UmaDog
Carpal Tunnel
Reged: 09/15/10
Loc: Long Island, NY
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Focuser tilt: did I do the right thing?
#5216332 - 05/10/12 10:29 PM
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Hi all,
I have a question about an alignment choice I've just made. Is the following reasonable?
The secondary on my 18" is only just large enough to see all of the primary, so positioning is fairly critical. One edge of the primary was consistently being clipped by the secondary and I finally decided to do something about it. Adjusting the spider vane length was not an option: no adjustment room. Instead, I aligned as normal then rotated the secondary until all of the primary was visible. This obviously destroyed the focuser axial alignment, so I tilted the focuser until the FAE was minimised. Now I see the whole primary in secondary and the axial alignment is good (cheshire and autocollimator).
I had second thoughts, however: I took it out under the stars and noticed that heavily de-focused star images (i.e. with the secondary visible) looked slightly oval if the star was in the center of the field. Moving the star to one side if the field produced a circular image. So something was off... I noticed this with paracorr and it was less obvious (but present) without paracorr. I haven't seen this before, but guessed it was due to the secondary not being rounded. I tweaked the secondary rotation a little and re-aligned. The defocused image was now almost circular.
Is what I did sensible or could the tendency for me to see the ovalness be due to the focuser and secondary being at a slightly odd angle. Then again, how can they be at an odd angle if what I measure with my tools looks right? Might I be better off returning the focuser to where it was and trying to re-position the primary? IIRC (scope's not set up now) loosening the sling should be the adjustment I would need to do. But maybe that's a bad idea?
Thanks!
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Jason D
Postmaster
Reged: 10/21/06
Loc: California
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Re: Focuser tilt: did I do the right thing?
[Re: UmaDog]
#5217037 - 05/11/12 12:19 PM
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Rob, what you stated is not making sense. If your focuser and primary axes are aligned and if the secondary mirror is centered and rounded under the focuser then you have good collimation. Focuser tilt is irrelevant in this case.
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UmaDog
Carpal Tunnel
Reged: 09/15/10
Loc: Long Island, NY
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Re: Focuser tilt: did I do the right thing?
[Re: Jason D]
#5217346 - 05/11/12 03:11 PM
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That's what I assumed must be true but then I started to doubt myself.
So the ovalness was just the secondary being a little out of round?
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FastMike
sage
Reged: 09/22/10
Loc: Florida 27° 15' N
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Re: Focuser tilt: did I do the right thing?
[Re: UmaDog]
#5217974 - 05/11/12 10:36 PM
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From your original post it seems you are trying to fully illuminate the center of the focal plane. But when the secondary is properly centered in the focuser and the telescope is collimated you are unable to get 100% illumination in the center because the primary is being clipped on one side?
Have you determined which edge of the primary is being clipped (top - bottom - left - right)? An easy way to determine which side is which, is to stick a post-it-note on one of the mirror clips for reference.
I don't know if you're not getting 100% illumination because the primary is not centered in the tube but you may find this helpful.
On my Webster which uses a cable sling, the primary is free to move from side to side in the mirror cell (there is no centering mechanism). During transport the primary mirror can become off center (shifted to one side). To center the primary mirror in the cell, point the telescope at the horizon. This will allow the primary to hang on the cable sling, with zero weight on the mirror cell. Gravity will center the mirror in the cell. Then raise the tube up to your normal collimating altitude.
If you need to adjust the up/down centering of the primary, you can shorten/lengthen the steel cable sling by adjusting the turnbuckle.
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UmaDog
Carpal Tunnel
Reged: 09/15/10
Loc: Long Island, NY
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Re: Focuser tilt: did I do the right thing?
[Re: FastMike]
#5218452 - 05/12/12 10:07 AM
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Exactly Mike. I will check if the primary is centered in the cell. I'm pretty sure it's the sling length I would need to change. It's not shifted to one side. Thanks for the input!
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: Focuser tilt: did I do the right thing?
[Re: UmaDog]
#5218474 - 05/12/12 10:19 AM
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advantage goes to a wiffle tree on this issue. 
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UmaDog
Carpal Tunnel
Reged: 09/15/10
Loc: Long Island, NY
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Re: Focuser tilt: did I do the right thing?
[Re: Pinbout]
#5218525 - 05/12/12 11:03 AM
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I'm missing something: why is that?
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Jason D
Postmaster
Reged: 10/21/06
Loc: California
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Re: Focuser tilt: did I do the right thing?
[Re: UmaDog]
#5218559 - 05/12/12 11:19 AM
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Quote:
I will check if the primary is centered in the cell
It is a good practice to do so but centering the primary in its cell is not a requirement for good collimation
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FastMike
sage
Reged: 09/22/10
Loc: Florida 27° 15' N
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Re: Focuser tilt: did I do the right thing?
