73 replies to this topic

[apfever]

OK, so you drilled out the center of the holder so the main bolt could pass through and take an acorn inside?  No clearance concerns or special short acorn to clear the glass?  The acorn nut would be a full threaded bore so it can go on 'backwards'?    I LIKE this. I'd still use allen head cap screws for collimation so I can rest the allens in place and have simultaneous adjustment on two screws.  The larger collimation screws can be rounded or pointed on the end so they nest in the original holes in the holder. Nesting the screws would keep them from walking the holder around when they are turned. 

 

The ones I'm working on are all different on how the center bolt is attached. They all have the set screw but one has one nut as you described, one has two nuts with one on each side of the spider hub, and one had no nuts - only the set screw. They all looked original with factory finish over them. One nut had a star lock washer. Typical variations that might have changed daily or even the same day!

 

Nice idea but I'm going more original and avoid the drilling and tapping. deSitter's work is well thought out and part of my decision to stay original is to simply offer another idea. I will get the allen bolts. I'll leave out the main bolt set screw - Yep, bad idea - and lock the bolt with a nut tightened to the hub.  

 

Original is not the best design for sure but not horrible, not the worst by far. The vane hub could have been larger and used as the back plate, with the mirror holder moved much closer to the hub. Hindsight. 

[deSitter]

OK, so you drilled out the center of the holder so the main bolt could pass through and take an acorn inside?  No clearance concerns or special short acorn to clear the glass?  The acorn nut would be a full threaded bore so it can go on 'backwards'?    I LIKE this. I'd still use allen head cap screws for collimation so I can rest the allens in place and have simultaneous adjustment on two screws.  The larger collimation screws can be rounded or pointed on the end so they nest in the original holes in the holder. Nesting the screws would keep them from walking the holder around when they are turned. 

 

The ones I'm working on are all different on how the center bolt is attached. They all have the set screw but one has one nut as you described, one has two nuts with one on each side of the spider hub, and one had no nuts - only the set screw. They all looked original with factory finish over them. One nut had a star lock washer. Typical variations that might have changed daily or even the same day!

 

Nice idea but I'm going more original and avoid the drilling and tapping. deSitter's work is well thought out and part of my decision to stay original is to simply offer another idea. I will get the allen bolts. I'll leave out the main bolt set screw - Yep, bad idea - and lock the bolt with a nut tightened to the hub.  

 

Original is not the best design for sure but not horrible, not the worst by far. The vane hub could have been larger and used as the back plate, with the mirror holder moved much closer to the hub. Hindsight. 

The key point of my rebuild was to make dimples in the cell plate so the pushers can push and not wander. I filed down the ends like a pencil and they went into the dimple and pushed. The tiniest adjustment of the pushers caused a visible change in collimation. I also remember thinking I needed 10-28 screws for a finer adjustment but didn't have a tap for that Ah the fun of homebrew engineering! If I could make money on spiders, I would be making them today!

 

-drl

[mfoose]

I'm still thinking about what to do for the secondary mirror holder. I may end up just keeping it as is and putting a nut to act as a stop on the collimation bolt that could damage the mirror. That way it would stop before any potential damage and I would simply loosen the two other collimation bolts and try for collimation again. I don't love the design, but I don't want to spend too much time on it if it holds collimation fine as is. 

 

With the spider removed, I cleaned the tube today. Any tips on how to straighten the spider vanes after it's put back in?

 

I also worked on the focuser today. It cleaned up easy and the brass came out looking nice. Time to re-install. 

[apfever]

Heads UP.  More metric and it looks like discontinued JIS. 

The collimation screws are all over the chalk board for thread size. I've found SAE 5-40 and Metric M3X0.6 so far.  Two spiders had metric, one spider was SAE. 

 

The M3X0.6 is still available in taps and dies, and I already had them. This is how I identified the metric collimation screws using a bino microscope for accuracy, and calipers.  The M3X0.6 does not show up on metric thread charts so this is likely a discontinued JIS thread. 

 

The 5-40 is an off size for SAE but I did find one tray of them at Ace Hardware. They replaced the originals in the secondary perfectly. 

 

The length may not be standard. I found most had factory ends. I did find one set that had been clipped to a custom length. They were obviously clipped using something like dyke plyers and the end cleaned up fairly well but not great. 

