OOUK OMC-140 Mechanics, thermal properties, collimation, focuser

[alex_stars]

Hi fellow stargazers,

I just recently got an OMC-140 (carbon tube, deluxe version) second hand as my new telescope to play around with. It came with the mechanics intact but the collimation was way off (let's blame the shipping, but I am not sure 😉). Anyhow it's an interesting Mak to look at and through. However I did not find much information on it's focusing mechanism and how to collimate it, so I started to research the net.

Here is the Mak before I started to wrap it in aluminium bubble wrap to thermally insulate it, but that's a story for a bit later.

So I though I post here all the info I can find on the scope (in addition to the usual reviews such as http://www.scopeviews.co.uk/OMC140.htm) to have an archive of what is known (mostly mechanical and such).

Everybody is of course welcome to add to this list.

Clear skies,

Alex

[iantaylor2uk]

I once had one of these and found you could roughly collimate in daylight by adjusting the collimating screws on the back until everything looked symmetrical when you stood about 6 feet or more in front of the scope.

[dweller25]

Nice scope,

It will probably needs two layers of bubble wrap for best results.

 

[alex_stars]

Collimation:

First I want to start with collimating the Mak. Mine came way off which I realized on the first night out. So I set out to find info on collimating it.

A method I really like to collimate a Mak is this one: http://www.mira.org/ascc/pages/lectures/collim.htm which works really well to get the collimation right by daylight. One can refine at night at a real star, but this method does the trick quite well.

On the back of the carbon deluxe version, four Allen (Hex) head screws the size M5 are exposed. If you own the normal metal tube version you have to first remove the back cover. Around the visual back (which has an SC-thread) there are three M5 screws with washers, the same distance away from the central axis. These are the collimation screws.

(image above from https://www.litepc.de/astronomie/Orion-OMC140-01.htm)

Using these you can move the whole inner plate carrying the baffle tube, focus mechanism and primary mirror.

Here is my sketch of the mechanics. And and image from the net I found showing the collimation mechanism:

(image above from http://www.astrosurf.com/jcanelhas/OMCOPEN.html) To the left and right you see the nuts that "close" the M5 Allen screws so that you can not ruin the scope by collimating to far inward.

All in all a well designed mechanical way to hold the mirror cell and easy to collimate with a M5 Allen key.

 

[alex_stars]

4 minutes ago, iantaylor2uk said:

I once had one of these and found you could roughly collimate in daylight by adjusting the collimating screws on the back until everything looked symmetrical when you stood about 6 feet or more in front of the scope.

Thanks for the fast reply. 👍 That is what I ended up doing to collimate mine. See post above.

4 minutes ago, dweller25 said:

It will probably needs two layers of bubble wrap for best results.

Also thanks for the hint and fast reply. That is exactly the way I insulated mine. Will post a few images soon.

This is such a great forum, always appreciate the input from the active members.

[alex_stars]

Focus mechanism

Another interesting mechanical detail is the focus mechanism of the OMC-140. I got to uncover that one when I tried to work out what the fourth "mystery" Allen screw was for on the back of the scope. Directly opposite to the micrometer focuser, there are two Allen screws in a row, not just one. The inner one is for collimation, but what does the outer one (without the washer) do. Hmmm.

This is my sketch of the focusing mechanism. In principle it works like that:

• The primary mirror sits in a piece of tube that can slide up and down (all the green lines above)
• A spring on the baffle tube pushes downward and places tension on the setup so that the mirror always gets pushed down (or back if the scope is horizontal)
• On the moving piece of tube a lever is mounted (inclined lines above) and that lever is pushed upward by the micrometer focus screw (to the left of the image)
• However that focus lever is tilted, so that the side of the micrometer focus screw is further up then the other side. This tilt distributes the force (from the spring) between the micrometer screw and the fourth Allen screw which you find on the back (which is opposite of the micrometer screw)
• With the fourth Allen screw you can control the force that pushes on the micrometer screw. Should you experience a non-smooth focus mechanism (mirror jump/flip) loosen this forth Allen screw a bit to change the force distribution.
• Side fact: in between the primary mirror and the focus lever there is a cork (the net says, I did not open mine yet) disk to keep things well padded.

Also a nice system. My scope focus works really well, much better than other Maks I had (SW 127 and 180), but I never tried the focus on an INTES 😄

Here a few images I found on the net, showing the mechanics:

Image above from https://www.cloudynights.com/topic/764174-omc-140-focuser-troubles/ Here you see the fourth Allen screw on top (well just the thread of it)

Image above from http://www.astrosurf.com/jcanelhas/OMCOPEN.html This source also stated the use of the fourth Allen screw according to communication with OO.

