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Equipment Discussions >> Reflectors

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azure1961p
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Am I missing ANYTHING by not using a Cheshire?
      #5769132 - 03/31/13 04:28 PM


Hi Guys,

Till now - some 19 years - Ive used simple eye balling alignment with no Cheshire eyepiece, auto collimated, laser or the like. If it looks good through the open draw tube in the focuser, I make sure concentric rings are had on diffraction in out of focus stars and call it a day.

Am I missing anything? I like high power when the sky allows it, Galilean moon study etc. could Inbe fit from a sighting tube for collimating?

Its an f/9 of course.

Pete


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jgraham
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: azure1961p]
      #5769153 - 03/31/13 04:40 PM

Not really. I've been collimating Newtonians for 40 years using the ol' eyeball technique. Alignment aids like Cheshires and lasers are great for fine tuning and tweaking, but not required and you're not missing anything if you know what you're looking at. The final proof is what you see at the eyepiece.

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george golitzin
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: jgraham]
      #5769390 - 03/31/13 06:03 PM

I agree, especially given the f/9 scope. Assuming the diffraction patterns are symmetrical when centered in the field, you're just fine.

I feel people make a huge deal over collimation, spending lots of money on lasers etc. In the old days I used a film can with a hole drilled in the cap. Now I use a simple cheshire. I just don't see the point of anything more than that, even at f/4.2.

-geo


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Starman1
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: george golitzin]
      #5769433 - 03/31/13 06:20 PM

Well, collimation tolerances are different at different focal ratios.
For example, at f/9, your primary center marker could be off as much as 3.6mm--over 1/8" and still be within exacting tolerances.
But, at f/4.2, the maximum error is 0.37mm, or close to 10X as stringent!
In the first case, you could eyeball it with a 'collimation cap' and probably get it close enough. In the second case, nope.

The primary collimation tolerances are a lot stiffer.
At f/9, the coma-free zone is as big as the field stop in a lot of eyepieces, so as long as that coma-free zone is within the field of the eyepiece, you probably won't see an aberration.
At f/4.2, though, the coma-free zone is only 1.6mm wide (about 1/10 as wide as at f/9), so maintaining the coma free zone near center in the field is essential.
And since f/4.2 is likely to use a coma corrector (at least 95% of f/4.2 users probably use one), the tolerances are tighter still. So where the f/9 scope user can 'eyeball' the primary alignment, the f/4.2 user needs to use a cheshire or barlowed laser to get the primary collimated anywhere near close enough.

A simple 'combination' collimation tool, that combines a sight tube (peep hole and crosshairs) with a cheshire (peep hole with reflected bright annulus with dark center, ignoring the crosshairs) is really cheap and is pretty much adequate for most scopes. It's easy to use and has excellent results in the field. It probably will improve the crispness of those lunar and planetary images, too, albeit only slightly.

For the scopes faster than f/5 that use coma correctors, you might add an autocollimator to the mix for even greater accuracy, but I see no particular reason to use one at f/9.

George's reference to a 'cheshire' was probably to one of these combination tools I mention. In some of the high-end tools, the sight tube and cheshire are separated into two tools.


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amicus sidera
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: Starman1]
      #5769465 - 03/31/13 06:43 PM

Over the years I've collimated countless fast reflectors, in the f/4 to f/5 range, using a Cheshire; the caveat being that the mirror must be accurately center-spotted. A final fine-adjustment while star-testing for best diffraction pattern, and... done.

This presupposes that the diagonal mirror has been set up properly beforehand, but even if it has not been, it takes but a little longer to set that to rights.

Lasers and crosshair collimators can increase the precision of collimation by a miniscule amount, but I feel the ROI (Return On Investment) is vanishingly small, and is in almost all cases subsumed by atmospheric instability and the limitations of the observer's eyesight.

As to the OP's question: are you missing ANYTHING? Perhaps, but it is such a vanishingly small anything that you would likely not realize that you were missing it, even if you had seen it in the first instance.


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Starman1
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: amicus sidera]
      #5769495 - 03/31/13 06:55 PM

Quote:

Over the years I've collimated countless fast reflectors, in the f/4 to f/5 range, using a Cheshire; the caveat being that the mirror must be accurately center-spotted. A final fine-adjustment while star-testing for best diffraction pattern, and... done.

