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Tips for accurate Cass mirrors centering?

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#1 AlphaGJohn

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Posted 25 June 2013 - 10:59 AM

So, having exhausted the subject of primary/2ndary spacing, I'm interested in tips for achieving the accurate alignment of the centers of a Cass-type primary and 2ndary.

The primary I have is intended for central mounting (the mirror's central inch or so around the perforation is flat, but the rest of the back is a shallow cone). It goes on a plate with the baffle tube attached and is held to the backplate by a collar around the baffle tube (see attached picture--which isn't as clear as I'd hoped). I do not see any way to adjust the centering of the primary. So I'm thinking that a gap between the ends of the 2ndary spider and the inside of the tube would provide some adjustment (picture attached to next post), but I'm not sure how to judge when the centers are properly aligned. I'm hoping for some suggestions.

(I have flocking to apply in all the appropriate places.)

Thanks!

John

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#2 AlphaGJohn

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Posted 25 June 2013 - 11:00 AM

Here's the gaps between the spider ends and the current tube (which I'm planning to replace w/ something of more uniform roundness).

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#3 mconnelley

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Posted 25 June 2013 - 02:02 PM

Hello:

You're getting into the issue of cassegrain collimation. For the telescope to be collimation, you need to get the four optical elements (star, primary, secondary, and eyepiece) centered on the same line (the optical axis). You can choose how you define that line. When I made my Dall-Kirkham, I chose that line to be defined by the center of the primary and the center of the secondary (2 points define a line). Defined this way, the primary and secondary are automatically on the optical axis. You then need to get mount the focuser so that it is centered under the primary, which usually isn't too hard. When you assemble the telescope, you then need to tilt the elements to make them square to the optical axis. First, tilt the focuser to center the secondary center dot on the focus axis (using either a laser or Cheshire). Tilt the secondary to return the laser back onto itself, or to center the reflection of the cheshire in the center dot. Finally, you need to tilt the primary. Look at a star, and tilt the primary to zero the coma, adjusting the pointing of the telescope to center the star. When the star is in the middle of the field and coma-free, you're done.

Cheers
Mike

#4 GlennLeDrew

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Posted 25 June 2013 - 06:04 PM

I would be severely tempted to design around a fixed primary, basing all adjustments around that immoveable object. Perhaps a means to make the primary square to the focuser mechanical axis would be a concession to absolute immoveability.

#5 MKV

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Posted 25 June 2013 - 10:27 PM

That mirror looks savagely chipped around the edges. I'd try to smooth that edge before it starts to crumble.

regards,
Mladen

#6 Ed Holland

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Posted 27 June 2013 - 11:04 PM

For centering & rough collimation, would not the method that works well for Maks and SCTs work here?

One views the telescope from the front, and examines the circular patterns & reflections created by the primary and secondary mirrors, adjusting until everything is concentric. With a good setup e.g. a makeshift optical bench and a carefully arranged peep sight, this can be pretty precise, as the now untilted mirrors in my 127mm Mak and C8 now attest.

Just food for thought,

Ed

#7 don clement

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Posted 28 June 2013 - 08:33 AM

I have always been intrigued as to how the late Max Bray made the secondary on his F/10 mak-cass scopes. The secondary shown below is a third curve ground, polished, and aluminized into the corrector. The secondary is permanently aligned, never needs separate collimation from the corrector.

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#8 Ajohn

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Posted 30 June 2013 - 03:52 AM

I've been giving cass alignment a lot of thought. To me it seems sensible to put most of the alignment on the primary mirror. What that means is the the mirror cell needs the usual tilt adjustments and centring adjustments. One thing is for sure unlike a DK the mirrors do need aligning axially and there is far more space and rigidity available at the primary end. There is another problem as well and that is mirror spacing. To me that is best adjusted and used for focusing by moving the 2ndry mirror. It could run on a shaft carried by 2 well spaced linear bearings and the parts can be made in a way that means no other alignment is needed other than a method of aligning the shaft the 2ndry runs on with the axis of the mirror. That could be done by spider alignment or an adjustment in the housing.

;) Curiously I vaguely remember seeing mention of this sort of set up mentioned in old boys own type astro books with the 2ndry being moved by a pulley arrangement and string. I also believe that this sort of arrangement is used on the "big" scopes. To me given the equipment likely to be available for a home brew scope it's also the easiest way to achieve something that will work. A lathe wouldn't be a bad idea though.

One other factor comes into it as well. The most precise arrangement what ever it is wont work out if the tube isn't sufficiently rigid to maintain the adjustment.

John
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#9 don clement

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Posted 30 June 2013 - 04:55 AM

Much easier said than done.

Don Clement

#10 PrestonE

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Posted 30 June 2013 - 10:17 AM

John, Not sure what you mean by adjust the secondary distance for focusing...

The primary and secondary spacing are critical and once attained should not be changed with an RC.