[alex_stars]

Thermal insulation

As @dweller25 pointed out above, two layers of alumina bubble wrap should be applied for best thermal results. This is what I did also to my OMC-140, also to the back. Here are some images:

Scissors, bubble wrap, duct-tape, normal clear tape and double sided tape. Let the crafts project begin.

I use the lens cap as a template to cut out two back plate insulation pieces:

And the visual back for the inner circle. this leads to those two pieces:

Which I suck to the back of the scope with normal tape:

To start the tube wrapping, I used three stripes of double sided tape to start with (just under the removed dovetail bar).

Wrapped twice and closed with a duct-tape stripe.

The back I finished off with duct-tape so that everything is nice. Two additional hints:

• You can cut radial slits in the back insulation, just at the locations where the collimation screws are. This way you can collimate ever with the bubble wrap one.
• You can cut two thumb sized pieces out of the insulation on the front of the scope, opposite to each other. This way you can remove the dust cap better.

So far I think this should do the trick for thermal insulation. Hope it inspires you to wrap up you Mak as well.

[dweller25]

Good job 👍

[dweller25]

Just for interest, here is my (long gone) OMC200 Deluxe, it had an Strelh of 0.987

The deluxe version OMC’s are at least 1/8th wave and you will see that at the eyepiece 👍

[gul1337]

This is great job!  I need to re-do insulation on mine the same way!

[alex_stars]

Thermal properties, cooling data

The other day I had time to make a cooling experiment with my Mak. I placed three temperature loggers on and in the scope:

• one inside the baffle tube at about the location of the primary mirror (called tube in the graph below). This shows the cooling of the OTAs centre.
• one in between the carbon tubus and the two layers of insulation (called insulation in the graph below). This shows the raw effect of the insulation.
• one air temperature sensor just outside the tube (called air in the graph below).

The scope looked like this, outside on my balcony on a very cloudy day:

and here is the thermal data graph:

The time scale (time=0) is as such that I first left the scope inside my office for an hour to rest (minus time data) and then moved it outside at t=0 and left it there for more than three hours.

As we can see the cooling inside the baffle tube starts 10 minutes later than the air temperature. Also the cooling is slowed down substantially by the insulation.

I guess the idea of the insulation is to slow the thermal changes inside the OTA so that no large temperature gradients form and thus no large scale thermal tube currents. The purpose of the bubble wrap insulation is not to completely insulate the OTA, that does not work as we can see in the graph.

Also interesting is the increase in air temperature at about 140 min. That jump (probably the sun coming through) did not effect either the carbon tube temperature under the insulation (green curve) nor the central baffle tube temperature (red curve). Cool!

Hope this is interesting to others as well.

Edited January 9, 2023 by alex_stars

[CraigT82]

3 hours ago, alex_stars said:

I guess the idea of the insulation is to slow the thermal changes inside the OTA so that no large temperature gradients form and thus no large scale thermal tube currents. The purpose of the bubble wrap insulation is not to completely insulate the OTA, that does not work as we can see in the graph.

Exactly that… the insulation is to keep the temperature of the tube wall as close as possible to the temperature of the air inside the scope. 
 

Without the insulation the air in the tube will contact the cold tube wall and this will create temperature differences inside the tube which can ruin the image. 

[alex_stars]

Collimation add-on and first impression:

I have a recommendation for making the "white card" mentioned in the collimation advice http://www.mira.org/ascc/pages/lectures/collim.htm :

• use squared graph paper on carton the make the "white card"
• draw a crosshair with a black fine liner on the paper
• punch a small hole at the center of the crosshair where the horizontal and vertical lines meet
• level the scope on the table/tripod and level the horizontal crosshair line on the "white card"

Now when you look through the punched hole at a focal length distance towards the OTA you see the reflections of the crosshair in mirror rings. Especially the magnified version in the first reflecting ring (counting from inside out, secondary mirror spot, shadow of the secondary mirror, first reflecting ring, another shadow ring, ....) helps you to align your sight with the optical axis of the telescope. This makes the described collimation procedure faster and more accurate.

After I collimated my scope like that I was surprised that I did not see any need for re-adjustment on a real star test later that night with good seeing conditions.

first night out:

after playing around with the collimation and having a great night ahead I thought I take the scope out and test how it performs. Using my 17.5 mm Morpheus (114x magnification, 1.2 mm exit pupil) I was starting off with

Jupiter

which was a very worthwhile target that night. At times of good seeing it was no problem to resolve the two main cloud bands of Jupiter, with a hint of finer structures at times. However the conditions did not really warrant higher magnifications. Nevertheless quite comparable views to my 125 mm APO doublet I used to own.

Rigel and Beta Monocerotis

later that night, Rigel was bright in the South and the scope definitely cooled enough, so I tried to resolve Rigels companion (9.4 " separation). It was no problem whatsoever for the scope at 114x and the ease of resolving the double stars was again comparable with my APO. So I got ambitious and searched for Beta Monocerotis. What a nice view in the large FOV of my Morpheus. Resolving beta mon A and B (7.1 " separation) was easy and when the seeing was steady, I was able to resolve beta mon B and C (2.6 "  separation). That was indeed satisfying, given the low magnification and a Rayleigh limit of the scope of about 1 ".

