How To Center Spot A Primary

[Tim]

My primary mirror has a donut. I'm happy with that. I am not gonna mess with something that aint broken.

Unless you check it, you wont know if its broken!! Even 1mm or two out makes a difference

[TopHouse]

Unless you check it, you wont know if its broken!! Even 1mm or two out makes a difference

Have to say I gave this one some thought, you've also just reminded me of something that I probably should have added.

Firstly, I checked the position of the existing ring on the primary on my 200P before removing it, all credit to the Chinese, it was actually bang on, BUT these are no doubt fitted using an automated process and as such doesn't guarantee that they 'all are'.

What I should have added is the fact that when re-fitting the primary, DO NOT fully tighten your clips, this can cause what are known as 'pinch errors', basically meaning the pressure on the mirror created by the clips 'can' cause some minor viewing distortions. General consensus is that you should be able to slide a thin piece of paper between clamp and mirror.

BUT

When you come to fit the primary in it's cell, there is a very slight amount of play, which I would say theoretically 'could' affect a perfectly centred spot, in effect being the same as having the spot out by half a mil or so, therefore there is a 'tiny' amount of tolerance.

I said 'theoretically because to be honest, I don't know for a fact, it was just something that occurred to me whilst i had the mirror in bits.

[GJBC]

well written up. a good tutorial easy to follow

Grahame

[swashy]

So what is the benefit of the 'winged' marker?

...and how can you be sure you have placed it centrally if using a template from the diameter? how can you guarantee the 'optical' centre is in fact in the centre with this?

[catseyeman]

So what is the benefit of the 'winged' marker?

When used with a CATSEYE Cheshire tool, the diameter of the segment arcs match the ID of the Cheshire ring (See 1ST attachment on left). When used with the 2-pupil autocollimator, the inverted reflection mates with it's up-right twin like two puzzle pieces when axial parallelism is achieved (See 2nd attachment on right]. When the segments are aligned with the collimation screws, Primary adjustments quickly become intuitive based on the visual queues.

...and how can you be sure you have placed it centrally if using a template from the diameter? how can you guarantee the 'optical' centre is in fact in the centre with this?

.

According to the "experts" that make fine Primary mirrors, because of the natural symmetry of the rotational process to create a parabolic shape, in lieu of no easy method of determining the optical center, the "mechanical" center (referenced off the circumference) is the "best" estimate for where the optical center is actually located. In 12 years in the collimation business, I've yet to hear a complaint of an accurately placed mechanically centered spot causing a scope performance issue.

[Jason D]

Attachments are real photos

Triangle vs HotSpot -- same mirror different center spots

Via Blackcat cheshire

Via autocollimator XLK central pupil

Via autocollimator XLK offset pupil

Via autocollimator XLK offset pupil (yellow spot)

[irishbloke]

Jason had you put the hotspot over the top of a blackened donut or is the dark spot to the bottom left of the hotspots centre the pupil of the cheshire? How did you take these pics? I can't figure out how you did them. They're very helpful.

Thanks

Brian

[Jason D]

Brian,

I removed old donut spots. I did not blacken them. What you see is adhesive residual outlining the position of the old donut. The white and yellow HotSpots in my photos are on two different mirrors. See attachment. You can see how the old donut was placed off center at the factory.

I take my photos with an average camera mounted on a tripod. I use manual aperture, speed shutter, ISO, and focus settings to get these photos.

You will find more info and photos about the HotSpot at the following link:

Telescope Reviews: Concise thread about autocollimators+improvements

Jason

[Tim]

This is a great thread btw

[irishbloke]

Jason,

since you have the white and yellow hotspot what is the advantage of yellow over white or red?

[Jason D]

Jason,

since you have the white and yellow hotspot what is the advantage of yellow over white or red?

Had there been a clear winner between white, yellow, or red. Catseye would be selling a single color today. Different individuals have different preferences and opinions. Based on my opinion, the white color has an edge when used with the autocollimator because of better contrast visually. The red/yellow is better with the cheshire because of its contrast against the cheshire’s white ring. Under red light at night all colors seem to be about the same. Personally, I prefer the white color but that is based on my own preference.

Jason