 

I was able to replace the original SAE 5-40 with longer screws and build the spring loaded assembly. The concept works well. I tried it with two different springs and screw lengths. One set of screws took a philips head which made simultaneous adjustments easy. The springs provide plenty of tension to hold collimation. Adjustment is smooth in both directions.

 

Collimation using two screws on a 3 screw system with springs:

A system that pivots on a center bolt must have springs on all three collimation screws, but only two have to be adjusted.  

 Always turn BOTH screws the same amount. Turning both screws the same direction will move a laser on a line up and down between the two screws. Turning each screw in opposite directions will move a laser side to side on a line through (parallel to) the screws. 

Attached Thumbnails

• 

Edited by apfever, 05 December 2024 - 05:59 PM.

[apfever]

Assembly can be frustrating. This is not meant to give a full procedure. It is meant to show the concept (and it worked good).  The basic design remains a bit clunky and chunky in use. You will have to find your own spring stiffness, embedment into the glass holder, spacing between holder and back plate, and more.  Disassembly and reassembly with the new concept is hindered by the collimation screws not being able to come out or go in directly. The screws will hit the spider vane hub. 

I will be fine tuning the mechanics and cosmetics, and using this for a test run eventually. That might be a long time. I don't recommend doing all three screws. One of the screws is directly under a spider vane and not good for access with a spring spacing. Not good for access period..

 

Factory original works too. I just had to check it out. 

 

Cost for one spider was between 2 and 3 dollars. Finding a hands on selection like my local Ace Hardware will be tough. 

Attached Thumbnails

• 
• 

Edited by apfever, 05 December 2024 - 02:02 AM.

[deSitter]

I'm still thinking about what to do for the secondary mirror holder. I may end up just keeping it as is and putting a nut to act as a stop on the collimation bolt that could damage the mirror. That way it would stop before any potential damage and I would simply loosen the two other collimation bolts and try for collimation again. I don't love the design, but I don't want to spend too much time on it if it holds collimation fine as is. 

 

With the spider removed, I cleaned the tube today. Any tips on how to straighten the spider vanes after it's put back in?

 

I also worked on the focuser today. It cleaned up easy and the brass came out looking nice. Time to re-install. 

I could not flatten my mangled vane. So I replaced them all. It is strange so many RVs have mangled spider vanes.

 

-drl

[DAVIDG]

 Here is  a picture of the spider on my RV-6 that I have had  in and out of my RV-6 many times since I bought back in 1976. The key is that you carefully bend the vanes into arcs as  you twist the spider and the vanes will pull free from the tube with no damage.

 

               - Dave 

Edited by DAVIDG, 06 December 2024 - 11:02 AM.

[mfoose]

Well, I got some good news and bad news.

 

Good news is I have the OTA back together. Twisted the spider back into place, reattached the focuser, finder bracket, and remounted the secondary mirror. I just put it back together like how I found it. When the weather gets warmer and I redo the tube I will mess with the spider more. I also center spotted the primary mirror and remounted it in the tube. Once I get the finder cleaned and remounted all the OTA will need is to be collimated!

 

Now, to bad news. When I was twisting the spider back into place one of the bolts on the end of the spider caught onto the hole it was supposed to slide through, but inside of sliding it stuck. I tried my best to be gentle, but as I was fitting it in I heard a slight crack. I inspected it, but did not find anything wrong. However, once I started to tighten the spider vanes down, the damage was revealed. One of the vanes broke off the mounting bolt. Not great, but it does slide right back into place and it feels tight. However, I know if I tighten the opposite side more it will slide out. Right now the spider is rigid, so I am leaving it as is.

 

I am nearly done cleaning it up and it works so I will have some fun observing with it this winter. Once it begins to thaw out here I will pull the OTA apart again to fix the tube damage, do a once over on the mount, and replace the spider with a new one. 

Attached Thumbnails

• 
• 
• 

[Jon Isaacs]

Thanks Jon!

 

I have heard nothing but praise for the RV-6. Excited to start using it soon. How would you rate the optics compared to your other premium mirrors?

