Mak-Cass cooling time?

[jtc]

Hi Folks,

Question regarding mirror cooling times - apologies if already covered thoroughly (having issues with search function, gives blank page?) -

Recently picked up an old russian Mak-Cass to be used specifically as a camera lens - MTO-11CA 10/1000 - (http://www.rugift.com/photocameras/mto_11_ca_lens_for_canon_eos.htm)

Had good results on first use - http://vimeo.com/66691779 - but now having issues getting crisp focus, and am wondering if it's due to not letting it cool down enough.

Based in Brisbane (outside temperature probably 12-15° or so at night), and it's currently stored in an unconditioned room which pretty much always has a window open. Would've thought 30mins should be enough?

Or, would another option be to build a resistor based dew-heater (have built a few for smaller diameter camera lenses) & wrap around the mirror area, in the hope of eliminating thermal differentials by just bringing it up to a higher constant?

Thanks!

[Wavytone]

Hi Josh,

I've had one of these before (and stripped and refurbished it). They are small enough that you won't notice tube currents and there is no cool-down time. If you look very closely at the reflections in the front meniscus it should be a doublet lens - two perfect menisci with the tiniest of air gaps. All up about 3cm of solid glass up front.

Mine also cane with a screw-on dewcap extending another 12-15 cm out the front and with this fitted I never found any problems with dew. If the scope has been a bit warmer during the day it should take many hours for all that glass to cool down to the dew point.

The mirror inside any mak should never dew and that isn't the place to put heaters. Where a Mak can sometimes dew is the front of the corrector, when the front surface cools below the dew point. But these things have such a massive amount of glass up front that if it was warm to start with it shouldn't dew over all night. If you are really really determined to add warmers, I'd unscrew the front (it comes off easily) and fit insulated nichrome wire around the inside of the tube just behind the corrector, and bring the wires out through a small hole in the side plugged neatly with silicone glue.

If you are having focus problems its mostly likely the helical focusser is sliding slowly under its own weight with time - that is quite common if it has the original icky grease. A simple way to stop it moving is a piece of gaffer tape around the helical focusser, then fine focus at the back of the scope.

I modified mine for visual use, as follows.

If you unscrew the front part holding the corrector off the back half holding the primary, carefully look inside and you should see a screw protruding in from the side about 1 cm which is the infinity-stop for the helical focussing thread. If you retract or remove this screw it will allow the lens to focus past infinity a tad, moving the secondary about 1 mm closer to the primary. This has the effect of giving you another 1cm of backfocus to play with out the rear of the lens, just enough to accommodate a star diagonal and a 1.25" telescope eyepiece. You can also use this screw to lock the focus into position by screwing it in gently. This screw is hidden under the outer ring around the barrel that has the tripod screws set in it. To remove this ring, you will need to remove the rear mirror cell cover. This mirror cell cover screws off easily but take a very close look at it first as from memory there may be some tiny setscrews locking it in place.

If you do take the front off, when reassembling it take care not to get dust inside, nor get grease on the corrector or mirror. In addition, it takes some patience to get the helical threads to re-engage all round. For lubricant you can use ordinary vaseline.

[bojan]

You can find some info on those lenses by googling for "russentonne".
They are very popular in Europe..
Also, here you can find something about ways to improve the performance:
http://www.strickling.net/russentonne.htm
http://www.harpoint-observatory.com/deutsch/eigenbau/russentonne.pdf
(NB: This not always necessary.. however in some cases the improvement is dramatic).

I have MTO-1000A and MC Rubinar 10/1000 Macro.. quite happy with them - definitely good value for money (they are more expensive recently.. couple of yeas ago they were half price (in US$)).
My personal experience is that you have to allow for cooling .. 1 hour is OK. Of course, this is for photographing, where the problems are much more visible than if you are using it for visual.

Also, the way the lens is mounted is critical - do not squeeze the tube - the mirror sits in the tube with very little tolerance and it is easy to deform it (this was driving me crazy for some time long time ago, when I tried to mount it with bracket, tightly fastened around the tube. The image in eyepiece was horrible!
This is illustrated in the thread below..
http://www.iceinspace.com.au/forum/showthread.php?t=35180


BTW, I am using a trick when focussing with both MTO and Rubinar (by means of live view, Bahtinov mask should behave the same):
Since the front corrector assembly is heavy, it tends to "fall down" towards primary.
So I always focus by turning the collar from right to left, which moves the front with secondary mirror away from primary, in "up" direction - this prevents the focus to change by itself because of the helical thread backlash, it is already in lowest position.

[jtc]

Thanks for the replies guys!

Not sure i'm game to do the dismantling & reassemble.. maybe! Believe this unit has already been modified to focus past infinity at least - the barrel rotates about 50mm past the infinity mark. Wondering if this would give enough backfocus to fit some kind of micro-focus device between scope & still allow a dslr camera mount?

Think that's likely a large part of the problem - having difficulty focusing using the main barrel focus, simply because as you touch it there's a great deal of wobble even on a sturdy tripod and heavy geared head. Might also try a Bahtinov mask - which would at least give a clearer indication of focus. Bojan - I also noticed that focus seemed to be easier to achieve from right to left, but didn't think about the cause being thread backlash, but makes sense!

Could also be the design issues mentioned though, with the mirror mounting causing stresses.. more testing required i think.

[bojan]

If the focus ring goes 50mm beyond the infinity mark, then the focus stop is already removed... It should give you more than enough back focus, possibly additional 10cm +.
The optics may have been relaxed as well (or it doesn't need any intervention) - try with high power eyepiece (5-10mm), the astigmatism (if exists) will be visible on stars, at 100x and higher magnifications.
Have a look at what I am doing about focus here:
http://www.iceinspace.com.au/forum/showthread.php?p=991124#post991124

The main issue is backlash..