OMC 200 problems

[fatblokestu]

Hi,

Finally got a chance to use this somewhere reasonably dark, down in Dorset last week but couldn't manage to get a decent image out of it. It was ok with a 40mm eyepiece (Lanth) but anything higher magnification was hopeless. As far as I could tell it was cooled properly, and the skies were decently steady but it was very disappointting. Need to contact OO but if anyone has any experience with colimation or other issues I would appreciate your feedback. de-focussed star tests were very uneven - looked ok one way, then had a big black hole in the middle the other way. Any ideas?

On the plus side, the MK1 eyeball was working well and saw plenty of Perseid shooting stars, and I think the ISS and shuttle?

Cheers

[jakey]

It might have been the seeing. The omc 200 is a f20 which means that you will get silly magnification with standard f/l eyepieces. I owned the same scope and I had all sorts of issues with seeing because of the microscopic field of view. It was like looking upwards and looking through a drinking straw.

Try the scope over a few differnt clear nights, then contact oo. However I imagine that oo will give you the same advice.

[fatblokestu]

Thanks for this.

The seeing actually didn't seem too bad. I did speak to oo and they said it was likely to be temperature stabilisation, particularly in the narrow baffled tube running up to the eyepiece. They recomended cooling the scope with the eyepiece out (which stupidly I did not do!) and then blowing (?) into the scope to make sure there was no trapped warm air in this section just before putting the eyepiece in. Might try and find another fan to do this a bit more effectively. Will let you know how I get on.

I did by it specifically for planetary use so was expecting/wanting the high magnification. Did you give up with it in the end? Any recommends on eyepieces? I have a 40, 10 and 7mm Lanth, as recommended by oo.

Thanks

[anthony]

f20!!

I have not used any scope higher than f10. I have noticed that a longer FL eyepiece, (say 32-40mm) will show the secondary mirror as a shadow in the middle of the field of view.

Anthony

[JOHN MILLER]

I sympathise - I have experienced similar problems. OO have told me it is very sensitive to seeing and collimation . Also, it takes a long time to cool down! I usually use the fan for half an hour or so. I have found collimation very tricky - those small grub screws on the secondary cover are a bit of a nightmare IMO, coupled with the long tube. Collimation really has to be spot on. But when everything is ok you will get some excellent views with it. Barry at OO has told me that in most seeing conditions you're doing well to be able to use a 20mm EP and my best to date has been 15mm on one night with very good seeing ( a very rare occurrence in suburban Glasgow). Generally a 25mm EP works well. It's less picky on the moon and I've had some fantastic views. Very narrow FOV of course and it really is a lunar/planetary scope.

[fatblokestu]

Thanks for this John. I find it all a little frustrating because my only reason for buying this sort of scope was to get high magnifications on lunar and planetary views. OO recommended a 7mm and 10mm and never mentioned this sort of trouble. With a 20mm eyepiece, it is only 200 x magnification which I could get on any number of different type scopes - OO talked of 400 x with no issues about seeing. Given the supposed quality of OO optics I am not sure why it doesn't perform very well?

Other than pure magnification, does anyone know if/why an F20 scope is more sensitive to seeing?

Thanks for the comments. Will update you if I have any better luck, otherwise I may choose to move it on and get something else!

[James]

With a 20mm eyepiece, it is only 200 x magnification which I could get on any number of different type scopes - OO talked of 400 x with no issues about seeing. Given the supposed quality of OO optics I am not sure why it doesn't perform very well?

...

Other than pure magnification, does anyone know if/why an F20 scope is more sensitive to seeing?

I find that 200-250x is about the highest magnification one can get in any scope without too much image degradation due to the normally fairly turbulent seeing in the UK. I've only had a couple of nights where I could push things up to 400x. I've no idea why an f/20 scope would be more susceptible to seeing at the same magnification as any other scope. I suspect its more the design of a scope that affects this (i.e. closed systems like catadioptric scopes take longer to cool down etc).

James

[fatblokestu]

Thanks James.

When you were able to use higher magnification, did the scope perform well? What sort of targets were you looking at?

Cheers

[barkis]

From what I have read about your particular instrument, collimation will have to be spot on. But I would think OO would have ensured it was perfect when you got it. At such a long focal length you have an inherrent high magnification, so I would think lunar and planetary targets are the realm of this instrument. Was it the moon you observed last night? If so it would not render much in the way of definition due to the phase angle.

In fact the moon was almost full, because it would go into eclipse today, although not seen from here, more in the Americas.

I would not become too despondent. I am sure the answer is probably simple. I can't see any other outcome but superb views of both the moon and planets with that baby. All you need is the right conditions and full thermal equillibrium. That system has four optical surfaces dont forget, and they all have to be in harmony to give a blistering performance.

Don,t worry. It will work out fine.

Ron.

[James]

Thanks James.

When you were able to use higher magnification, did the scope perform well? What sort of targets were you looking at?

Cheers

The scope was a 10" f/4.7 Newtonian that I'd painstakenly spent an hour or two collimating to the best of my ability that afternoon before using it. It was back in April at the Kelling Heath Star Party - I was able to push the scope to 400x looking at Saturn and some globulars (M3, M5). It was at the absolute limit forr the conditions, Saturns disc was faded somewhat but there was impressive ring detail. The globulars were well resolved.

Usually at those magnifications any seeing problems just get magnified too and any distortions render the image a mush. The collimation was pretty good too and that must have helped and the scope had had time to cool down. I reserve high magnification for planets and globulars when I can (sometimes its useful/necessary on planetary nebulae)...

james

[John]

To echo what Maccers (James) says, I found with my well collimated 8 inch SCT that I was rarely able to use powers above 275x despite the theoretical capability of the scope to provide reasonable images up to 400x. I guess that the issue that you are facing is that your scopes capabilities exceed the limits imposed on us by the viewing conditions in the UK.

In that respect I'm suprised that OO did not sound one or two words of caution when talking about very high power viewing - unless they were under the impression that you were going to be observing from a mountain in the Canaries or Hawaii !.

On the plus side, most of the time you will be using the scope well within its capabilites which should lead to very nice images and using longer focal length eyepieces with longer eye relief for more comfortable viewing.

John

[fatblokestu]

Thanks again all for your comments and re-assurance. The most surprising thing was the recommendation by OO of eyepieces that give very high magnification. It's not a problem because I have other scopes which I can use them on. From what I can see of the scope, the collimation is pretty much spot on, I think it was purely an issue of thermal equilibrium so will persevere, follow your suggestions and see what happens!

My main targets are the moon, planets and globulars so I don't think I've got the wrong scope, just need some more patience!

Cheers