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Old 22-08-2021, 09:30 AM
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Making an oversized pitch lap.

Hello,

I am about to start my 1st lap and have a question in regards to the shaping of oversized laps.

Reading Mel Bartels website, I have the following:
"When working an oversized lap, move the mirror around every few seconds as you can see in the second image." - the image shows a MOT with an overhang of about 1/4(or less) of the diameter of the glass.

My question is, am I picking up the mirror and repressing or am I sliding the mirror around every few seconds similar to the smoothing strokes to define my edge? Do I press till I get a rough shape and use the smoothing stroke for the finish before it firms up?


Thanks
Steve

Last edited by mura_gadi; 22-08-2021 at 09:46 AM.
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Old 22-08-2021, 12:52 PM
Rod
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Hi Steve

I’ve used oversized laps a couple of times. They work well to prevent or reduce a turned edge. But they are tricky to press evenly. Mel suggests warm pressing by warming up the pitch tool and mirror in warm water. You then move the mirror around over the tool as everything cools. I tend to use a circular stroke, making sure the mirror moves over all sections of the lap.

For me a tool that is about 6 mm larger than the mirror works well. I tried a larger one once and found it almost impossible to get good contact.

A normal lap the same size as your mirror is ok too and easier to press. If this is your first mirror, there’s probably a good argument to use that - it keeps things simple. You can always make a larger lap later if you need it.

Hope that helps,

Rod.
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Old 22-08-2021, 02:42 PM
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Hello,

Thanks for the reply.

I had hoped to have some Hydrostone by now, but either travel restrictions or price of postage has kept me from getting any!

I have an oversized tool already, (200mm blank and a 220mm tool - both soda glass) I was thinking of making the lap from the tool. I have found using the oversized tool very easy so far on my 8" F9. (nothing to compare it to either though)

The 220mm will present a fairly large overhang... is it too large? Should I consider making a 210/205mm plywood lap? (The wood and sealants are not a problem...)


Thanks
Steve
Ps. Have you tried attaching pitch squares over a full pitch lap with pressed grooves?

I am reading "How to make a Telescope" by J Texereau and he makes the suggestion of fixing squares on, over a fully covered lap and pressing the grooves in. The squares seem to allow easier trimming and for the lap to be easily modified by changing the pattern or reducing/increasing the squares in any particular zone.

But certainly not in favour anymore as you never seen a webpage using anything but a fully pitched lap. It seems like a good idea, but I'd like to know why it fell out of use with ATM'ers.

I see Mel creates a full pitch lap and then a secondary pitch lap using the "Pitch lap Calculator". I would prefer 1 lap as I have limited rosin to work with and it would be nice to use the modifiable pitch lap squares idea.

Last edited by mura_gadi; 22-08-2021 at 03:20 PM.
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Old 22-08-2021, 04:06 PM
Rod
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Hi Steve

You can use Plaster of Paris or cement. I often mix cement with plaster. The plaster helps it dry a bit quicker. If you are in a hurry casting plaster might be an option. For an F9, a flat surface is probably ok as the curve is so shallow. So a flat piece of thicker glass, metal etc would be ok. Normally you use the oversized tool for polishing not grinding. If it passed the sharpie / pencil test it’s probably fine though.

Personally I would use a smaller lap. I think others have used something your size successfully but as I said I found it hard to get good contact, leading to a rough surface. So I would try about 206 mm. Plywood might be ok but it would need to be very well sealed.

Are you referring to Texerau’s method of making individual pitch squares and attaching them to the tool? You can do it that way but it seems like a lot of extra time, work and more pressing than other methods. I don’t see any advantage in the extra effort. It just looks neater which is unnecessary. Gordon Waite has some good videos, one of them is on making a pitch lap. I like to use a mould to make mine eg. a piece of a laundry basket with suitable size square holes. I cover it with soapy cerium oxide and water then press it into the warm pitch to make the grid pattern.

Mel uses more than one pitch lap because he makes ultra fast mirrors - sub F3. For an F9 one lap is all you need.

Rod.
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Old 22-08-2021, 04:57 PM
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Hello,

Thanks again Rod, do you seal the Plaster of Paris(PoP)/concrete mix? What sort of ratio are you using? I assume more concrete than PoP and you shouldn't need a sealant?


