The South Cave Collimator System
Posted 17 December 2009 - 03:50 PM
Posted 18 December 2009 - 09:43 AM
First, the unusual terms, "slewing, elevating, and shunting", are equivalent to skew error, tilt adjustment (confined to the plane defined by the primary mirror axis and the mechanical focuser axis), and offset adjustment (moving the secondary mirror closer to or farther away from the primary mirror only). What I commonly refer to as rotation, tilt, and offset adjustments.
Correcting "error 1" by adjusting secondary mirror rotation to align the pupil and primary mirror center spot with the vertical axis (assuming the focuser is positioned on the top of the OTA) can be a useful signature when secondary mirror adjustment screws 1 and 2 are set equally to minimize skew error (the most common, combined tilt/rotation error). You can use the crosshairs on a good sight tube to accomplish the same adjustment.
Using the center mounting screw to balance adjustments to screw 1 (step 3, correcting vertical or horizontal ovals), is potentially useful to reduce the possibility of creating a skew error (by balancing screw 1 with equal, but opposite, adjustments to screws 2 and 3), but it further impacts the offset adjustment closer to or further from the primary mirror. (Four screw tilt adjustment secondary mirror holders, like the AstroSystems holders, simplify the tilt adjustment.)
I don't quite understand step 4, in particular, the implication (and the meaning) of "splashy" ovals.
Also, assuming an accurately placed center spot, I don't understand why there should be a "displacement along the vertical axis...between the peephole reflection and primary marker...for reflectors shorter than f/6...when full optical alignment has been achieved". Since the alignment of the peephole reflection and the primary marker is essentially equivalent to a Cheshire alignment, the author seems to be advocating "better" secondary mirror alignment over precision primary mirror axial alignment, which, especially at shorter focal ratios, is the most critical alignment of the three Newtonian alignments (primary mirror axis, focuser axis, and secondary mirror alignments, listed in the order of their importance).
The author also notes his difficulties using a Cheshire with an f/4.4 optic, which I suspect could be alleviated with a "calibrated" primary mirror center spot.
Finally, aligning the crosshairs on a translucent front cover in the method described is akin to aligning to the spider vanes (with a carefully centered spider), which can be problematic with the commonly used offset procedures. If, however, the secondary mirror is mechanically offset away from the focuser side of the OTA, the method described could be used to improve coaxial alignment between the primary mirror axis and the OTA axis. While this can be important for precision DSC performance (or for some Newtonian applications with front mounted correctors), it should not be perfected at the expense of the more critical, primary mirror axial alignment.
Posted 18 December 2009 - 12:13 PM
Here's how it was done:
--the secondary mirror was removed, leaving a spider in the tube with a hole in the center. This hole was meticulously centered in the tube. Looking through this tube toward the primary, one centered the eye's pupil in the reflection from the primary and centered the pupil with a dot or ring on the primary. After this, the primary was not touched and all adjustment was done with the secondary.
--the secondary was replaced and a peephole eyepiece was placed in the focuser. The peephole, the peephole reflection, the inside of the peephole eyepiece's reflection, and the primary mirror's reflection were all made concentric. This resulted in having the primary's reflection in the secondary appear offset toward the end of the secondary nearer the primary. Collimation was decent, but edge-of-field illumination was far from even. That uneven illumination would get worse for shorter f/ratio scopes (remember, most scopes of the time were f/8 at shortest, and typically f/10-f/12)
The reason I bring this up is that the template-on-paper routine reminds me of the older collimation. Unless I misunderstand what's going on, it seems this aligns the primary axis to the tube's center axis, and ignores any necessary offset for the secondary (which would have the secondary shadow appear off-center if mechanical offset was built into the secondary holder), which is probably why the author comments that it seems to have poor results at shorter f/ratios--the primary reflection in the secondary would appear dramatically offset from concentricity.
Maybe I'm mis-reading this. I think I'm more confused than Vic.
