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# 6in Dall-Kirkham

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### #1 Matthew Paul

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Posted 30 June 2017 - 09:47 AM

I started grinding on a new 6" mirror last week. I had got a borosilicate float glass disc a while ago with the intention of involving the significant other in making a newtonian telescope as she showed interest after Stellafane 2016. Long story short, I rough ground it to an F4 with the intention of making a small travel scope. I have always wanted to have a go at making a Cassegrain scope, so I figure why not give it a go. I used a calculator "Originally written by Jean Texereau. Translated and modified by Will Remaklus. JavaScript version by Tomáš Maruška 26-11-1999. Removed bug in computing, added 4 decimal places into output 4-8-2006." and came up with the following. As it sits, the only thing that I am unsure of is if 8" of backfocus will be enough to use a star diagonal at the focuser.

I'm not brilliant with mathematics as the furthest I ever got in school (with math) was 10th grade, but I try to learn/relearn as I go. To offset this lack in mathematics I do have an extensive background in mechanics and fabrication. I am a backsmith by trade with some machine experience, and was a mechanic for General Motors in a past life - figuratively speaking. I've so far built two telescopes, one 3" f10 in which I did not make the optics, and one 6in F8, that I just finished the optics on.

I plan on coring the primary 90% of the way before I go past 120gr. I'm thinking 1.25" hole in the primary. The secondary I plan on grinding from 3" rounds and also coring 80-90% of the way, and I'm thinking of slightly oversizing it to 1.5" to allow for some movement, and dealing with the little bit of extra light loss and diffraction.

I'll attach photos of the potential physical measurements on the scope, as well as the calculations on the optics. I'm planning to figure the primary through traditional methods using the Foucault knife-edge test. The secodary I plan on testing by checking the tool with a Foucault knife-edge test, and the mirror to the tool using a simple Fringe tester. Then, checking the whole system with autocollimation, and star testing. I can silver the mirrors in shop, so silvering the secondary for testing is a non issue.

We will see what happens. It might be a failure, it might be a sucess. However there is no way to know if I can or can not succeed if I do not attempt it.

I just need to figure out if the back focus is sufficient before I can start on the secondary.

It would be wonderful to hear your thoughts on the subject.

Thank you,
Matt

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### #2 DAVIDG

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Posted 30 June 2017 - 10:12 AM

I double checked your numbers and  yours agree exactly with what I got. The conjugate foci for the ellipse are 25.77" and 349.23" if you want additional test to double check your Foucault results.

As for the secondary, you might want to consider grind two larger disks , say 3" in diameter and then once you get the concave curve ground into the larger disk,  grind the smaller one that you plan to make the secondary from into it. I have done this a number of times and it is easier to control the smaller disk. Also you don't need to do a complete polish on the larger concave disk to use it as a test plate and you won't have to worry about a turned edge on it as well since it will be larger in diameter then the secondary your making.

For coring the primary I recommend diamond coring bits from THK Tools. I have used them many times and they work great and are very inexpensive. It will take maybe 15 minutes  to core your blank with one. http://stores.ebay.c...?_fsub=15599558

- Dave

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### #3 Matthew Paul

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Posted 30 June 2017 - 11:48 AM

Thank you Dave. The conjugate foci is a useful bit of information to have.

Do you think that I should figure in some additional back focus?
The cell and mirror will take up 3.5" and the focuser (That I have) will be 3 1/4" to 5 1/4"
I'm guessing from what I have seen that a diagonal will need about 2" of back-focus?

So, that would be 8 3/4" racked all the way in, so I would not be able to use that focuser with a diagonal, and the focsuer is a plastic rack an pinion pile of junk anyway.

If I switched to a flat base JMI Mini1, the minimum height would be 1.9" , if I left an additional .6" of inward focus to allow for some cell adjustment and material spacing, plus the cell space of 3.5", that would give me a total of 8", if the diagonal does indeed need 2in of light path - which will be perfect with the 8" of back focus that I figured in. The JMI focuser has a travel of about 1.6", so with the .6in of extra inward focus that would leave me with 1" of focuser travel, and I can always space the focuser off the cell if I find that I need more back focus.

Does that seem correct? Seems like I probably just answered my own question...

### #4 DAVIDG

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Posted 30 June 2017 - 12:18 PM

Hi Matt,

Here is a  link to an old Coulter Optics catalog and they made Dall Kirkham sets. All their sets had back focal lengths of 11" and longer so 10" might be better then your original 8"

- Dave

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### #5 macdonjh

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Posted 30 June 2017 - 12:36 PM

Typically a 2" diagonal has a 100mm (4") optical path length.  I can't remember the OPL for a 1 1/4" diagonal right now.  I'm pretty sure I asked Tele Vue when I needed to know.  They are friendly and quite helpful.

