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OMC140 stuff


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After reading that some thought that the OMC140s were a mixed bag, I began wondering what part of the bag mine came out of. Now as the one I have is my only experience of triple pass optics (with the exception of photo camera Cat. lenses) I am very much a novice when it comes to assesing these things. Here are a few of my thoughts and observations, perhaps someone who knows could enlighten me / us. TIA.

a) When focussing the scope, if i go from screwing the knob in to screwing it out, the image moves. Is that normal or a sign of backlash in the system?

:lol: If I look at Saturn I get slightly less sharpness with the Mak. than I do with the Skywatcher 200 Newt plus a 2X Barlow (standard Skywatcher issue variety). Is that due to the bigger aperture of the Newt. or do I have a problem? With camera lenses it is almost always the case that a cheap long lens will outdo a more expensive short lens plus teleconverter so that's where I'm coming from here.

c) When I stick the DSLR on I get colour fringing across the image sometimes. Could that be indicative of a problem or more to do with the streetlights shining into the tube at an angle.?

d) The instruction booklet describes collimation as being accomplished by peering through the eyepiece hole. Is that sufficient or should I get a Cheshire eyepiece / laser / something else to check it out?

e) Anybody know of a good website describing the collimation technique? I have found several sites for Newts from the links section and elsewhere but very little for Maks.

I think that's about enough for today, I'm going to have another play tonight, as I am bothered that there might have been slightly more high cloud when i tried the OMC than later when i did the Newt. It could also have been dewed up slightly, I only thought to check after I'd packed it away and it was too late.

Captain Chaos

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After reading that some thought that the OMC140s were a mixed bag, I began wondering what part of the bag mine came out of. Now as the one I have is my only experience of triple pass optics (with the exception of photo camera Cat. lenses) I am very much a novice when it comes to assesing these things. Here are a few of my thoughts and observations, perhaps someone who knows could enlighten me / us. TIA.

a) When focussing the scope, if i go from screwing the knob in to screwing it out, the image moves. Is that normal or a sign of backlash in the system?

:lol: If I look at Saturn I get slightly less sharpness with the Mak. than I do with the Skywatcher 200 Newt plus a 2X Barlow (standard Skywatcher issue variety). Is that due to the bigger aperture of the Newt. or do I have a problem? With camera lenses it is almost always the case that a cheap long lens will outdo a more expensive short lens plus teleconverter so that's where I'm coming from here.

c) When I stick the DSLR on I get colour fringing across the image sometimes. Could that be indicative of a problem or more to do with the streetlights shining into the tube at an angle.?

d) The instruction booklet describes collimation as being accomplished by peering through the eyepiece hole. Is that sufficient or should I get a Cheshire eyepiece / laser / something else to check it out?

e) Anybody know of a good website describing the collimation technique? I have found several sites for Newts from the links section and elsewhere but very little for Maks.

I think that's about enough for today, I'm going to have another play tonight, as I am bothered that there might have been slightly more high cloud when i tried the OMC than later when i did the Newt. It could also have been dewed up slightly, I only thought to check after I'd packed it away and it was too late.

Captain Chaos

Im no expert but i hope some of this helps a little:

A) this is the dreaded image shift, it's a function of how SCT's and Maks use the primary mirror to focus by moving it in the tube. It can be reduced by design but can't be removed. The workaround is to install a second crayford focuser on the back of the Mak and use that to focus instead. But this is an added cost that should have been sorted in the factory IMO.

:) The mak will have a larger central obstruction than the newt, hence the lack of contrast. You might find that collimation will improve matters a little but only to a certain point. If you want high contrast you have to move to a refractor.

C) Not sure about colour fringing, there is a lens in the mak system, the corrector plate, but im not sure if this give chromatic aberation or not. Other more experienced can tell you i hope.

D) the best collimation is done against a star at very high magnification. Examine the intra and extra focal images and adjust. There are links on the this forum if you search, i dont have one at hand though unfortunatly.

E) Not sure, but a search on this forum and maybe on cloudy nights might reveal some answers.

Sorry i can't be of more help.

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Cloud willing I'm going to check the collimation tonight using the before and after focus methods. On the websites that I have looked at (could only find Scmidt Cass. type collimation techniques) there is some good info. about how to do this in the dark. They (some of them) go on to say how critical the collimation is on these types of reflectors, so that could be the cure. It came well packed but the parcel guys might not have treated it with the utmost respect in transit, so it may be in need of some tweaking.

A quick peek down the hole showed some off-centredness so here's hoping.

Captain Chaos

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Good luck CC,

Just a quick recap on some of your original questions,

a) is normal for scopes that move the primary mirror to focus, a way around is to attach a focusser to the back of the scope and use that instead.

:lol: sounds normal, Maks are great designs but not miracle workers, the bottom line is that the OMC140 is a 5 1/2" scope with a 33% (ish) obstruction.

c) you should n't get noticable coour fringing from a Mak, that is something to look into.

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:lol: sounds normal, Maks are great designs but not miracle workers, the bottom line is that the OMC140 is a 5 1/2" scope with a 33% (ish) obstruction.

Hmmmm. Thinking I might reduce the inventory a little then. I got this for planets initially, as I thought it would do a lot better than the Newt. If it turns out that it won't beat the Newt, then I might have to let it go.

Plan A at the moment is to have it indoors on a photo tripod to look at Jupiter from the bedroom window. If I go outside the house is in the way and I can only see high up stuff to the East and South. If I go out of the front of the house I'm on the road with lots of street lighting.

I'll see how it goes tonight. Thanks for the replies guys.

Captain chaos

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Update on the collimation testing.

Last night I lined it up on a star and screwed the focusser well past the focus point and then slowly eased it through focus. The iinitial donut shrank and shrank to a point, with no signs of elongation, and then did exactly the same out the other side. I was using a 4mm eyepiece, which would give x500 magnification, so that fits in with what I have read.

Seems to me that the collimation is OK on the beast, next thing to look at is the front glass corrector plate, if that's the correct term. It looks like it has some kind of oily film on it. It looks different to normal glass. I haven't tried to clean it yet as I'm wondering if this might be contamination or it could be some kind of coating which I shouldn't clean off.

I think I might get in touch with Orion Optics in Crewe to see if they offer a collimation check and clean service so that I can be sure that it's all set up properly before I decide that it's staying or going.

Captain Chaos

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Thanks Arthur, I thought I'd better find out first! Wouldn't be the first time I've messed something up by going in feet first. My first response to something not doing what I want is that it needs taking apart, but on this occasion I've RTFM and decided that the alignment of the optics is something I'm not kitted out for, not until I get my head round what it is I'm trying to acheive.

Now I've got a laser, there's a mirror in the bathroom, how hard can it be??

Captain Chaos

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Update on the collimation testing.

Last night I lined it up on a star and screwed the focusser well past the focus point and then slowly eased it through focus. The iinitial donut shrank and shrank to a point, with no signs of elongation, and then did exactly the same out the other side. I was using a 4mm eyepiece, which would give x500 magnification, so that fits in with what I have read.

Umm - you're not looking for elongation as such here, but more that the dark centre of the do-nut shape (when out of focus) is in the exact centre of the outer ring.

Arthur

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