meade ldx75 8"

[dave710]

hello all and this is a first post i have 2 questions firstly i have a meade lxd75 8" schmidt-cassegrain and would like to know the best collimation tool to buy any recomendations.... and maybe a silly question but when i look through the scope during the day its fully clear view but when i look at the moon at night or other objects at night i get a black round circle in view...in the middle ..as you can tell im new to this and need any help possible..

thanks dave

[Oddsocks]

Hello Dave and welcome to SGL.

You don't really need any tools to collimate an 8" SCT.

Because of the way the moving mirror is only centrally supported, allowing some slop and tilt movement depending on the angle of the OTA, then just ordinary star shape collimation on a bright de-focussed star close to the object you want to observe is enough.

I bought and tried various laser collimators and cheshire eyepieces when I owned both a 10"and later an 8" version of the Meade LX200 SCT series scopes, but it was all a pointless exercise in how to waste money, since as soon as I moved the OTA to point at a different angle the mirror would flop a little and all the careful laser alignment would be out again!

All you do need to help you is a set of quick collimation knobs for the secondary so that you can keep those potentially damaging metal tools away from the front corrector, with the knobs fitted and adjusting for concentric diffraction rings around the defocussed star you will be properly collimated in just a minute or two.

Here is a link to a dealer in Belgium that stocks quick collimation knobs "Bob's Knobs", according to the manufacturer Altair Astro is a UK dealer but I couldn't find any on Altair's website.

If you do order a set from Robotics in Belgium then make sure you choose the correct set of knobs for your 8" SCT (either f10, F6.3 and either three or four knobs to match what your have on your OTA)

http://www.robtics.n...h=31_180_61_117

The black blob you see in the middle of the image is probably the shadow of the secondary mirror.

I'm assuming you have properly focussed the telescope to make the moon image sharp, the difference between a daytime image of something a few hundred metres away and bought to focus and the night time image of the stars or moon which are effectively at infinity may require (from memory) about ten turns or more of the focus knob.

If the Moon/star image is sharp but the black blob remains then this may be, depending on your make and type of eyepiece, either:

1: Using an eyepiece with too wide a field of view, I seem to recall getting the black blob with a 24mm Televue Panoptic eyepiece, but not with a Meade 32mm standard Plossl eyepiece that has a narrower field of view.

2: Not getting the correct distance between your eye and the rear lens of the eyepiece.

You shouldn't see any black blob with the standard Meade 25mm Plossl and certainly not with anything smaller, 15mm, 12mm etc.

William.

[dave710]

hello william and thanks for taking the time to reply and for your advice ..