Concise thread about autocollimators+improvements
Posted 28 January 2011 - 07:49 AM
Posted 28 January 2011 - 08:23 AM
with machine-figured mirrors, the precision of figuring must depend on centering the blank (the periphery - ignore any asymmetry of the bevel) on the machine table - I have no guess here. With hand-figured mirrors, there may be even less precision! But anyway, while reports of badly centered spots are common, reports of actual decentering, large enough to call attention, seems rare to non-existing (I'd be interested to learn of any case).
I can't think of any method other than careful star-collimating under excellent seeing to find the optical center to any useful precision (I believe casual star-collimation as often recommended to finish instrument-collimation is a total waste of effort). I have no good data myself, but for a f/4.5 mm mirror, a 2 mm error would mean a Strehl ratio loss of 0.2 for a perfect mirror - should be readily discernible. A 1 mm error (there is half the error at the focal plane) probably discernible in good conditions. A few tenths of a millimeter - I would not believe so.
Perhaps a good research project for someone blessed with good seeing (and willing to waste a night of it on the altar of Science) and a good mirror? Done "blindly" by deliberate precision miscollimation (in different directions to balance a possible innate optical centering error) not known to the observer.
Or has anyone here done any systematic studies already?
Posted 28 January 2011 - 12:44 PM
Posted 28 January 2011 - 01:35 PM
The bevel on my XX12 is appalling and this made placing the centre spot rather tricky. Did it OK in the end, I think.
Jam a geometrically-sound block against the mirror with the proper height then use the block edge as your reference.
Posted 29 January 2011 - 05:34 AM
I have been having a long spate of bad seeing here, everything was just fuzzballs for months and months. Nothing over 120x. Then last night something Nils said jogged my memory.... Hmm. The last time I center-spotted my mirror was about four month ago. I had used the template for the first time and it placed the center spot about 1.5-2mm away from the engraved center of my OMI mirror. Could very well be my own human error but I had been reasonably careful so I went ahead and kept the center hotspot in this new position. Hmmmm...! Today I cleaned the mirror and moved the center hotspot back to the factory engraved center about 2mm away - and the difference in view is *dramatic*. I am looking at Saturn right now this morning at 250x and it is absolutely razor sharp. The mirror is performing as well as I have ever seen it, just gorgeous and super sharp. Wow. I'm very happy and the moral of the tale is, place the center spot *very* carefully a few mm can make a crazy amount of difference. What I thought was months of bad seeing (and maybe even incurable astigmatism) was just a very slightly shifted center spot.
Posted 29 January 2011 - 05:59 AM
Posted 29 January 2011 - 10:33 AM
Nils Olof's earlier comments about how the mispositioning of the center marker will influence the Strehl ratio of the final image is certainly apropos.
Say you have a superlative-quality mirror and your collimation isn't right--you will never see what the mirror is truly capable of.
Seeing the most out of a mirror is the best justification for collimation accuracy, center marker positional accuracy, cooling fans, and coma correctors.
When everything is as good as it can be, and you see superb seeing conditions also, it's magic.
Burned into my brain is an image of Jupiter through my 12.5" at 228X, wherein the amount of detail seen was close to a truly superb photograph, only sharper. White swirls in the GRS, multiple small storms in the EQ band, and polar shading broken up into a mackerel sky pattern of storms and eddies. All in color.
I could not even have begun to draw what I saw.
And my centermark is probably off by about 0.5mm.
And therein lies my interest in making the center marker accuracy as high as possible--if having a .5mm error results in near-perfect star images and that kind of detail on Jupiter, what would be the results in similar seeing of having an even higher accuracy on the center marker's position?
Posted 29 January 2011 - 11:07 AM
I recommend the technique I described in multiple posts earlier.
I had used the template for the first time and it placed the center spot about 1.5-2mm away from the engraved center of my OMI mirror. Could very well be my own human error but I had been reasonably careful so I went ahead and kept the center hotspot in this new position.
1- Place blocks around the mirror with about the same height -- maybe little lower
2- Place the template and place weights on the top of the blocks to hold the template in place
3- When it is time to press, press a little vertically and ensure the template is still centered before committing the center spot.
John, I do not doubt you centered the template but I believe when it was time to press and commit, the template slipped.
Posted 29 January 2011 - 11:46 AM
It's 0.06 mm from centre along L/R but 3.35 mm away along U/D.
I suppose I'd better go back and do it again...
Posted 29 January 2011 - 12:04 PM
It is not clear how such a large error can be introduced.
3.35 mm away along U/D
Yes, you need to redo it.
3.35mm error will introduce ~1.6mm PAE with is significant.
If you redo it, consider the method I have described.
