Collimation close...but no cigar...yet

[northcanadian]

Hi all:

I have another question of you SGL members/mentors/friends. There is no one in my area that can answer all these questions that keep popping up.

When collimating in fine focus with my sct I notice a fuzziness (slight flaring(?) on  one side. The airy disc seems to be centered fairly well. My understanding is that the collimation screw opposite this soft side needs a tweak.  Is this correct?

Thanks for your advice...again!

Cheers

Roger

[alan potts]

Roger,

Thats basically how I do it but with the star out of focus, choose something around 30-45 degrees above the horizon that is at least 2nd Mag, it's easier to work.

It matters not as far as I can see which side of focus it is and I find making the de-focused star fairly large helps, I tend to go long side of focus. I start with a magnification of about X80 and get that shadow of the seconary in the middle,it is not going to be a mile out normally, it seems hard but it is not after a few times. Then turn the screws like you say but small movement, check, if you made a mistake turn it back or a bit more, whatever. Leave the alen key in the screw so you know which it was and work carefully. We don't what the key lost in the grass. Continue with the other screws until you see the shadow of the mirror in the centre of the de-focused star and then up the power.

Then I go to about X150 and fine tune, movement of the screws needs to be very small because if you give it a big tweak the star could vanish off the FOV.

I finish off at about X400 of more but the tiniest key movements are required and when I am happy I just try a focus and see what we look like, then drop the power to a more reasonable X100 or so and check out the airy disc, should be fine. It is really easy but I have to say fisrt time I was very nervous.

Hope this is of some use.

Alan

[barkis]

Roger, Alan mentioned leave the key in the screw, from which I surmise you don't have Bobs Knobs fitted to your secondary.

If you haven't, it's well worth doing the Mod. It makes tweaking a might easier, and you don't have to fiddle with an Allen wrench in the dark .

Apologies if you do have B.K.'s, but I thought I would mention it in case you don't.

Ron.

[alan potts]

Good point Ron, I don't have B K's as I can't get them here but they sure would make the job even easier. It really is worrying the first time though because you never get it right straight away and think you have messed up the scope.

One thing I should stress is never turn the Allen screw or BK more than a full turn in any direction, a bit more than this ( Maybe 2 turns) could have screw coming away from the mirror and we don't want that as this mean major work.

[northcanadian]

Thank you Alan and Ron for your advice....very helpful and appreciated!

My sct actually has Philips head screws.  I think it might help if my right arm was half a foot longer...lol.

Cheers

Roger

[knobby]

Don't forget it could just be cooling, the area of the tube near the dovetail holds heat longest and will be the last bit to fully acclimatise

[alan potts]

Roger,

If you get the chance PM me and let me know how you got on, you have inspired me to get the LX 12 inch out to-night, I have a clear night, I dare not say it so I have writen it. It is only the third in May, it's been terrible here.

BTW I know what you mean about arms not being long enough,

Good luck,

Alan

[northcanadian]

Roger,

If you get the chance PM me and let me know how you got on, you have inspired me to get the LX 12 inch out to-night, I have a clear night, I dare not say it so I have writen it. It is only the third in May, it's been terrible here.

BTW I know what you mean about arms not being long enough,

Good luck,

Alan

Hi Alan:

I was using Spica to tweak my drift alignment last night (before the clouds rolled in).  It seemed like a good choice for collimation also...nice and bright and right between Saturn and Mars for focus checking.  I will check it out tonight..if the cloud gods permit me...and let you know how I made out!

Cheers

Roger

[Mr Spock]

Those awful screws are the reason I fitted Bob's Knobs - best bit of cash I've spent

My routine is similar to the above.

If I've had the scope to bits, I do a rough alignment by looking down the big end from about 3m away.

Once outside, I find a bright star at about x100 and defocus. I get to a wide 'doughnut' and tweak the screws until the dark centre is symetrical to the outer bright ring. Increase then to x200 and do the same again.

Lastly, in good seeing conditions, I find a star about mag 4. I get fine focus on the star at around x400 and adjust until the diffraction ring is perfectly symmetrical.

That last step makes all the difference when you are looking for fine planetary detail or splitting a close doubles

[alan potts]

Michael,

I may give that last part of your system a try as I find my scope which was not cheap awful on tricky doubles, I do not tink I have ever split Sirius with this my largest scope and I have done it many times with smaller apertures I have.

Roger

Spica will do just fine or Antares late on.

Alan.

[barkis]

Roger, here is the website.

You may be able to order directly from the site.

Assuming they have stock for your particular Instrument.

They really are worth the investment.

Ron

http://www.bobsknobs.com/

[m37]

I've heard about BobsKnobs, would they offer any advantage over something like these?

http://www.wdsltd.co.uk/product/4562/wing-grip-traffic-red-thermoplastic-with-steel-screw-wds-8460

[barkis]

I've heard about BobsKnobs, would they offer any advantage over something like these?

http://www.wdsltd.co.uk/product/4562/wing-grip-traffic-red-thermoplastic-with-steel-screw-wds-8460

The wing dimensions look too large. The smallest is 2", therefore three of those would make it difficult

on a secondary Cell.  They would do for a Main mirror cell on a Newtonian, but the OP's telescope is a SCT,

which only requires secondary mirror collimating screws.

Ron.

[Tinker1947]

I bought some stainless steel knobs 3mm thread i believe spaced them out with some washers so the thread goes a good way into the secondary housing.....there were less than £7 including postage i think....... 

http://a2a4.com/acatalog/KNURLED_THUMB_SCREWS_HIGH_A_2-1.html