 

 

I am not really qualified to comment.  It has been quite a while since I owned an RV-6 and the aperture difference and focal ratio differences are quite major.  DavidG has tested a number of them and I would trust Dave and others.

 

Your scope is a 6 inch F/8.3..  A spherical 6 inch F/8.3 is 1/4 wave as a parabola,  

 

I think the OTA design is very effective compared to the typical Asian scope. I think the tube is superior from a thermal point of view and the primary cell is nice and clean. The air must just flow through the OTA with ease.

 

Jon

Edited by Jon Isaacs, 06 December 2024 - 07:36 AM.

[deSitter]

I am not really qualified to comment.  It has been quite a while since I owned an RV-6 and the aperture difference and focal ratio differences are quite major.  DavidG has tested a number of them and I would trust Dave and others.

 

Your scope is a 6 inch F/8.3..  A spherical 6 inch F/8.3 is 1/4 wave as a parabola,  

 

I think the OTA design is very effective compared to the typical Asian scope. I think the tube is superior from a thermal point of view and the primary cell is nice and clean. The air must just flow through the OTA with ease.

 

Jon

 

6" f/8.3 is 1/2 wave, not 1/4 wave, and would be a poor telescope. See Sam Brown page 99 Table 3. f/10 is 1/4 wave and f/12 is 1/6 wave. A 1/2 wave scope of any kind would be a very poor performer on planets.

 

My RV-6 was absolutely great on planets, so not 1/2 wave, so not a sphere.

 

-drl

[DAVIDG]

6" f/8.3 is 1/2 wave, not 1/4 wave, and would be a poor telescope. See Sam Brown page 99 Table 3. f/10 is 1/4 wave and f/12 is 1/6 wave. A 1/2 wave scope of any kind would be a very poor performer on planets.

 

My RV-6 was absolutely great on planets, so not 1/2 wave, so not a sphere.

 

-drl

 Sam Brown is wrong and it was been documented that the chart is wrong by 2x on the wavefront. There is a  chart in  Texereau's " How to make a telescope"  that is correct at shows a 6" mirror spherical mirror with a 50" focal length  is  just at a  1/4 wave.  Here is also the wave front analysis from OSLO also showing that it is 1/4 wave system. The Strehl is just above 0.80 which is 1/4 wave system the PV wavefront also show it below 0.25 which again is just at a 1/4 wave.

  This is why people rant and rave about RV-6s having  great optics. The odds are they are observing with diffraction limited optics vs poorer optics they thought were much better. Criterion understood that if they made a 6" mirror with a 50" focal length and  if  they put the least amount of correction on it, or  none at all, the result would be a diffraction limited system. The secret was just the get the figure smooth and it would work.

   I have tested many many RV-6 mirrors and  a very large percentage test as under corrected but even with that small amount of correction the result is a 1/4 system.  So the odds that  you have 1/4 wave optics in your RV-6. My own RV-6 that I purchased in 1976 came with basically a spherical mirror with turned edge that I enjoyed  and  years later I refigured it. 

 

                 - Dave 

 

Edited by DAVIDG, 06 December 2024 - 09:59 AM.

[deSitter]

 Sam Brown is wrong and it was been documented that the chart is wrong by 2x on the wavefront. There is a  chart in  Texereau's " How to make a telescope"  that is correct at shows a 6" mirror spherical mirror with a 50" focal length  is  just at a  1/4 wave.  Here is also the wave front analysis from OSLO also showing that it is 1/4 wave system. The Strehl is just above 0.80 which is 1/4 wave system the PV wavefront also show it below 0.25 which again is just at a 1/4 wave.

  This is why people rant and rave about RV-6s having  great optics. The odds are they are observing with diffraction limited optics vs poorer optics they thought were much better. Criterion understood that if they made a 6" mirror with a 50" focal length and  if  they put the least amount of correction on it, or  none at all, the result would be a diffraction limited system. The secret was just the get the figure smooth and it would work.

   I have tested many many RV-6 mirrors and  a very large percentage test as under corrected but even with that small amount of correction the result is a 1/4 system.  So the odds that  you have 1/4 wave optics in your RV-6. My own RV-6 that I purchased in 1976 came with basically a spherical mirror with turned edge that I enjoyed  and  years later I refigured it. 