Posted 29 January 2011 - 12:10 PM
Posted 29 January 2011 - 12:14 PM
In my case, I used 2x4 wood blocks (1.5" width). Weigh one side but shift the template 0.1 away from the weighted side. The 0.1mm shift is a recommendation but not a requirement.
Posted 29 January 2011 - 12:53 PM
That mirrors a comment I made a few years back: that I was amazed at how much the seeing improved once I learned to use the autocollimator to achieve good alignment.
Another thing that tipped me off was, I had recently started using the Paracorr and I noticed it was displaying some field curvature or something, the image was shifting in parts of the field. After I shifted the center spot on the mirror the Paracorr performed great and for the first time I am able to describe the view as near "refractor-like". Incidentally my new 10mm Ethos was amazing in the Paracorr at 200x, the Trapezium including the E and F star were perfectly sharp and visible all the way to the edge, with very little distortion if any at the edge. Saturn's Cassini Division was a very thin black line, the rings are only slightly tilted yet but I could see glimpses of what I call the "record groove" effect. the disc showed some detail and beautiful color, and Saturn's tiny moons were sprinkled like salt around the planet, it was mesmerizing. Again thanks to everyone in this thread, especially Nils for jogging my memory with his comment on 2mm shifted center spot causing noticable strehl loss.
Posted 29 January 2011 - 03:01 PM
have you measured the OMI indicated center spot placement?
Just wondering if you've determined whether it was "human error" or is the engraved center actually 2mm off of the mirror blank's measured center?
...The last time I center-spotted my mirror was about four month ago. I had used the template for the first time and it placed the center spot about 1.5-2mm away from the engraved center of my OMI mirror. Could very well be my own human error but I had been reasonably careful so I went ahead and kept the center hotspot in this new position. Hmmmm...! Today I cleaned the mirror and moved the center hotspot back to the factory engraved center about 2mm away - and the difference in view is *dramatic*...
Posted 29 January 2011 - 03:03 PM
However, I'm having enormous difficulty verifying that with the Catseye template. The reason is that the mirror slightly larger than 12" and the coating is not a consistent distance from the bevelled edge. No matter how much I re-measure, the current spot position seems to be right one to under a 1 mm.
Ah! It turns out that the spot isn't so far off after all. What matters, it seems, is that the camera is pointing straight at the middle of the mirror. If it isn't, the calculated spot position is off. Using an older image where the camera was positioned in the focuser and the scope was collimated, I derive a spotting error of about 0.48 mm. I've gone on to verify that with the mirror removed from the scope. Based on a previous thread, I think this puts me within tolerance even for a Paracorr. So I'm calling it a day on this one!
Posted 29 January 2011 - 03:32 PM
Posted 29 January 2011 - 04:13 PM
This is pretty much what I suspected--thanks for the confirmation!
...At the time I had called OMI and spoke to someone there about the engraved center spot, they said it should be the same as physical center...
Posted 29 January 2011 - 04:35 PM
I am afraid without verification; the X location should not be accepted as the unquestionable center location.
Posted 29 January 2011 - 06:22 PM
Posted 29 January 2011 - 07:02 PM
The marks I've seen are diamond etched into the glass before the mirror is aluminized. I'm guessing it's done on a measuring table of some sort. If you were to ask OMI, I'm guessing they would put the accuracy somewhere around +/- one or two hundredths of an inch...
Posted 30 January 2011 - 01:04 AM
So, I took out my Orion stock primary mirror to assess the placement of my yellow HotSpot. I found the placement to be really good. See attachment.
Posted 30 January 2011 - 01:11 AM
But 2mm and 3mm errors sound excessive and should not be expected when Catseye template is used correctly. Again, I suggest using blocks and weighing down one side of the template.
Posted 30 January 2011 - 01:12 AM
(In the photo, the ruler spacing is 0.5mm)
Posted 30 January 2011 - 01:48 AM
Obviously, the technique used to place the X wasn't very accurate, in my case.
Yours may not be any better.
Posted 05 September 2011 - 05:07 PM
Recently, I pulled out my Stevens premium mirror to install and evaluate a Zambuto loaner mirror.
Before I returned my Stevens mirror back, I re-evaluated the centerness of my Hotspot which was originally placed using Catseye template. See attached photos in this post and the following one.
I am happy with the accuracy of Catseye template.
The reason I chose to add my new comments to this old thread because the last few pages were dedicated to this subject. I did the same evaluation with respect to my stock mirror Hotspot. Two separate mirrors were spotted using Catseye template and both mirrors were re-evaluated for placement accuracy and both have passed the evaluation with flying colors.
Bottom line: If it is done right, Catseye template is a cheap, easy, and an accurate method to center spot primary mirrors.