SW Pro 7" Mak

[bmitchell82]

Okay, this thread is about confirming my readings and fuddlings :D

After my quest for a better F ratio i was chatting to a mate who should i say has a few scopes, he said oh i have a 7" Mak that i have been waiting to get collminated for close to 5 months (after another mate said that he could do it and stuffed it). Said if you can collminate it you can use it basically when ever you want.

Yeow bewdie, im up up and in!

So i got a hold of the scope that had finger prints on the inside of the front corrector plate, and primary mirror and not to mention that the lockin ring for the front corrector plate was loose aka corrector place could audiably be heard clunking in the front cell :(. so i have pulled the thing down and cleaning it as i would clean the 10" mirror have got it nice (damn SW black grease gave me a headache).

Now the method im using for the collmination process.

Collminated Laser collminator! (home made with a ROUND laser dot :D)
2x Barlow
EQ6 pro mount
20m accross my back yard to a fence
a Target on a piece of paper
and a 12mm illumiated rectical.

Now this is my proceedure so far

put the rectical in and center it to the target.
while hanging the rectical from the visual back insert the laser in.
Focus the secondary mirror spot on the paper to see the mirror spot ~5-10mm diameter.
now bring this spot no more than say 40% of the way back to the middle using the primary mirror adjustment screws.
re insert the rectical while hanging the laser off the focuser tube, recenter the target in the rectical.

Repeat untill very little movement is needed. (unlikely to get dead center).

Now after i did this i did a star test, but noticed that the diffraction rings where not concentric and no matter what i fiddled with the rings did not change. untill..... I thought about the light cone and what influenced it... the front corrector place must not have been 100% flush hence fiddling with the direction of the cone.

Im i right in saying that i have to muck around with the Front corrector plate untill i have concentric rings then go back to my collmination method to align the primary with the secondary spot.?

[DavidU]

Try this.
http://www.company7.com/library/orion/Inst_makcasscollim.pdf

[bmitchell82]

So it is as i thought anyhow.

Star test
Collminate
Star test
Collminate
Star test.

The front corrector plate i believe is out of whack aka is tilted (only a pooftinth) but enought to make the rings "un concentric" (not perfectionist). get this right before i "move onto the next step"

[Wavytone]

Brendan,

I have a Black Diamond 7" Mak. I'd suggest instead you ditch the laser tomfoolery and do it the old tried-&-proven way used for the previous 50 years. You need:

a) a "cheshire" eyepiece (old one with a small opening and no lenses, eg an old 4mm ortho minus the lenses);
b) medium power eyepiece, say 25mm;
c) higher power eyepiece, about 13mm; and
d) a steady mount, preferably one that tracks (doesn't have to be well aligned).

1. Start in daylight with the scope pointing at the sky and insert the cheshire eyepiece. Looking through this, concentrate on what you see in the secondary mirror, which will be a reflection of the primary. If the corrector is aligned properly the perimeter of the primary should be visibly concentric with the perimeter of the secondary. If not, adjust the tilt of the corrector till it is. To work out where to make the adjustment, hold a finger in front of the scope and move it to where the gap between secondary and primary appears greatest. Find the adjustment for the corrector nearest this point.

Repeat until the reflection of the primary appears concentric in the secondary.

http://www.robincasady.com/Astro/myimages/collim.html

2. Now the primary, do this on a clear night . Insert medium power eyepiece and find a bright star, centre on this and look at the out-of-focus rings. When properly aligned, the rings will be concentric. If not concentric, hold up a finger in front of the scope and you will see this in the out-of-focus rings. Move your finger to the point where the rings are most mis-aligned, and use the corresponding screw to align the primary.

Repeat until the rings are concentric.

3. Swap to higher power eyepiece and repeat 2.

Note: A 7" is big enough to suffer internal tube currents and you may have to wait till these die down. Another solution is to stir up the air inside by swinging the scope around a bit, I find this helps a lot.

All told it can be done in 30 minutes or so at dusk.

[bmitchell82]

Thankyou for your words.

The front corrector was my biggest "*******" so to speak i wasn't looking forward to tracking a star and fiddling with the corrector plate in the dark. i will use this method to get the front plate right.

the laser pointer in the method i have has only one job to do, and that is to project a shadow of the secondary mirror spot. There is no tomfoolery :) and is extremely accurate as your not relying on your eye your looking at a target with a spot that moves when you adjust the primary, hence making the adjustment is quantitative. i found that after 3 iterations the spot didn't deviate from the target as compared with the rectical.

Apart from the front corrector being skewed i belive that i had 13 diffraction rings, though 12 confirmed diffraction rings that ran all the way around at even distance untill the bow caused by the correction plate when i star tested on monday evening (thankyou sky gods for steady skies).

Ahh the good old tube currents. as a owner of a reasonable size newt, these don't affect me though i was sitting there in aww looking at the laser swirling around in the Mak, funky :) after about 30 min it settled down to acceptable levels.!