This presupposes that the diagonal mirror has been set up properly beforehand, but even if it has not been, it takes but a little longer to set that to rights.

Lasers and crosshair collimators can increase the precision of collimation by a miniscule amount, but I feel the ROI (Return On Investment) is vanishingly small, and is in almost all cases subsumed by atmospheric instability and the limitations of the observer's eyesight.

As to the OP's question: are you missing ANYTHING? Perhaps, but it is such a vanishingly small anything that you would likely not realize that you were missing it, even if you had seen it in the first instance.




So far as I can tell, the combination sight tube/cheshire tools cost less than separate cheshires. And a decent combination tool is only ~$40, so the return on investment is superb, even with an inexpensive reflector.


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amicus sidera
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: Starman1]
      #5769527 - 03/31/13 07:22 PM

I made mine from bar stock, so cost was minimal; that said, your point is well taken. My concern was more for the time many spend endlessly chasing "perfect" collimation, when simply "excellent" collimation would serve them as well.

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george golitzin
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: Starman1]
      #5769535 - 03/31/13 07:27 PM

Quote:

So where the f/9 scope user can 'eyeball' the primary alignment, the f/4.2 user needs to use a cheshire or barlowed laser to get the primary collimated anywhere near close enough.





Well I don't think that's quite so--one can use a bright star. If you know what a comatic star looks like, you're not going to leave the scope out of collimation. So what if the coma-free zone is dinky? That just means the comatic error declares itself to the eye all the more readily. So one tweaks until stars on axis are coma-free. Generally, what happens once you get close is that the coma-free star image appears somewhere in the field of view. Collimation can be thought of as moving that star toward the center of field.

I do use a sort of mini cheshire on my 18--a little refractor collimating eyepiece--I double check the final result on a star. (I use a somewhat nicer cheshire on my 12.5, because of its large center mark. And on both I use a sight tube to get the secondary right for starters--but this almost never needs adjustment, so it doesn't travel with my eyepieces.) Again, I think requirements for collimation tools, and the difficulty of collimating fast scopes, are generally overblown. The point of fancier collimating tools is that they make the process perhaps a little bit easier, that's all.


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george golitzin
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: Starman1]
      #5769550 - 03/31/13 07:34 PM

Quote:


George's reference to a 'cheshire' was probably to one of these combination tools I mention. In some of the high-end tools, the sight tube and cheshire are separated into two tools.




Well no, I don't own one--don't like 'em, but I'm sure they do the job. The "nicer" cheshire mentioned above is a tectron that came with some used telescope deal that I've forgotten, and the refractor collimator is this doohicky from scopestuff.

-geo


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azure1961p
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: Starman1]
      #5769685 - 03/31/13 08:51 PM

Don ,

What's a center market? (Heh). Ill be honest I never spotted my mirror. Is this worth it at F /9 even a little. Miniscule is important, and Ive done fine up till now but after nearly twenty years Ive never actually collimated the thing with any accuracy you guys do - even if it needn't be so stringent in tolerance. Ill get a Peep and crosshairs.

George - thanks for the comments. Just got through reading your Saturn accounts . Sounds like a very rewarding session. I've got to get my platform operational already. Its just a matter of having the drive bearings -stainless steel rods machined to mateto the synchronous motors.

Don thanks for the very detailed reply.

Pete


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Ed Jones
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: azure1961p]
      #5769859 - 03/31/13 10:55 PM

This reminds me of a story over 40 years ago, my old friend Stan had a 10 inch Cave and was having trouble getting good alignment using a Cheshire. There were no lasers back then but I had learned a neat way of aligning my 6 inch f/8 and I offered to do his the same way. I would put an 1.25 inch brass tube in the focuser and I would look a foot or more away. I could tell when my eye was aligned with the axis of the tube by the reflections in the tube. I could also see the reflection of my eyeball and found that when my scope was properly aligned I could see my eyeball centered in this tube. I offered to align Stan's scope the same way and afterwards we were looking at a perfect diffraction pattern at 600X or more. He was quite pleased and his Cave had an awesome mirror. After lasers were available I no longer used this method but actually the principle was the same. Guess I'm showing my age.