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### #6 Matthew Paul

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Posted 30 June 2017 - 12:56 PM

Fantastic. I just got off the phone with them. The 1.25 diagonal has an optical path of about 2.5" So that would use a total of 7.9" of the 9" back focus leave me 0.1" of additional inward focus. I think I should figure on an additional 1/2" of back focus and re work the math. Shortening the cell by 1/4" would also be easy enough.

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### #7 Bomber Bob

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Posted 30 June 2017 - 07:50 PM

3B made the mirrors for my 4" F15 D-K.  At optimal spacing the back focus is 7.25" -- pretty tight!  I use a short bodied helical focuser, and vintage 1.25" prism diagonals, and the range is just enough to use all my 1.25" Orthoscopic eyepieces.  To use my spectros .965" Plossls, I can't just use a reducing adapter, I have to swap in a .965" prism diagonal.  I experimented with moving the primary about 5mm closer to the secondary to increase the back focus, but the image quality suffers.

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### #8 Matthew Paul

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Posted 01 July 2017 - 11:29 AM

I partially perforated the blank this morning. I left 5/32" glass to be bored still. The hole appears to be wildly off center when viewed head on, and that made me not happy. However I measured several times and it is .015" off center on one axis, and centered on the other, which I think will be just fine. I used a carbide tipped hole saw as it was far less expensive than a diamond coated one, and I used 60gr silicone carbine abrasive and water inside of the putty dam. It took about 30 minutes to reach the 1/2" mark. That includes making a pot of coffee and taking pictures.

I'm not super happy with it being off center like it is, but considering how I did it, I suppose it's not that bad.

#### Attached Thumbnails

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### #9 Matthew Paul

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Posted 01 July 2017 - 11:31 AM

Here is the "finished" work for today, and a short video.

I've got a few rounds in at 120gr, so far - with a 24.5" focal length.

Edited by Matthew Paul, 01 July 2017 - 11:32 AM.

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### #10 DAVIDG

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Posted 01 July 2017 - 04:40 PM

The secret to getting hole centered is to rotate the blank as your boring the hole. Any off centering of the bit gets averaged out and you just bore a slightly larger hole it  also averages  out the straightens of the walls of bore if the blank isn't sitting perfect flat on  the drill press table or the table isn't perfectly 90 degrees to the bit.

When your done and core the plug out, you can put a professional looking bevel on the hole by taking a glass bottle placed in the hole and grinding with some 320 grit.

- Dave

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### #11 Matthew Paul

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Posted 02 July 2017 - 08:25 AM

Thanks for the tip Dave.

Last night I finished up with 120 gr, and got started on 220. About an hour and a half into 220 gr I gave a quick polish with some finer grit and put it on the test stand. 48.25" Radius of curvature. So, I'm still about .125" over my ideal focal length. The last mirror that I worked on I over shot the focal length by a lot, being mostly careless from not knowing any better. My first goal on this project is to hit the focal length as dead on as I can, so I don't have to go fudging numbers around later on. Being able to accurately hit a focal length when finished will pay off later on with other scopes as well.

Back to grinding....

Edited by Matthew Paul, 02 July 2017 - 08:28 AM.

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### #12 Ed Jones

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Posted 02 July 2017 - 09:12 AM

To center the hole cutter I've always cut a hole through a piece of regular window glass and then take the center plug and pitch it on to where you want the hole.  As the pitch cools you can move it around and center it exactly with calipers.  This plug will then act to keep the cutter centered.  Also you can find diamond hole saw really cheap on Ebay but they are usually in millimeters.  They cut much faster.

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### #13 DAVIDG

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Posted 02 July 2017 - 10:39 AM

When cutting the hole you also need to be sure that the drill press table is square to the bit and the blank has surfaces that are flat and parallel to each. If  not the hole will be bored at an angle and come up the bake side off center.  By rotating the blank as you bore thru, these issues will get averaged out and you'll bore  a straight hole that is well centered.

The THK diamond coring bits are very inexpensive and don't require addition grit, just a little water and they cut very quickly. They are also great for cutting out the secondary from a large piece.

- Dave

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### #14 Ed Jones

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Posted 02 July 2017 - 07:47 PM

I forgot to mention that when you are ready to finish cutting through pitch this plug to the front side for safety.  The cutter can't touch the curve until it is centered.

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### #15 Matthew Paul

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Posted 04 July 2017 - 03:50 PM

I'm about halfway through 25 micron, and exactly at 48in ROC, which is nice. Last mirror I over shot the ROC, this time I left 1/4" over for the start of fine grinding.

There are a few pits from larger grits or bubbles in the surface, scattered randomly about. We'll see what happens as I do another hour on this abrasive. Worst case I step back a grit or two, if there are just a few after this set I'll just leave them.