Depending on the coefficient of expansion of your tube with changing temperatures, you will already be getting some change in the optimum spacing unless going to a properly designed carbon fiber tube.

Best Regards,

Preston

#11 Ajohn

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Posted 01 July 2013 - 03:52 AM

The movements due to temperature and construction errors make me think that axial movement of the 2ndry may as well be used for focusing. Linear bearings and ground shaft are easily available so I don't see making something like that up as much of a problem. Driving it is more of a problem in some ways. These bearings may have tiny amounts of play but that can be overcome if needed by using 2 spaced apart so that the max angular error is acceptable.

In principle if the main mirror is tilted in the usual way and centring added 2ndry adjustments of any sort aren't needed in theory providing it's assembly is reasonably aligned but who would build a fixed focus scope? One way or another the focus will shift in practice even if it's only down to temperature effects on the glass.

Good focusers are expensive. :grin: An aspect I always think about and I feel that the 2ndry focuser is much easier to make. The downside of just adjusting the 2ndry for focusing though is eyepieces. I assume that their focal distance from their flanges varies but Celestron and Meade get away with that so it shouldn't matter too much. I sometimes wonder if people who use a mix of eyepieces on SCT's and find that some need a significant focus shift realise that they may be messing things up.

One things for sure a Cass is a challenge even to make the mirrors. That's my 1st step can I do that. Then I intend to try and make up a scope as I have outlined. If I have to go DK then I would still be inclined to do it the same way. Actually I suspect this area is why sct's move the main mirror with the added bonus that the scope will focus to shorter distances plus in their terms it's easier and cheaper to manufacture. Trying to do that at home would be more difficult than doing the same thing with the 2ndry.

The other point is where I would prefer the alignment controls to be - at the bottom of the tube where I can get at them while looking through it. :cool: That sort of takes care of things needing tiny tweaks in practice. A sort of fall back if the idea doesn't work out as well as I would hope. I would also try and make the 2ndry so that it had zero wedge so that axial angle adjustments on the shaft that carries it wouldn't be needed. I doubt if that area is a problem really unless some one wants to use it as a spotting scope and needed a lot of focusing movement.

John
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#12 Ajohn

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Posted 01 July 2013 - 04:05 AM

One thing I meant to add after looking at the photo at the start of the thread is that 0.5mm pitch taps and dies can be obtained fairly cheaply now in several sizes thanks to some board camera lenses. UK model engineering 40 tpi taps and dies are still available here as well in a wide range of sizes and probably still a cheaper option than 0.5mm. They are generally listed as ME threads. These sorts of pitches should be good for making robust adjustment screws if needed where standard screws aren't ideal.

John
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#13 PrestonE

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Posted 01 July 2013 - 08:31 AM


This is not a Cass!!! Which can be focused by moving either the primary or secondary...

Why do you think that Mike Jones showed you what happens when you vary the spacing of the primary and secondary with the Ronchi drawings...straight lines are the correct spacing and then with a focuser behind the primary you focus... :tonofbricks: :tonofbricks: :tonofbricks:

Regards,

Preston

#14 don clement

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Posted 01 July 2013 - 08:55 AM

Linear bearings and ground shaft are easily available so I don't see making something like that up as much of a problem.


Having made secondary focusers myself with linear bearings and ground shafts I wouldn’t say “making something like that up as much of a problem”. As has been stated on this thread, there is the issue of optimum spacing between the primary and secondary with a Cassigrian. Then there is the added requirement of the magnification of Cassigrain secondary to a secondary focuser. Just ask an SCT owner how that pole-dancing moving mirror focuser works for them. I invented my compliant focuser to solve the problems of focusing a Cassigrain (8” F/10 Mak-Cass) that was focused by moving the primary using a pole-dancing clone as is used on popular SCTs. Instead of using the stick-slip properties(stiction) of rolling or sliding bearings that depend on external surfaces (i.e. ground shaft with linear bearings), I incorporated the atomically smooth internal solid movement of flexures to produce linear motion of my secondary focuser.
MIT Professor of mechanical engineering Alexander Slocum described this process of internal solid movement of flexural bearings on P521 of "Precision Machine Design" ISBN 0872634922:
'Sliding, rolling, and fluid film bearings all rely on some form of mechanical or fluid contact to maintain the distance between two objects while allowing for relative motion between them. Since no surface is perfect and no fluid system is free from dynamic or thermal effects, all these bearings have an inherent fundamental limit to their performance. Flexural bearings (also called flexure pivots), on the other hand, rely on the stretching of atomic bonds during elastic motion to attain smooth motion. Since there are millions of planes of atoms in a typical flexural bearing, an averaging effect is produced that allows flexural bearings to achieve atomically smooth motion. For example, flexural bearings allow the tip of a scanning tunneling microscope to scan the surface of a sample with subatomic resolution. See for example, G. Binning and H. Rohrer, "Scanning Electron Microscopy." Helv. Phys Acta. Vol. 55, 1982, pp.726-735." '

Don Clement






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