Posted 01 January 2010 - 06:14 AM
a gift from God in plastic, hard cheese to those in metal.
So, cracklin' Meade and Celestron Office gid on board before the 60's return leaves you behind at light speed.
Yes, I'll be delighted with a gift of the 1000th collimating front cover off your production line and it doesn't have to be in solid silver!
Posted 01 January 2010 - 06:31 AM
Posted 01 January 2010 - 06:52 AM
Posted 04 January 2010 - 11:34 AM
From your commentary, it appears you have access to an older edition of New Perspectives.... I apologize if you found it difficult to understand--the fifth edition is a complete rewrite and is a bit clearer regarding secondary mirror alignment.
Your comment recommending "the 3rd paragraph of Testing Optics, by P.J. Violin", was also a bit confusing. Are you commenting on aligning spherical mirrors or did you mean the 4th (...get the collimation DEAD ON) or 5th (Here's how I collimate a scope. First, I use a steel ball...) paragraph?
Regarding the Cheshire, you noted in your original article, "...at f 4.4 its peephole surround, primary doughnut and cross hairs combine into one large unclear black blob..." and "...difficulty with collimating ‘fast’ ‘scopes had been news to me until the day of the Cheshire..." My comment in response, advocating a calibrated primary mirror center spot, stands. If the cross hairs are problematic, using an actual Cheshire eyepiece instead of a combo tool will resolve that issue. CatsEye Collimation offers calibrated Cheshires and primary mirror center spots that should resolve any other issues (their center spots also work quite well with other commercially available Cheshire derivatives, including combo tools, collimating caps, and Barlowed lasers).
Posted 04 January 2010 - 01:58 PM
....in making collimation even more incomprehensible!
Listen guys, I have a stash of rare, vintage model "Rosenstock Collimation Wizard Tools", that I'm selling for $25 each, while they last.
Each RCWT (formerly used in the photographic industry to store rolls of an archaic medium, "film") has been precisely center-punched with a peephole that permits trouble-free alignment of your optics in just minutes.
RCWTs are available in three basic colors; black, grey, and white. I'll be producing a small run of "Gold anniversary models", but these will be strictly limited to however long my can of gold spray paint lasts...
Let me know that you read about it on CN, and I'll knock $5 off the price. Except for the Gold anniversary models...everybody pays full price for them!
Posted 09 January 2010 - 07:11 PM
Can somebody else open it and post it in this thread?
Posted 10 January 2010 - 05:39 AM
and is for any required movement to achieve 90 degree reflection optically. n.b. if collimation writers confuse, just pretend you are Isaac Newton doing it for the first time, but with modern materials around you. Peter C.
Posted 10 January 2010 - 09:39 AM
Thanks. My first blog for the popular press begins,'Look through the special pephole at the reflection of its red painted peprimeter, the primary mirror centre douhgnut and the crosshairs of my transluscent CFC invention. If their centres afe displaced to one side, Fig.2, rotate the seconary mirror until there is no displacement, Fig.3. This ends Newtonian secondary mirror collimating difficulties for amateurs at last. With the drawings, the practical have a choice of a good read on, or to roll their sleeves up and get on with it.
The popular editorial preference for this subject matter is naturally first for acceptance by the BAA as a long winded thorough good paper or long letter, edited by them, agreed with me, then published in the Journal of the BAA.
Posted 10 January 2010 - 09:45 AM
Posted 10 January 2010 - 11:33 AM
Posted 10 January 2010 - 12:35 PM
I read your article several times. I believe I understand – though I am not 100% sure I do.
Your device is akin to a cheshire. Actually it has the exact functionality. However, instead of adjusting the primary mirror to line up the primary's center spot reflection with the tool’s face reflection (cross-hairs in your case), you adjust the secondary mirror. Basically, the device and methods you have described only aligns the primary mirror axis. They do not align the focuser axis.
Refer to the attachment.
