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Equipment Discussions >> Reflectors

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Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Am I missing ANYTHING by not using a Cheshire? new [Re: Ed Jones]
      #5771148 - 04/01/13 04:35 PM

Quote:

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.




The late Jim Carrol in Chicago collimated my old 10" f/9 scope the exact same way on several occasions - and it was always as good as I could do with tools! I think Dan Joyce did it that way too, and with faster scopes. Unfortunately I never could get the hang of the technique.

One evening at Astrofest we say spokes in Saturn's rings, the only night I have ever seen that. (The seeing might have had a little to do with that though ...)

Back to the OP - the tools aren't that expensive, the only thing you apparently are missing is peace of mind.


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buddyjesus
Carpal Tunnel
*****

Reged: 07/07/10

Loc: Davison, Michigan
Re: Am I missing ANYTHING by not using a Cheshire? new [Re: Jeff Morgan]
      #5771251 - 04/01/13 05:07 PM

I think the big advantage to the laser is that you don't need two people or multiple trips to the eyepiece to collimate. I only used a sight tube(made from posterboard) on my old 3" reflector and worked fine for me. I only just made my current laser collimator for the refractor for a buck.

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cordite33
member


Reged: 02/13/13

Loc: western montana
Re: Am I missing ANYTHING by not using a Cheshire? new [Re: Starman1]
      #5771279 - 04/01/13 05:20 PM

Quote:


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.




Thanks Don. That is very enlightening!


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azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Am I missing ANYTHING by not using a Cheshire? new [Re: Vic Menard]
      #5771917 - 04/01/13 10:53 PM

Just Ganymede. Callisto I clearly see color in it - that dirty cast about it but I still havent made a concerted effort for detail, and Io Ive seen the egg/effect created by its darker poles and bright equatorial region. Europa Im thinking is just beyond hope with what Ive got. Seemingly even huge apertures only dhow a blank disc as well.

At anyrate its particularly with observing like that where I have the nagging idea that some sort of tool might help me be more thorough.

Thanks!!


Pete


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Datapanic
Carpal Tunnel
*****

Reged: 10/17/09

Loc: Tucson, Arizona
Re: Am I missing ANYTHING by not using a Cheshire? new [Re: azure1961p]
      #5771942 - 04/01/13 11:14 PM Attachment (8 downloads)

Back in the old days, I too would just eye-ball it to get the secondary centered and then work the adjusters on the primary and then later do a star test to get it just right.

When Cave shipped their scopes, the primary was removed from the tube along with its cell and these instructions were all that was given to collimate the scope. I do appreciate lasers!


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james867
sage


Reged: 02/07/11

Loc: Southern Illinois
Re: Am I missing ANYTHING by not using a Cheshire? new [Re: Datapanic]
      #5772935 - 04/02/13 01:17 PM

I do like the cheshire for the secondary mirror alignment and its crucial when I collimate my bird-jones scope but to each his own.

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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Am I missing ANYTHING by not using a Cheshire? new [Re: james867]
      #5773147 - 04/02/13 03:29 PM

Quote:

I do like the cheshire for the secondary mirror alignment and its crucial when I collimate my bird-jones scope but to each his own.



Semantically, a cheshire is a tool used only for primary mirror alignment. It is a bright circle with a dark center, viewed in reflection through a peep hole in the middle of the tool.

If you refer to a tool that combines this with a sight tube (peep-hole plus crosshairs), it is called a "combination tool", or "sight tube + cheshire".

Collimating a Jones-Bird scope with the lens in place is an exercise in frustration. They are much easier to collimate by removing the lens, collimating, then replacing the lens.[In that case, simple newtonian collimation techniques are used] Some, though, have the lens glued in place in the focuser or mounted in front of the secondary mirror. These are best collimated by removing the secondary, aligning the primary with the front of the tube, then replacing the secondary and using the eye to collimate the secondary (for that, a sight tube helps, but the crosshairs don't help much at all--it's mostly making everything except the shadow of the secondary concentric).
A version of the barlowed laser collimation technique can be used once the primary is properly aligned with the tube center. Then the centermark shadow can help align the secondary. Because of the double pass through the lens (if the lens is mounted in front of the secondary mirror), a bright laser is needed. If the lens is in the focuser, then the standard barlowed technique can be used, but to align the tilt of the secondary instead of the primary. In both cases, the "barlow" used is the internal lens--no external barlow is used.

As for aligning the primary mirror by itself, this can be easily done with a transparent or translucent tube cap with a 1/4" hole in the center. Or, the central hole of a spider with the secondary removed. You align the center mark on the primary with the reflection of the opening of the tube cap (your pupil fills the hole). This was a standard technique back in the '50s and '60s. It assumes the primary is centered in the tube, so check that first.

Edited by Starman1 (04/02/13 03:31 PM)


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donnie3
Carpal Tunnel


Reged: 12/15/04

Loc: bartlesville oklahoma
Re: Am I missing ANYTHING by not using a Cheshire? new [Re: Starman1]
      #5773336 - 04/02/13 04:57 PM

I don't know how true this is but I've been told that if you can see all the primary mirror clips that hold the mirror in place your secondary mirror is real close aligned. any comments on this. donnie

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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Am I missing ANYTHING by not using a Cheshire? new [Re: donnie3]
      #5773408 - 04/02/13 05:16 PM

If the secondary mirror is adequately sized, you will be able to not only see the clips but a fair amount of ring of space around the edge of the primary.
If you can *just* see the clips, then the secondary is only adequately sized enough to illuminate the very center of the field to 100%, and this small a secondary will significantly vignette the field in longer focal length eyepieces.

What is true is that the secondary, if properly centered under the focuser and collimated, will have the outer edge of the reflective surface concentric with the outer edge of the reflected primary. But that presupposes the centering of the secondary under the focuser, and the best tool for doing that is the sight tube.


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