 

                 - Dave 

 

6 f8.3 sphere.JPG

OK so please explain WHY he is wrong. Where did he go wrong? Use the book and show us. It's free online here.

 

https://www.edmundop...elescopes 2.pdf

 

-drl

[DAVIDG]

 If that was my spider with the broken end I would repair it  by soldering a thin brass strip over the broken end and re-drilling the hole in it. I believe that will be easier then trying to replace the whole vane.

    The vanes should be brass as well so soldering a thin piece of brass over the end will be no problem and very strong.

 

              - Dave 

[DAVIDG]

OK so please explain WHY he is wrong. Where did he go wrong? Use the book and show us. It's free online here.

 

https://www.edmundop...elescopes 2.pdf

 

-drl

  On page 18-19 of Texereau's book "How to make a telescope" he goes through the math.  So if you want you can follow the derivation in the book   Also  any raytracing program will show that same results like I did with OSLO. So two independent results show that Brown is wrong and a 6" f8.3 mirror if left spherical is 1/4 wave system. As I said this is the reason why many give praise  to  the image they are viewing in a RV-6.

  

                  - Dave 

 

[davidc135]

Also, Telescope-Optics.net at bottom of p4.1.2 in the white box gives the example of the 6'' f8.15 sphere having 1/4 wave under-correction at best focus. 

The focus of smallest blur suffers 1/2 wave but that doesn't take ray distribution and diffraction into account.

 

David

[deSitter]

Also, Telescope-Optics.net at bottom of p4.1.2 in the white box gives the example of the 6'' f8.15 sphere having 1/4 wave under-correction at best focus. 

The focus of smallest blur suffers 1/2 wave but that doesn't take ray distribution and diffraction into account.

 

David

Thanks, I don't have Tex so can't follow his argument. I suspect this is a matter of definition. I would like to see the actual argument.

 

-drl

[apfever]

The wave front will be twice the error on PV as the mirror surface. A 1/4 wave mirror surface will create a 1/2 wave front.  This is where most 2X errors occur. 

 

The mirror surface isn't a problem. It's the "wave front" that some don't quite get, including me. Imagine a flat sheet of paper as the wave front. It hits the mirror first at the edges and continues to travel into the curve. The reflected sheet will be shaped like the mirror surface but stretched out two times longer. The curve will be twice as deep (Sagitta), and every mirror imperfection will be twice as long.

 

The other possibility is that the incoming sheet of paper is shaped like a perfect paraboloid to the mirrors specs. It would hit the mirror simultaneously everywhere. The reflected sheet (wave front) would be the same shape (Sagitta not doubled) but every imperfection would be doubled in length. 

 

You photo guys are already on your own. I think us visuals have more leeway in how we envision a best image than a lot of the miniscule numbers like best focus, corrected focus, best Strehl, and all that.

[deSitter]

The wave front will be twice the error on PV as the mirror surface. A 1/4 wave mirror surface will create a 1/2 wave front.  This is where most 2X errors occur. 

 

The mirror surface isn't a problem. It's the "wave front" that some don't quite get, including me. Imagine a flat sheet of paper as the wave front. It hits the mirror first at the edges and continues to travel into the curve. The reflected sheet will be shaped like the mirror surface but stretched out two times longer. The curve will be twice as deep (Sagitta), and every mirror imperfection will be twice as long.

 

The other possibility is that the incoming sheet of paper is shaped like a perfect paraboloid to the mirrors specs. It would hit the mirror simultaneously everywhere. The reflected sheet (wave front) would be the same shape (Sagitta not doubled) but every imperfection would be doubled in length. 

 

You photo guys are already on your own. I think us visuals have more leeway in how we envision a best image than a lot of the miniscule numbers like best focus, corrected focus, best Strehl, and all that.

You are concerned about an object known in differential geometry as an "envelope". You find surfaces that are everywhere normal to the EM field momentum vector. Those are the wave fronts.

 

-drl

[DAVIDG]

 You are not taking into account were the best focus is. You need to look at the optical paths difference  So both Texereau and OSLO are doing this and calculating the actual wavefront. The Strehl ratio  from OSLO is showing this that the wavefront at the eyepiece is just below a 1/4 wave at 0.82 while while 0.8 is exactly 1/4 wave. 

