Focussing a Mak127... is it normally this difficult?

[Takahashi]

I recently acquired one "on the cheap", but I cannot achieve even slightly acceptable focus through the camera (500D @ prime). If I focus through the viewfinder, or even using live-view zoom, it looks "ok", but the photo-output seems to be nowhere near what I see through the viewfinder, i.e. it's worse. Do these scopes have any foibles I should be aware of, other than the obvious generally clumsy focuser (compared to 10:1 Crayfords I'm more used to)?

[Merlin66]

The longer focal ratio actual gives you a little more latitude in focussing, the Critical Zone is reasonably forgiving. I think if everything is collimated, then it's just practise, practise, practise.

[Takahashi]

That's the problem though... my other scopes; a 110mm/775mm fl, a 100mm/900mm fl (on loan), and a Megrez 72mm/436mm fl, all have very forgiving focusers, but seem to have a larger margin for error, or focal "bracket", for want of a better word, than the Mak127. As far as collimation is concerned, I'd be more inclined to lay the blame there, as this poor wee scope had obviously seen some rough treatment before it got to me. However, I tried a few minor tests after I got it, on advice from some folks here, and the results seemed to say that if collimation was "out", it wasn't by much.

What I need is for someone experienced with Maks or other catadioptrics to come in and say "Yep, it's knackered" or "Here, I'll tweak this, adjust that, and hey presto!".

Failing that, the first person to offer me £120 can take it off me hands.

[Takahashi]

Can anyone help me with this? Either I'm doing something wrong, or there's something amiss with the optics/focuser.

[Merlin66]

Ivor,

Do you have a Ronchi grating? This can be used to find the precise focus of the telescope and show, at the same time any optical errors.

Does the star image look round and shows an even brightness doughnut close to focus?

Ken

[Mr Spock]

With Maks you have to be very careful with collimation. A slightly mis-collimated Mak can look awful. My 140 has to be spot on or planets look a blur!

The only real way to get it right is to collimate using a star test. Admittedly my 140 has 1/8PV optics and the last ounce of correction is worthwhile, but you must spend time getting your star test right.

For a long while I thought near enough was good enough, but once I'd seen doubles at x313 with my Mak properly collimated I was convinced the time and effort is worth it.

[yeti monster]

My 90 suffered from grease "migration", the primary was clearly coated giving the most aweful views. After a strip down and clean up (removing virtually all the inky, black grease) everything was hunky dory. Does your primary appear "fuzzy"?

http://stargazerslounge.com/primers-tutorials/103520-skymax-90-stripdown.html

[Takahashi]

Sorry to have taken so long to reply guys, been away for a bit.

Merlin, I haven't used the scope for a week or two, but I will try the star-image test as soon as I get a clear sky. I don't have a Ronchi grating, unfortunately.

A slightly mis-collimated Mak can look awful. My 140 has to be spot on or planets look a blur

This sounds suspiciously familiar. The problem is more noticeable in daylight, where if I point at an object, well outside the minimum focus distance, turning the focuser gets you to almost the sweet-spot, but just as you're expecting it to snap into focus, it starts to go the other way.

Star-test looks to be the way, but I do have one other, critical thing to do first; probably the cause of the focus issue; the rear cell isn't tight in the tube... part of the reason I got this on the cheap is that it's ex-display, and hasn't been treated too well. A combination of shoddy workmanship and kack-handed handling has resulted in the entire rear-cell (the assembly that actually screws into the tube itself) wobbling on it's threads, never quite achieving tightness, even at the end of the threads. If I can't get it to seat solidly, and if the star-test never comes to fruition, I suspect it'll be sold for parts or project, and the once-bitten/twice-shy logic will apply to future purchases.

[OzDave]

sorry to hijack this thread without being able to offer any advice, but I was reading above about collimating MAK's. I thought that MAK's didn't basically need collimation. Is this a myth?

Does anyone know how to go about doing such a collimation? Or how to verify if one is needed? I'm a beginner, so try not to bamboozle me. I'll have a google around and see what I find, but if anyone knows any links to tutorials on this, it might be useful.

David

[OzDave]

Oh, one thing I was considering with my own Mak127 was to forget about the built-in focuser and fit a moonlight (or other brand) SCT focuser instead. I could do this with a MAK->SCT adapter ring. Going this route would be relatively expensive depending on the choice of focuser (probably more than the scope is worth), but might provide extra fine adjustment.

The other thing I have read about until my budget will allow the above luxury, is to fit something like a clothes peg over the focusing knob so that you get finer angular control over the focuser. Unfortunately I haven't yet found a peg that will fit, but I can see how in principle this might help.

Though it sounds like in your case, the root of the problem is actually not with the focusing as such, but more with the mechanics of the rear cell.

Dave

[GazOC]

sorry to hijack this thread without being able to offer any advice, but I was reading above about collimating MAK's. I thought that MAK's didn't basically need collimation. Is this a myth?

Does anyone know how to go about doing such a collimation? Or how to verify if one is needed? I'm a beginner, so try not to bamboozle me. I'll have a google around and see what I find, but if anyone knows any links to tutorials on this, it might be useful.

David

Maks do need collimating but there are 2 popular types of Mak, the Gregorian and the Rumak, the former tends to need a lot less collimating than the latter all other things being equal.

If you are careful with a Gregorian such as the Skywatchers then you might never need to collimate it.

EDIT: If you are looking at a new focuser to attach to the back of the 127 make sure there is enough clearance between the centre and the focus knob to accomodate the new focuser and adapters. I've used the clothes peg trick with very good results.

[Merlin66]

You can make a large fine focus knob from a large plastic lid ( Vegemite!) just cut a central hole to suit the original knob and slip it over. I did that on my PST - works a charm!!