### #16 Matthew Paul

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Posted 14 July 2017 - 04:10 PM

From what I can see, all but five of the pits/bubbles are gone. The focal length has shortened to 23.9" so I'll need to do a couple tool on top wets to bring it back, and then I can move back on my way through the grits.

In the coming weeks I am hoping to get the substrate for the secondary. I still have not decided on how I want to do that yet, without spending a ton of money. I could get plate glass locally and inexpensively but Id rather not. I could get borcalite discs from McMaster for \$30 each in 3/8" thickness, 3" rounds, that I would core the 1.4" secondary from.

I could get two 1.5" x 3/8" discs from McMaster for \$13 each, not have to core anything, and try like hell to not get any edge defects.

Or, I could get a Borosilicate test plate blank measuring 3" and 1" thick from FirstHand Discovery for \$20, and the will be mirror blank from ether McMaster for \$30. The McMaster glass says that it meets MIL Spec G-47033.
.

Another option would be 2" x .25" BK7 Discs from Ebay for \$12 each, which would work fine and allow me to still core the lens if I were to embed it in pitch, maybe...? that would prevent edge defects, but I'm not sure what effect the pitch would have on a finished mirror after taking it off and cleaning with turpentine. None? Stains? I might be able to use the pitch lap to support the substrate and carefully clamp the 1/4" edge? This would probably be the best option financially as I would be able to get both for \$24 shipped, and still eliminate the edge after grinding and polishing.

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### #17 Matthew Paul

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Posted 20 July 2017 - 03:18 PM

I just started on 15 micron, did half an hour last night. I figured this afternoon I would cast a tool and pour the pitch lap so that I could do a quick polish and accurately check the focal length before I got too far. As it sits now I'm 1/32" shy on the focal length. Hopefully some tool on top fine grinding will bring that back by the time I finish up.

I had to return to 220 gr from my previous 25 micron to remove those pits. It looks nice now and Ill continue to fine grind then polish.

Edited by Matthew Paul, 20 July 2017 - 03:20 PM.

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### #18 Matthew Paul

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Posted 24 July 2017 - 07:18 AM

I finished up fine grinding last night and started polishing. I've got about one hour into polishing and the mirror forms an image. Still many pits to polish out but it is polishing much more evenly from edge to center than the last mirror that I made. I thought that I used Gugoloz 73 on my last mirror but I guess that I did not. Whatever I used on the last one was very hard and did not flow much at all, and was clear. This 73 feels and acts just right on this mirror, so it seems. We'll see how it works figuring the fast mirror. Ive never done a fast mirror but I did accidentally figure my 6inF8 like an F4 and f6 while learning how not to do that :O

It should be good. I'm very exited. Things are starting to come together. Grinding is such a drag, figuring and building the scope and testing is the fun part for me.

Edited by Matthew Paul, 24 July 2017 - 07:19 AM.

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### #19 Matthew Paul

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Posted 28 July 2017 - 12:31 PM

The mirror is polishing out nicely. The focal length is a bit off on the primary, by 1/16" But, I did some tweaking in OSLO by modifying the conic of the primary and spacing of the secondary, and also shortening the ROC of the secondary slightly to a more measurable number. In addition to gaining some more back focus that I was after, the Longitudinal Spherical Aberration has improved a bit as well. The trade off was the smallest amount more of astigmatism, but nothing much to speak of. This is all assuming that I can properly grind and polish the secondary, and get the spacing to be exact when assembling the scope.

The back focus is not around 9 3/8" instead of 8"
The ROC of the secondary is set to 13.25 instead of 13.3333333"

I'll attach two photos, one is of the original design, and the second where I am at now. I think that it will be a good idea to grind and polish the secondary before figuring the primary, as I may need to adjust the conic and spacing to match any departure from ideal in the secondary ROC.

### #20 Matthew Paul

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Posted 25 October 2017 - 11:43 AM

Life has become crazy, but I should be back at polishing the primary in a couple weeks, and then on to grinding the secondary. hopefully. Just figured Id check in and say Hi.

### #21 Matthew Paul

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Posted 30 June 2018 - 10:33 PM

It's been a while, but here is the secondary and tool / test plate ground to the approximate focal length. I'm working 220 now.

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### #22 macdonjh

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Posted 01 July 2018 - 08:34 AM

Good luck.  I assembled a classical Cassegrain and it took me three years of off-and-on effort to finally get it to work.  And I bought my mirrors.  I am impressed you've been able to do so much glass work in a year.

### #23 Matthew Paul

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Posted 02 July 2018 - 10:29 AM

Thank you. I was hoping to have it finished by now, but life sometimes gets in the way of living.

### #24 Augustus

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Posted 02 December 2018 - 08:24 PM

### #25 Matthew Paul

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Posted 31 December 2018 - 01:27 PM

Not yet, business is booming and time is short.

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