Collimation ocd -re centering spot-Ocal

[bottletopburly]

In my quest to feed my collimating ocd after  endless hours collimating my 200pds and couldn’t quite get final collimation just measured centre spot and it isn’t central would marking centre of mirror with a black marker pen be ok ? I have been using an Ocal and spent hours recentering secondary , tweaking  steeltrack focuser so secondary as as central as I can get it , seems it’s the primary donut throwing things out or should I just align the primary mirror outer in Ocal and use Ocal centre crosshairs to align  Ocal camera sensor and ignore donut ?.

[Captain Scarlet]

Centre spot accuracy  seems to be a largely unacknowledged problem with Newtonians. Both my newish expensive 1/10wave mirrors had misplaced spots, one grossly so. I removed and replaced them myself.

As you’ll know, Newtonian collimation amounts to the alignment and coincidence of the two optical axes, and that spot, its shadow and/or reflections is used to determine those axes. If the spot is wrong, miscollimation is guaranteed.

A marker dot will certainly help, but might be difficult to see under certain circumstances, and if you’re using a barlow method, might not get reflected as a shadow.

The outline of the edge of the mirror might be ok to use to roughly align the eyepiece axis (to the primary centre) if you’re using, say, a Concenter which has a central hole and concentric circles etched into its glass face: you’d use that hole to indicate the centre of the primary. But that won’t help aligning the primary back to the eyepiece axis, you really need a mark for that.

Cheers, Magnus

[bottletopburly]

Does anyone sell a centre spot @FLO showing cateseye spot unavailable 

[bottletopburly]

I suppose I could use an Airfix model decal , RAF or swazstika 🤣

[niallk]

2 hours ago, Captain Scarlet said:

Centre spot accuracy  seems to be a largely unacknowledged problem with Newtonians. Both my newish expensive 1/10wave mirrors had misplaced spots, one grossly so. I removed and replaced them myself.

Mine too - 15" from OMI has an 'x' etched marking the centre, and checks out as the geometric centre, but the 'O' donut sticker was a few mm off.  I respotted with a catseye using their handy template... particularly easy with the etched mark too.

To the OP, you can just peel the existing one off: you don't need to be overly precious about the centre of the mirror.  If you have a ring punch reinforcer, or punch a clean circular hole in a sticker  the outer edge doesn't matter for basic laser alignment,  only the inner circle placement, if using barlowed method.

 

Edited February 25, 2023 by niallk

[jetstream]

4 hours ago, bottletopburly said:

In my quest to feed my collimating ocd

You dont have OCD until youve check all your reflectors with a 2 pupil autocollimator... ask me how I know. These things tell all including if the scope is even capable of it as is...

Yeah I've checked the spots too, but the Ostahowski installed ones are always bang on. Others, well...

[davies07]

See my post on the Catseye collimator here

If it's of any help, I've also spent hours playing with the Ocal collimator (not on a Newtonian) but with little success. The key difficulty for me was in establishing a reference pointing direction (offset) that gave a collimated telescope.

David

[Coco]

Yes use the outside of the primary.

I bought an OCAL as I wanted to strip my GSO 12” dob down completely, fit a new focuser with gears etc, spider out the whole job!

Before I took the secondary out along with the spider and used a laser (collimated) to mark the tube on the opposite side to the focuser, I stripped everything out and took the old focuser off to fit the new one, using the new focuser with lazer in anticipation of having to shim it to my surprise despite being a different brand it was bang on, this could be due to making a card template from the new focuser.

I then started the procedure as per instructions and various you tubers,
Focus tube alignment – check!
Secondary alignment – check!
The centre spot was around 3/8th’s inch out maybe more! Ignoring the spot I used the third ocal camera ring overlay on the mirror edge, switched the cross hairs on and pop! The ocal lens was bang on centre of the Ocal lens.

The centre spot isnt an issue as its hiddn by the secondary however it stops me using a laser/cheshire out in the field.

Ive discovered that 60 minutes cool down isn’t enough, the little fan just doesn’t cool it fast enough, I now use a house fan.
I may strip it down again in summer and fit Aperture Mask as and re spot the mirror, I think mirror is showing signs it has a turned down edge.

I can hear the refractor boys laughing....

[Starflyer]

On 28/03/2023 at 11:34, Coco said:

Ive discovered that 60 minutes cool down isn’t enough, the little fan just doesn’t cool it fast enough, I now use a house fan.

 

I noticed this with my VX10 and then CT10, the fans were sucking air down the tube, I flipped them around so they blow up the tube.  Cool down time is much reduced, it breaks up the static layer of warm air on the surface of the mirror, and it has the added benefit of keeping the secondary pretty dew free.

[Coco]

5 minutes ago, Starflyer said:

I noticed this with my VX10 and then CT10, the fans were sucking air down the tube, I flipped them around so they blow up the tube.  Cool down time is much reduced, it breaks up the static layer of warm air on the surface of the mirror, and it has the added benefit of keeping the secondary pretty dew free.

I'll check the direction thanks 👍