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maknewtnut
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: azure1961p]
      #5769898 - 03/31/13 11:27 PM

Well worth it IMO. I've seen many scopes that provided good images, "for years" according to their owners. Even if it's just a pinhole in the bottom of an old 35mm film cannister, it's easy to see something like a secondary being not quite centered to a focuser. Standing some distance away and using just the drawtube with the naked eye allows for errors that are detectable with very inexpensive tools made for the job. Heck, even using a laser w/o insuring proper secondary to focuser alignment often results in offsetting angles or effective reduction of aperture.

When interested in assessment and improvement, owners who took the time almost always offer comments like, "She went from good to great", or "It's almost like the seeing got better more often" when you see them at the next star party.

It doesn't take much (and with a well built scope often means the next check idicates the scope is still properly collimated)...but it makes a difference.


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azure1961p
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: maknewtnut]
      #5770228 - 04/01/13 07:40 AM

Ed,

I must try that. Thanks for the details in the method and results of the efforts. I also want to say as an aside, (more thanks) your explicit instructions on building an equatorial platform were instrumental in the one Ive got nearly complete. I have two stainless stellar rods to have machined then a variable power converter for speed control and DC-AC conversion. I went with the 1/15 RPM AC synchronous motors from Herbach & R- that you recommended. I glassed the platform heavily to reduce flexure as folks who've done this on their own seem to all have to deal with the flexure demon.

Mark, your testimonials are appreciated, thanks!

Pete


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jgraham
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: azure1961p]
      #5770250 - 04/01/13 08:16 AM

I recently saw an example where a laser collimator showed good alignment, but the star images were awful. One peek through the draw tube showed the problem, the diagonal wasn't even close to being on-axis with the focuser. Once we fixed that we got good agreement between visual and laser alignment. Lesson learned... look first, laser second.

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Ed Jones
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: azure1961p]
      #5770260 - 04/01/13 08:26 AM

Pete,
Good luck on your platform. All Dobs should have one, they are not that difficult to make and so much nicer than pushing a scope around at high power.


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FirstSight
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: azure1961p]
      #5770485 - 04/01/13 10:54 AM

Quote:


Till now - some 19 years - Ive used simple eye balling alignment with no Cheshire eyepiece, auto collimated, laser or the like. If it looks good through the open draw tube in the focuser, I make sure concentric rings are had on diffraction in out of focus stars and call it a day.

Am I missing anything?





Do I correctly imply from your question above that in the 19 years you've been using tool-less eyeball collimation, you've *never* had chances to try out borrowing someone else's quality cheshire? If so, here's your problem: the CN reflectors forum has never been without a small handful of star alignment curmudgeons with a somewhat dismissive attitude toward the relative value of quality collimation tools. However, without some experience of your own using a quality cheshire, how would you know whether or to what extent they're correct or wrong? If you simply want assurance that you're not the only one getting by skimping on use of formal tools, you got it already in this thread and can go on satisfied, fortified by having gotten by so far for 19 years without the view being of a quality that discouraged you from continuing.

HOWEVER, if you really seek an answer to your question, rather than merely reassurance from a faction inclined to give you the answer you might prefer to hear, I suggest borrowing someone else's cheshire and trying it out for yourself. The experience will give you vastly better context to evaluate the feedback you've received in this thread. The insight will be more informative still if the person you borrow it from has a relatively fast e.g. f/5-ish scope, and you compare how helpful it seems with his scope versus your own more forgiving f/9. You may indeed possibly conclude that even though a cheshire is useful in faster scopes, you can satisfactorily continue getting by just fine without one for your f/9 scope. Or, you may conclude that even though you could continue getting by without one in your f/9 scope, it would just be so much easier and better if you had one.

19 years and you haven't tried this experiment a few times? I recommend that it's time to either borrow one for a trial or else risk $30-40 buying one and keeping it if it's worthwhile and selling it if it proves not sufficiently useful to your tastes.