[Re: Jason D]
#5218588 - 05/12/12 11:39 AM
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Quote:
centering the primary in its cell is not a requirement for good collimation
Yes it is required, for mine anyway.
I learned this the hard way.
When my telescope is moved across uneven ground the primary will rock from side to side in the mirror cell. The mirror cell pads have enough friction against the back of the mirror to hold the mirror off center in the cell until it's pointed near the horizon. If the scope is collimated *without* centering the primary (as described above), the scope will hold collimation fine until its pointed low (maybe 20-30 degrees). Then the primary will slip against the cell pads as gravity pulls the mirror to center of the cell. This of course makes the collimation goes kaput.
My 28" mirror can move about 1/4" each way (left - right) in the cell and say there until it's pointed at the horizon. It took me a dozen or so observing sessions to figure this out. I would collimate the scope without centering the primary. Most nights I don't go below 40-50 degrees so my collimation stayed solid all night long. But occasionally when I went to an object low in the sky my collimation would go bad, sometimes really bad as the primary moved in the cell.
So the first thing I do now once the scope is moved to my observing spot is to point it at the horizon and allow gravity to center the primary.
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Jason D
Postmaster
Reged: 10/21/06
Loc: California
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Re: Focuser tilt: did I do the right thing?
[Re: FastMike]
#5218595 - 05/12/12 11:44 AM
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Do not mix issues.
Centering the primary is not a requirement as I stated.
Stablizing the primary mirror in its cell to avoid shifts is a requirement which seems to be your issue.
Jason
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UmaDog
Carpal Tunnel
Reged: 09/15/10
Loc: Long Island, NY
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Re: Focuser tilt: did I do the right thing?
[Re: Jason D]
#5218636 - 05/12/12 12:17 PM
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When I picked up the scope, Eric advised me to give it a shake prior to collimation. i.e. lower it to about 20 degrees, grasp the OTA, and give it a couple of vigorous left/right shoves. Without that, the mirror can slip as you described and *BLEEP* up the axial alignment.
Jason, whilst centering the primary isn't a collimation requirement I imagine it should be pretty close to center or the mirror may deform. That's why I've not altered the sling length so far. Then again, I've not accurately measured whether the mirror is centered and I don't know the tolerances anyway.
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Howie Glatter
Vendor
Reged: 07/04/06
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Re: Focuser tilt: did I do the right thing?
[Re: Pinbout]
#5218694 - 05/12/12 01:02 PM
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A sling-supported mirror can be stabilized against sideways shifts by adding low friction restraints around 3 and 9 o'clock that just miss touching the mirror (or sling) at the sides.
I provide eccentrically adjustable side bumpers with my sling kits for this purpose.
UmaDog said:
"Eric advised me to give it a shake prior to collimation. ."
Related to this, there is a wonderful moment in the documentary movie The Astronomers, where John Dobson walks away from a scope aimed low that he just set up, and with an impish smile, lightly back-kicks the mirror box without breaking stride.
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michael_m
sage
Reged: 11/16/10
Loc: Mount Vernon, Wa
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Re: Focuser tilt: did I do the right thing?
[Re: Howie Glatter]
#5218774 - 05/12/12 02:05 PM
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Mike, I notice this effect you mention of the side to side shift of the primary mirror. My shift is potentially very tiny though. Like Howie just said his Delrin side bumbers limit that shift. First I made sure the mirror is centered by gravity, like Mike does. Then I adjusted my Glatter Sling side bumpers to allow only a sliver of daylight while the mirror is pointing low in the sky, like 20*, and the sliver of daylight is adjusted to the thickness of a playing card on both sides. Then I did the whole secondary mirror adjustment and focuser tilt to establish a baseline proper collimation.
What's amazing is that even this slight shift is enough to unstack the autocollimator. After moving my scope I do give it that bump at a low angle to settle the mirror. Then I go to the back of the mirror cell and look for the daylight on both sides being equal. If somehow the mirror is still touching the sling on one side I phyically push the mirror over the tiny distance required to equalize the gap.
So far so good. After doing this for a while now with my newly built scope I am seeing why a lot of people like the wiffle tree. If the wiffle is implemented really well to nearly match the mirror figure benefits of a sling, then the mirror is always in the same place. IMO the secret to the sling is to limit the side to side movement of the mirror to very small distances like the thickness of a playing card...or less. Howie's Delrin bumbers work very well.
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Starman1
Vendor (EyepiecesEtc.com)
Reged: 06/24/03
Loc: Los Angeles
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Re: Focuser tilt: did I do the right thing?
[Re: michael_m]
#5218813 - 05/12/12 02:39 PM
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Illuminating.
Fortunately, the 18 points of support behind my much-smaller 12.5" mirror keep the mirror from accidentally sliding sideways.
But, like you all, I lower the scope toward the horizon before collimating to insure the mirror is sitting on the cable.
In case I didn't say it before, Howie, great design on the sling!
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