 

             - Dave 

[apfever]

You are concerned about an object known in differential geometry as an "envelope". You find surfaces that are everywhere normal to the EM field momentum vector. Those are the wave fronts.

 

-drl

That I can deal with.  Then an incoming wave front is a flat plane and a reflected wave front is spherical. 

[davidc135]

If a 6'' f8.15 sphere touches (equal radius of curvature) a 6'' f8.15 paraboloid at their centres then they will differ at their edges by 1/2 wave. Likewise if their edges have a common radius of curvature there is also 1/2 wave difference in sags at their centres. Images corresponding to the infinity foci of both edge and centre of the sphere suffer 1 wave.

 

If the sphere and parabola each share the same RoC at the 0.707r zone then the maximum deviation between both curves is 1/8th wave at both centre and edge, hence 1/4 wave at best focus of the sphere. Or the sphere can be visualised as touching the same paraboloid at centre and edge in which case there is a maximum surface deviation of 1/8 wave at the 0.707 zone.

 

The 1/2 wave error for the disc of least confusion focus corresponds to the two curves sharing a common RoC at the 0.866r zone.

 

My understanding of wavefronts is pretty shaky.

 

David

Edited by davidc135, 06 December 2024 - 05:29 PM.

[DAVIDG]

 Here is a spot diagram calculated with OSLO for 6" f8.3 sphere. The black circle is the size of the Airy disk so it shows how much light comes to focus inside the Airy disk. As the spot diagram , Strehl ratio and PV wavefront results show the result is a wavefront that is just a little better then 1/4 wave.  Again this is why Criterion decided to make the RV-6 with  a 50" focal length 6" mirror. They understood that if they just made a smooth surface from a sphere to have any small amount of correction and the you had diffraction limited image.  So they didn't need to spend a lot of time testing and figuring. A quick look at the mirror with a Foucault test will show if the mirror was "OK"  and that is what they were doing. Time is money so they found the quickest and cheapest why to make their optics to keep cost down and profits up. 

 

               - Dave 

 

[deSitter]

 Here is a spot diagram calculated with OSLO for 6" f8.3 sphere. The black circle is the size of the Airy disk so it shows how much light comes to focus inside the Airy disk. As the spot diagram , Strehl ratio and PV wavefront results show the result is a wavefront that is just a little better then 1/4 wave.  Again this is why Criterion decided to make the RV-6 with  a 50" focal length 6" mirror. They understood that if they just made a smooth surface from a sphere to have any small amount of correction and the you had diffraction limited image.  So they didn't need to spend a lot of time testing and figuring. A quick look at the mirror with a Foucault test will show if the mirror was "OK"  and that is what they were doing. Time is money so they found the quickest and cheapest why to make their optics to keep cost down and profits up. 

 

               - Dave 

 

6 f8.3 spot diagram.JPG

Yeah that's not great - I'm sure i could see this.

 

So to clarify - when Brown says 1/2 wave he means that when the centers are tangent, the sphere and paraboloid differ by 1/2 wave at the edge, while if they intersect at the 70 percent zone, the center and edge are 1/4th wave closer and farther away respectively. Do I have that right? So peak to valley is still 1/2 wave.

 

-drl

[mfoose]

 If that was my spider with the broken end I would repair it  by soldering a thin brass strip over the broken end and re-drilling the hole in it. I believe that will be easier then trying to replace the whole vane.

    The vanes should be brass as well so soldering a thin piece of brass over the end will be no problem and very strong.

 

              - Dave 

Thanks for the suggestion, Dave. I will keep this in mind when I restore it in the coming months. 

 

I am also interested in testing the optics. I don't have much experience doing a star test and skies here a in the Upper Midwest are pretty turbulent this time of year. How hard is a Foucault test?

 

Had first light last night. It was brief as we had a friend for dinner, but got to show them the Moon and Jupiter. After they left I got a quick peak at M42. Wanted to stay out more, but I had to be up early. The sky was clear, but turbulent. I used a 24 Panoptic. It was looser than I liked in the focuser. I think the undercut in the barrel caused it to not have as much surface area giving friction. I tested an eyepiece with a barrel that has no undercut and it stayed in better. 

Attached Thumbnails

•