SCT collimation?

Hi everybody,

I've had my 10" sct a number of years but have never felt it needed collimation...or at least when I defocus on a bright star, it usually seems failry ok. I've just noticed recently, for some reason more with my 2" eyepieces than the 1.25" pieces a 'darkish' spot at the centre of field of view...as if the secondary mirror is in the view faintly. Is this a result of poor collimation?
Any help greatly aprreciated.

Mark

Mark,

I'd bet my last buck thats not a collimation issue. It's down to the fact that the light cone is not wide enough to fully illuminate a 2" wide eyepiece. With the 1.25" being narrower you wont see it so much. Poor collimation would result in focussed stars flaring in one direction, like little comets. You will always see this efrfect near the extremities of the eyepiece field even in a well collimated scope but if it were poor;y collimated you would see it in the center too.

As for the the actual problem which is that your 2" eyepieces are not fully illuminated right to the edge, well, im afraid there's not much you can do about that:( Its just inherint in the scopes design.

If I'm reading you correctly, then I've seen something similar to this with my scope, its much more obvious in lower powers and with a brighter background.
If you push the magnification in a SCT too low a dark area will appear in the center of the FOV, its the shadow cast by the secondary.
You should not be able to see it at higher powers.
Are you 2" eyepieces low power eyepieces?? \
SCTs do have a minimum workable magnification limit.

Hi Guys,

What you both say makes sense....just been out and definatley not as noticable in dark sky...early on in evening had it on moon but the sky was still quite bright and it was quite apparent but now its not.
Yes they are quite low power...ones a 27mm panoptic and the other is a 17mm nagler. I suppose if its not going to be very noticable at night then there's really no big issue. Just checked for collimation and it looks perfect...not bad as I've the scope for about 7 years now without EVER having to collimate it(!).

Thanks again

Mark

Thats good news Mark,
Can I ask is it a F6.3 or a F10 SCT, the F6.3 has a bigger secondary which exaserbates the problem.

Hi Guys,What you both say makes sense....just been out and definatley not as noticable in dark sky...early on in evening had it on moon but the sky was still quite bright and it was quite apparent but now its not. Yes they are quite low power...ones a 27mm panoptic and the other is a 17mm nagler. I suppose if its not going to be very noticable at night then there's really no big issue. Just checked for collimation and it looks perfect...not bad as I've the scope for about 7 years now without EVER having to collimate it(!).Mark

Mark,

The effect you are seeing is indeed the shadow of the secondary mirror, an effect particularly noticeable at low power in daytime or under bright night skies. Probably only really noticeable where the secondary obstruction exceeds 25% of the main mirror's diameter. In the case of the Meade 10" SCT, the secondary obstruction is 37% so no surprise that its visible with this scope but 92x and 147x are not exactly low power. Are you seeing it while using a f/6.3 reducer-corrector?

On the other point discussed, full illumination to the edge of the field-of-view, neither of these eyepieces will restrict the light cone and cause vignetting. The diameter of the field stops on the 27mm Panoptic & 17mm Nagler are 30.5mm and 24.3mm respectively, both well within the 38mm diameter of the Meade Schimdt thread adapter. It's only when the diameter of the field stop of the eyepiece exceeds 38mm (such as my 31mm Nagler with a field stop of 42mm or a 41mm Panoptic - 46mm field stop) that any loss of illumination at the edge will occur. An Eye Opener adapter can solve this.