Focus aid for Mak/SCT's...?

[AndyUK]

Being very new to the planetary imaging world through a "cat", can I ask if there's a trick to bringing them to fine focus?

The reason I ask is that I have a "Morgan's special" SPC880 (+ filter, flashed to SPC900) but on my first night out with my Mak 180 I was totally unable to focus it anywhere near enough on Saturn, let alone consider attaching a barlow (which is my ultimate aim).

Admittedly, the seeing through the scope wasn't good either visually or on the screen, but then again I was trying to shoot across a rooftop...

Is there an easy way to focus? I have a bahtinov mask that I use with my MN190 for DSLR imaging... could I reuse that?

[step_hen]

I find a close peg on the focus knob can help

[mrmoo]

Have a look for the 'Peanut Butter Lid' fine focus knob, no joke. You might try a crayford focuser with an sct backend (+adapter), or a Borg helical focuser.

[JamesF]

AIUI Bahtinov mask designs are slightly different depending on the scope aperture, but making something that should work for you is allegedly far easier than cutting loads of slots into a sheet of card or plastic. If you search for "Bahtinov mask" you should find a recent thread that covers the whole issue.

James

[AndyUK]

Cheers all...

I'd heard of the clothes peg trick, and even without looking I can work out how "the peanut butter lid" works (very ingenious!) both of which I can imagine will provide a much better (finer) way of adjusting the focuser, but what I'm really after is a way of being able to confirm exactly when I've achieved the best focus possible through the webcam...

I've had a quick flick around for Bahtinov masks and found a website that you can input FL, aperture and "frame" (here) - It does look pretty similar to the MN190 mask, although the "gaps" are a little bit wider. I'll try out the one I have and see if that works and, if not, I reckon I can probably make that one (even with my clumsy fingers!)

[JamesF]

Took me a while to find it, but this is the thread I was talking about. Definitely worth a read through the linked PDF file:

http://stargazerslounge.com/beginners-help-advice/134838-bhatinov-focus-mask.html

James

[AndyUK]

DOH! Thanks James - That's EXACTLY what I was looking for...! Thanks!

[Mak One]

Being very new to the planetary imaging world through a "cat", can I ask if there's a trick to bringing them to fine focus?

The reason I ask is that I have a "Morgan's special" SPC880 (+ filter, flashed to SPC900) but on my first night out with my Mak 180 I was totally unable to focus it anywhere near enough on Saturn, let alone consider attaching a barlow (which is my ultimate aim).

Admittedly, the seeing through the scope wasn't good either visually or on the screen, but then again I was trying to shoot across a rooftop...

Is there an easy way to focus? I have a bahtinov mask that I use with my MN190 for DSLR imaging... could I reuse that?

Hi Andy

I'm a long way behind you on the kit inventory and only have the 180's smaller sibling, but I've hit the same wall in terms of getting anything like a crisp webcam image with or without a barlow. I cooked up a Bahtinov mask based on a template for a C6 - fits the end of the 150 OK and creates the desired pattern/effect. I'll have a crack with it next time and let you know how I get on.

I'm also going to give the "peanut butter lid" a go. This all sounds a lot cheaper than adapters and Crayfords/Borgs (it may yet come to that though...)

[riklaunim]

There are some Baader adapters, like 1,25" eyepiece holder with simple helical focusing. This is cheaper than a cryford. I also use Farpoint Astro Bahtinov mask - works correctly.