You start off with a misaligned scope (figure A)
You adjust the secondary mirror until the primary mirror's center spot reflection aligns with the tool’s cross-hairs reflection (figure
Now if you perform a star test then you are only fine tuning the primary axis alignment which is the same as figure B
Figure C should be your target which aligns both the focuser and primary axes but there is nothing in your description the covers the focuser axial alignment.
Posted 10 January 2010 - 07:41 PM
I think it's easier for the other readers of this thread if I respond to your attachment directly--so I copied and pasted it here.
Hello again Vic. Why ‘attachments?’ Far cheaper than typing on line through my phone exchange old line and dial up. Great skies though, mag.5.8 often.
That's the tilt motion I refer to as "skew" (as it skews the diagonal aim away from the plane defined by the focal point, the intercept point on the secondary mirror, and the point defining the center of the primary mirror).
My ‘slewing sideways’ is the effect of turning sec. adj. bolts 2 or 3 singly or in opposite directions.
That's a fact! Gone are the days of 6-inch f/8 reflectors that were collimated by eye, making all of the reflections look "concentric".
Jeez, the reams we’ve all added since Norton’s 1950 2/3rd’s page for all of collimating.
You'll have to excuse me but I'm still confused, I assume this is the PJV article you're referring to: Testing Optics You had noted in the earlier attachment to me, "For when perfect collimation is alas clearly only the end of the beginning, I can do no better than to recommend the 3rd paragraph of Testing Optics, by P.J. Violin, on this web site." I fail to see what a turned edge has to do with "perfect collimation". I can't decide if you're implying that a turned edge limits the alignment read, or if in the presence of a turned edge, precision alignment is less important?
...PJV’s 3rd para. Begin reading at TURNED DOWN EDGE for list of symptoms when you realise even the SCC system has got you only to the end of the beginning of sorting a Newtonian.
Which final paragraph are you alluding to?
...You’re final para. Thanks Vic. I may be 50 years into being qualified in sextant optics, correction and use, but only 5 as a telescope amateur and have a history of good inventions which others make money from. A woman has just paraded my 15-year-old stand up urinal for women on Dragon’s Den! Complete list by request.
Actually, I believe Nils Olof was the first to comment on "calibrated" Cheshire eyepiece/center spots. One size works for any focal ratio (although two sizes are available from CatsEye Collimation to accommodate 1.25- and 2-inch Cheshires made by that company). And again, I miss the meaning of your closing remark, "...merciless to duds." Most Cheshire collimation tools I've used will work with any primary mirror center spot (a calibrated center spot clarifies the read and improves the resolution to about 0.005-inch). Before the Barlowed laser was invented by Nils Olof, the Cheshire eyepiece was the only tool available capable of reading just the primary mirror error as well as magnifying the error 2X. It was Tectron Telescopes most popular alignment tool, almost certainly because it was so easy to use.
Anyway, I will submit to editors of respected publications the following correction, …’until the day of the Cheshire when the type of primary centre mark I used at f/6 and fl 1200 was reflected at f/4.4 as a large unclear black blob. It seems that no one centre mark preference will do for all. It must be special/calibrated(your word) for each and merciless to duds.’
I think I'm less confused than I was after reading your review posted in this thread. And I'm no masochist, but if you'll direct me to the url to download V367, I'll try downloading it.
...Still confused? You ain’t seen nothing yet. Try downloading V367.
Posted 11 January 2010 - 05:41 PM
Thanks.I'd just realised your thoughts on axial alignment ommission and already put in the article for its development that axial's given by the intersection of the cross hairs on the collimating front cover. Acurate from the first collimating touch to align everything else to and hardly thought about in practice; a bonus. With a Cheshire I'd say. axial has to be worked at after the first round of full sec. and primary collimation. This is short enough not to do as an attachment!
Posted 11 January 2010 - 09:03 PM
Posted 12 January 2010 - 03:30 PM
Posted 12 January 2010 - 03:49 PM