Mak-90: Is this fixable or knackered?

[Pixies]

The black primary mirror end is loose/wobbly and the sealing ring is split.

With the ring in place, it keeps the black end piece (not sure what you call it) relatively secure. I'm not sure what would normally hold it in place.  There are 6 bolt heads - 3 big and 3 small - these are the collimation adjusters I understand. I have never had a cat up close before.

The sealing ring is hard - is it plastic, or was it softer rubber that has perished?

Anyway. Is this knackered or can it be fixed? I'm sure one of you have had these things in bits before.

 

Cheers

[Captain Scarlet]

The blue tube, as I understand it, screws onto the rear plate. The black rubber/plastic ring both seals it and acts as a sort of "spring-washer" to lock the thread. If the rubber ring has broken, the thread is freer to wobble around. See my detailed post here about disassembling my own Skymax 180, whose design is similar but has the middle tube bolted to the rear cell rather than threaded on. Your scope I believe will have a similar overall design, apart from the threading of the tube. The main collimation bolts, for example, hold the whole primary mirror in place and are its only connection to that rear cell. DO NOT undo those collimation bolts!

I think you can safely unscrew the whole tube from the rear cell and lifit it off to expose the innards and primary, but be careful when balancing it on just the visual back! You'll need to find a replacement large O ring or similar way of preventing the thread wobble, maybe some thread-lock?

My verdict: totally fixable. I hope that's been helpful!

Cheers

Magnus

Edited October 21, 2020 by Captain Magenta

[Captain Scarlet]

... further thoughts. Either a suitably large rubber band which means you don't need to unscrew the tube completely. Or some PTFE tape around the threads, which means you probably will. Or both.

M

[JamesF]

The seal wasn't a ring when I took my 127 Mak apart either.  I just put the same seal back when I reassembled it.  Seems to be fine :)  I can't recall how I kept it in place during reassembly though :(

In my case I think it was the black mounting ring at the bottom of the picture that screwed inside the main tube and the backplate was fixed to that with the "pull" collimation bolts.  If yours is wobbly I'd guess that either the collimation bolts have worked loose or the mounting ring needs tightening.  Assuming it is constructed the same way as mine of course, which is not necessarily a given.

James

[Pixies]

Well. I bit the bullet and unscrewed:

The threaded part (which obviously fixes to the OTA) is what the collimation bolts are attached to. So the outer plate must move slightly during collimation adjustments. It does appear to be attached to the baffle tube. So the opposite of the 180

[JamesF]

You can blow some of that dust off the mirror now, too

James

[Pixies]

@JamesFJust missed your post.

Yep - I see how it works now. I'll add some PTFE to the threads and a 'little' grease to the seal to see if that holds it in place.

I assume then, if I tighten it up when I screw the end cap back, I will then no longer be able to 'tighten'  the collimation screws, as there is no slack left to adjust.

[Pixies]

Just now, JamesF said:

You can blow some of that dust off the mirror now, too

James

Did that ages ago.....

[JamesF]

Just now, Pixies said:

I assume then, if I tighten it up when I screw the end cap back, I will then no longer be able to 'tighten'  the collimation screws, as there is no slack left to adjust.

I assume that's partly what the seal is there for.  If you screw up the back until the seal is nipped in place but not completely squashed, it will deform a little further during collimation, but hopefully stay in place.  I guess if you start with the "pull" collimation screws as far out as feels safe and then refit the back you'll probably only ever be tightening the back against the seal rather than loosening it, if you see what I mean.

James

[RT65CB-SWL]

@Pixies I just found this GB/UK supplier of 'O' rings... https://www.barnwell.co.uk/content/uploads/2015/11/Metric-Range.pdf

[Pixies]

Thanks both of you,

It's all gone back together fine. Now time to collimate it.

That star diagonal. It is pants, isn't it?

And I wonder why the gold parts on this Mak? Perhaps it's quite an early one.

[John]

The black anodizing goes gold when exposed to strong sunlight for long periods. It may have been stood in a window or a conservatory for some time during it's life.

