo I've got a small 12V cooling fan system that I knocked up from some left over odds and ends, which definitely reduces the cooling time. I didn't bother with that last night though, as it was a more diagnostic effort.

My main interest now is trying to reduce the play in the mirror when using the standard focuser. One thing I did learn last night is that a low power EP is the way to go when star testing. I started off using a 12mm, but the adjustments kept moving the star from the FOV. I've got a 40mm that was much easier to get on with, though I did have to do a fair amount of chasing via the handset...

[samtheeagle]

Ok, so call me mister fiddle, but I just couldn't resist taking the Mak apart Although I had improved the view considerably with some extensive star testing last night I still wasn't happy with the results. And not knowing how this bit of kit worked I couldn't figure out what I needed to do to improve things. My main concern was the image shift / loss of collimation when using the standard focuser, so out came the screwdriver!

My tried and trusted technique of removing screws until things start to get loose worked like a charm. After 5 minutes or so I had the mirror cell out. I had a couple of questions I wanted to answer:

(1) How does the focuser work?

(2) How do the collimation bolts work?

So the focuser. Seems to be a pretty simple arrangement of a large spring and a bolt. The two highlighted locking collars were rotating freely, which didn't strike me as a very good thing. This is perhaps the cause of the large amount of play in the focus mechanism? So I played with the range of travel, and adjusted the collars to suit the maximum extents.

Next the collimation bolts. These are arranged as 3 pairs spaced at 120 degree intervals. The outer bolt (closest to the edge of the back plate) extends into a smaller inner plate that holds the mirror/spring/focuser arrangement, so that the whole kaboodle can be tilted. The inner bolt I assume must presses against the back of the inner plate to act as a lock. I found that the outer bolts were pretty much at the maximum "in" position, probably why I was having trouble making the adjustments I wanted, I had no room left to adjust. So I slackened all 6 bolts off, and adjusted so that they were all set to approximately 50% of their length. Hopefully this will help when I recollimate, which is most assuredly required now!

Hopefully I haven't broken it

[beamish]

given the views and play-I think- no, actually I know I would've done the same.

Bottom line is you would never have been happy with it knowing you had'nt tried to get it right -

Good on yer Sam- hope it all goes back together and you can collimate it- Sure you can !! then........great views !

[samtheeagle]

Thanks Karlo, a kindred spirit in endless tinkering eh? Like you say, there would have always been a niggling doubt if I had not taken the plunge. Same as when I got my HEQ5 second-hand, it had to come apart so I could verify everything was in order.

So for now, I can at least focus on some TV aerials a few roads away, so it's not fundamentally knackered. And when I get a sniff of a star again I'll be back out for another calibration session...

[beamish]

Sam, don't know if this will help or just confuse you further ??

http://stargazerslounge.com/equipment-discussion/109736-mak-cass-collimation.html

[samtheeagle]

Well I'm always open to being confused! I shall read it carefully...

I got "Mak" out again last night, and rather surprisingly the optics were behaving pretty damn well considering he'd been taken apart and messed with. My by eye adjustment of the collimation bolts while he was in bits clearly wasn't too far off the mark

So the good news is that the mirror shift is now greatly diminished. It's still pretty bad mind you, but nothing like what it was. Using my 12mm EP I could use the coarse focuser and not have the target leave the FOV, which is nice.

The collimation is still something of a mystery to me however. I'm getting some lovely concentric circles on either side of the focus point now, MUCH more defined than before. And as far as far as I can tell they're pretty well centred. Using a 40mm EP I could get what appeared to be a really tight focus on a bright star, but swapping to the 12mm still left me with some fuzzyness.

My bahtinov mask showed that I was focused, but can that be trusted to report correctly if the collimation is out?

Anyway, great strides forward! I just dislike the imprecision of having to judge things by eye. A laser collimator on a newt was so easy to trust, unlike my own judgement

[LMC]

Just a thought here: Have you checked that the mirrored spot is actually centered in the light path?

Mine was a bare 0.3 mm out when I first received it and all the collimating in the world didn't help. Only when I looked straight through the back did I notice the mis-alignment. Loosened the meniscus, centred it, and hey presto, all was well.

Its a very easy thing to miss and makes a big difference.