All in all I am very satisfied with the scope so far.

[alex_stars]

Collimation check, artificial star

Today I had the time to check my collimation on the OMC-140 with an artificial star setup at home. I dropped a needle on some aluminium foil to make a "tiny hole", placed it in front of a strong LED light and then had a look at that artificial star with my scope from about 15 m away. A quick photo with my smartphone showed this:

The black cross-hair is from my 23 mm eyepiece, however the red, concentric circles I added later with software. To me this looks quite acceptable for the purpose of collimation. However the artificial star is not far enough away to test for defects in the optics, but that is some other task for later.

Comments are welcome!

[CraigT82]

That looks really good, though this method is a rough collimation, accurate collimation needs an in-focus star image to check the brightness distribution of the diffraction ring: Should be evenly bright all the way around. Ideally not on an artificial star either as there could be mirror/focuser movement when you then angle the scope upwards from horizontal. 

[alex_stars]

True, very accurate collimation should be done on a real star. Thanks for the feedback! I sometimes check visually the diffraction patterns on bright stars when I take out the scope, but on those occasions I don't image through the eyepiece.

[Mr Spock]

I used to have an OMC-140. It was at its best when collimated using a star test.

It was a devil to cool down. I'd almost recommend keeping it in a fridge... 

[alex_stars]

As I had my OMC-140 opened up today to clean the mirror (just blowing air) I thought I'd post some images for comparison with the other ones posted above:

Micrometer focusing screw and how it is mounted in the foreground. The screw to the left is one of the collimation screws.

Detail of the focusing mechanism. Noteworthy is also that the mirror now rests on a plastic washer and not only a cork washer as in the old days (mine is a 2017 model according to the previous owner).

When you look at the baffle tube from above, you'll see that a cork washer is still present. Anyhow all looked good to me and the mechanics really do work smoothly.

Disassembly of the back-side primary mirror is really easy:

1. mark the orientation of the rear cell with respect to the carbon tube (I used red tape as can be seen above).
2. Loosen all the screws at the rear end of the carbon tube, which are on the carbon tube. You do not need to loosen any screws on the backside of the scope where the visual back is mounted.
3. Start with the dovetail mount and its opposite counterpart. Those a four screws.
4. Continue with the three tiny screws which are at the same radial locations as the collimation screws.
5. Place the scope downward on the table (meniscus lens downward) and pull (gently) the back of the scope upward to remove the rear cell.

Assembly is just those steps backwards with the addition of a quick collimation at the end.

Clear skies everybody,

Alex

[alex_stars]

just a quick add-on to this collection of information. I wrote up my favourite daytime collimation technique in a different thread. That method extents the method described above.

https://stargazerslounge.com/topic/406851-precise-daytime-collimation-of-a-makutov-cassegrain-projection-method/

CS Alex

[alex_stars]

Thermal Insulation, part II

I was not completely satisfied with the thermal performance of my two layers of aluminium bubble wrap and recently redid my thermal insulation. The reason for doing so was that I compared thermal conductivity (k) values for different insulation materials and wanted to aim for the best insulation I could find in order to maximize the thermal inertia of the scope. Here are the values I looked at:

• thermal conductivity of aluminium coated bubble wrap: approx. 1 W / m K [that would be Watts per meter and Kelvin]
• PE, EVA and similar foams: approx. 0.035-0.04 W / m K
• air: approx. 0.025 W / m K

So air, if you could keep it still, would be best, but I went for a 1 cm thick EVA foam plate with aluminium coating on one side, which you can find in the form of camping mats. Here are some images:

Made a new finderscope mount (stainless steel) so that the insulation fits nicely in between.

Crafted a paper template for the back, including small holes for the collimation screws (just in case I want to work on those)

Double sided tape to fix it to the OTA

finished scope on the mount. Closed the back with aluminium tape.

Now I just need to wait for clear skies to test the thermal behaviour.

CS Alex

[alex_stars]

To test the new insulation, I recorded more thermal data last night. Similar to the post above, I recorded temperature just below the insulation on the tube (green line), inside the baffle tube at the location of the primary (blue line) and the outside air temperature (red line)

As we can see in the temperature difference plot, after about 3 hours of adjusting to the outside air, the difference between the OTA outside and the centre is at its minimum, and then raises to about 0.5 °C, where it remains constant throughout the 16 hrs.

Thus a couple of hours adjusting period before observing should just do nicely to avoid any major thermal currents in the tube. Still have to confirm visually if the tube currents are minimal, cause currently there is only 🌧️

CS,

Alex