I've seen this quite often with Skywatcher scopes and others. I have a "gold" Meade 26mm plossl

 

Edited October 21, 2020 by John

[JamesF]

My Mak has similarly gone "gold" in places.  In fact I think the corrector cell is partly gold and partly black where the end cap was in place but it doesn't completely cover the anodised area.  That particular OTA used to spend a lot of time outdoors on sunny days for white light solar viewing and imaging and I've always assumed that was what caused it.

James

[johninderby]

I had a scope that had some black parts turn a bronze colour while  it was stored on top of a wardrobe. 

Black anodised parts will turn a gold or bronze colour if not properly sealed which explains why only some things discolour. UV can make the parts turn a lighter shade.of gold.

Edited October 21, 2020 by johninderby

[Pixies]

It's proving hard to collimate with the o-ring being as hard as bakelite! I'll have to get it done roughly until I can get a new o-ring.

Just checking - it's a case of using a cheshire to get the cross-hairs over the eye hole? Then a star-test.

I'm doing it without the star-diagonal in place. Is that OK?

[John]

I collimated a mak-cass 90 recently. I used an artificial star. It was a bit of a fiddly process with very small adjustments needed but once you get it, it stays put. I then checked it at high power (180x) on Polaris at the 1st opportunity.

I'm not sure that the cheshire can really help with a mak-cassegrain ?

 

[Pixies]

16 minutes ago, John said:

I'm not sure that the cheshire can really help with a mak-cassegrain ?

Oh - it was so far out, I needed something to get it close. I've got an artificial star, so just about to fish it out of the man drawer.

[JamesF]

I seem to recall that when I did mine I set the OTA up on a tabletop and then went around the front end, trying to align my eye with the optical axis, about a metre or so in front of the OTA.  If you're not even close, I think you see reflections of the mirrors drifting off towards the edges of the OTA and you need to adjust the collimation to bring them into line with the optical axis.

I feel certain I also used a collimation cap for some purpose or other, but it was such a long time ago I'm afraid the reason escapes me for the moment.  Things are so bad these days that if I can remember what I did last week I count it as a bonus, to be honest :)

James

[johninderby]

Large o rings are available on Amazon.

https://www.amazon.co.uk/s?k=90mm+o+ring&ref=nb_sb_noss_1

[Pixies]

Managed to get it as close as I can do with the hubble artificial star. I bought that ages ago (from here) but have never been able to use it as my other scope's minimum focus distance is just too great.

But at about 10m it's fine with the wee Mak. It's so odd seeing the diffraction rings with no disturbance.

I'll do a proper star test when there's a clear night (hopefully in 2020 sometime).

 

So thanks everyone for all your help. I now have a working Skymax-90 for very little cash. I love this forum!

[Alien 13]

Nice result, its good to see the innards of these Maks as I have often thought about doing some mods to mine, have been thinking about making the focus tube inner/outer blacker than a black thing with some of that "Vanta black" type paint which supposedly gives a boost in contrast.

Alan

[Marvin Jenkins]

8 hours ago, JamesF said:

The seal wasn't a ring when I took my 127 Mak apart either.  I just put the same seal back when I reassembled it.  Seems to be fine   I can't recall how I kept it in place during reassembly though

In my case I think it was the black mounting ring at the bottom of the picture that screwed inside the main tube and the backplate was fixed to that with the "pull" collimation bolts.  If yours is wobbly I'd guess that either the collimation bolts have worked loose or the mounting ring needs tightening.  Assuming it is constructed the same way as mine of course, which is not necessarily a given.

James

Is it just me or are there just too many circles in this photo? There is supposed to be a telescope in there somewhere but I cannot see it. 😳

[JamesF]

2 hours ago, Marvin Jenkins said:

Is it just me or are there just too many circles in this photo? There is supposed to be a telescope in there somewhere but I cannot see it. 😳

I did wonder about that when I looked back at this photo   Fortunately we have a different tablecloth now.

James

[RT65CB-SWL]

11 hours ago, Pixies said:

Thanks both of you,

It's all gone back together fine. Now time to collimate it.

That star diagonal. It is pants, isn't it?

And I wonder why the gold parts on this Mak? Perhaps it's quite an early one.

It looks quite 'chav' / 'blingy' like! 😜

Seriously... the UV rays from the Sun changes the anodising finish and is perfectly normal if left out. I have a Meade 20mm Plossl and the chrome nosepiece is going the same, (unless it's me over polishing it)! 🤪