Edited by FirstSight (04/01/13 11:19 AM)


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azure1961p
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: FirstSight]
      #5770922 - 04/01/13 02:21 PM

Hi Chris.

Nope not even once. Your preloaded assumption that I made the query perhaps in order to gather support for a bias against collimating tools was peculiar . To be quite frank the tone of your entire post was negative and indirectly confrontational in a number if directions. But you entertained yourself obviously.

Pete


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cordite33
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: azure1961p]
      #5770975 - 04/01/13 02:48 PM

I find it interesting that Parks did not center spot their mirrors, at least mine isn't. They included an eyepiece with their scopes for collimation and their instructions focused on getting everything concentric.

Now I'm not suggesting that more advanced tools and techniques aren't worthwhile, I'm studying Vic Menard's book on collimation right now. But it is interesting to me that a company like Parks, who marketed their products as a cut above everyone else, would endorse a method that might not promote optimum clarity.


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Starman1
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: cordite33]
      #5771015 - 04/01/13 03:07 PM

Quote:

I find it interesting that Parks did not center spot their mirrors, at least mine isn't. They included an eyepiece with their scopes for collimation and their instructions focused on getting everything concentric.

Now I'm not suggesting that more advanced tools and techniques aren't worthwhile, I'm studying Vic Menard's book on collimation right now. But it is interesting to me that a company like Parks, who marketed their products as a cut above everyone else, would endorse a method that might not promote optimum clarity.




When I started working for Scope City, which was owned by the owner of Parks, one of the first things I did was to get Parks to start putting center marks (triangles) on their mirrors. So the later scopes and OTAs DO have center markers.

You see, how they did it was with translucent templates, laser reflections, and focuser transparencies, etc. They took a day to collimate each scope.

But they failed to allow for the fact that the transport of that scope would knock the scope out of collimation, even if only slightly, and that meant the user could not replicate the factory collimation easily. The center marker on the primary not only shortened the factory collimation to 15 minutes each, but also meant the end customer could also recollimate easily.

Their techniques were pioneered back in the f/7-f/12 era, and just hadn't kept up with today's f/4-f/6 standards. They didn't even realize that you couldn't simultaneously collimate a short f/ratio scope and have the shadow of the secondary appear concentric with the primary or focuser.

A Parks mirror is definitely worth owning, but the first thing I'd do is to center mark the mirror with either a triangle, donut, or Hotspot.


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Vic Menard
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Re: Am I missing ANYTHING by not using a Cheshire? new [Re: azure1961p]
      #5771034 - 04/01/13 03:17 PM

Quote:

...Till now - some 19 years - Ive used simple eye balling alignment with no Cheshire eyepiece, auto collimated, laser or the like. If it looks good through the open draw tube in the focuser, I make sure concentric rings are had on diffraction in out of focus stars and call it a day.

Am I missing anything? I like high power when the sky allows it, Galilean moon study etc. could I benefit from a sighting tube for collimating



If by Galilean moon study, you mean surface details on Ganymede and Callisto, it doesn't sound like you're missing anything.

An 8-inch f/9 has very comfortable axial alignment tolerances. Your fov should be pretty much coma free and your focus should snap comfortably. Plossls and Orthos may show some astigmatism near the edge of the fov, but you're probably mostly interested in what's in the center of the fov.

The whole premise behind collimation tolerances is minimizing the impact on your optic's Strehl ratio. If you're good with 0.8-Strehl (the old "diffraction limit"), you probably have lots of room for the axial tolerances. Similarly, if your seeing is rarely sub arcsecond, there's less opportunity to benefit from "high magnification" tolerances.

But there's a lot of discussion in these forums extolling the performance of refractors while deriding the performance of the "lowly Newtonian", especially since the arrival of the fast focus, "light bucket" Dobsonian. It's been my experience that properly executed, even a fast focus Newtonian is capable of besting a premium apochromatic refractor. It takes excellent optics, good seeing, and axial collimation that's kept inside the prescribed tolerances ("perfect" is not required).

FWIW, I regularly observe with a 22-inch f/4 Newtonian (with coma correction). I also have, and use, a 6-inch AstroPhysics refractor on a Byers 58 GEM (my imaging platform).


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