Thanks
Steve
Ps. I was referring to Texerau’s method of making individual pitch squares, just seemed an easy way to change the lap if I wanted to change the polishing across any of the zones.

Yes, the sharpie test works well, but that's all I have done and the wet mirror test for F/l of course. No Ronchi/Foucault testing till I get the first polish on and built a test stand etc. Just focusing on strokes and any uneven progress in smoothing between the zones, which so far there hasn't been.

One reason for the F9 was bugger all difference in spherical to paraboloid and only 1.41mm of glass to remove. Also to see if the refraction spikes are reduced nicely for what is essentially a star splitter/cluster/planet sort of project.
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Old 22-08-2021, 06:16 PM
Rod
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Hi Steve

The actual mix isn’t crucial. Roughly 75% concrete should be OK. If you are in a hurry a glazier might cut a rough circle in 12 mm glass for you and you could use that. You might need to smooth the edges with a carborundum stone.

Changing the lap is no easier / harder no matter which way you make the facets. At F9 you shouldn’t need to tinker with the lap anyway.

Not sure what you mean by refraction spikes?

F9 is a nice choice for a first mirror - if you can cope with the long tube. My first mirror was a 6 inch f10 sphere. In retrospect it was poorly made but the tolerances were so loose that I got an acceptable result. I had a lot of fun with it. I remember being astonished that it actually worked when I put it together. That was nearly 40 years ago.

Rod.
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Old 22-08-2021, 07:15 PM
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Hello,

Sorry, diffraction spikes was what I meant, I liked the thought they reduce as you get slower in a newt. I seem to remember at F9 they should be nearly eliminated, a feature I thought would be nice visually for clusters/planets etc.

I'll make a 205/6mm or so lap with the concrete and PoP tomorrow. The cure time will be a good chance to get started on the other test aspects of this endeavour.



Thanks again for your help and input
Steve
Ps.
The whole F/L idea is based on 1800mm roughly as I am 6'1"+ and closer to 6'4" in the moon boots. I wanted a F/L on a Dob that after the base was added and the eyepiece at close to apex I wouldn't need more than 1 foot step to use the scope. (Box with a pole on one side sort of step/support)

The starter is 8" at F9, the final a F5 16" (F4.5 18" if I can get away with a 25mm thick mirror) and a 12" in between to help learn how to work a steeper bigger mirror prior to the 16"/18". I wanted to see if I like the 1800mm F/L prior to committing to the 16"/18".

The scope building starts with my tester stand, so, the 1800mm maybe scrapped sooner... I'm hoping to get a cheap 2nd hand 8" GSO to strip parts from to build the mirror tester on a plank sort of thing for the star testing.

Last edited by mura_gadi; 22-08-2021 at 07:29 PM.
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  #8  
Old 22-08-2021, 07:40 PM
Rod
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Hi Steve

Diffraction spikes are inevitable with a conventional spider no matter what focal ratio you choose. If you want to get rid of them, think about a curved spider or a wire spider. The most effective solution is an optical widow but that is a lot of work and probably expensive.

For testing, I suggest you think more about building a Foucault / ronchi tester. You can star test with your finished scope prior to Aluminising.

Good luck!

Rod
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Old 23-08-2021, 05:01 AM
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Hello,

I have been looking at what is needed for both Ronchi and Foucault tester. The Ronchi eyepiece is the cheapest option and use with a star test. I need the dial indicator (Foucault) or a decent Ronchi screen, both are seen as expensive within my budget. (i have no budget atm tbh, so, its galvanized pipes and elbow bends if I wanted to setup a EQ pier out the back sort of attitude.)

The comment about the diffraction spikes comes from J Texereau again and tied in well with the plans for the F9 8" using a small secondary.

"At any rate the effect in the Newtonian reflector almost disappears if we make the focal ratio 8 or 10 and the diagonal mirror only large enough to accommodate the incident light cone"

Mind you he goes on to quote a secondary of about 1/8th of the primary, so, I need to get my head around the science of that statement. The effects of the obstruction on altering the diffraction figure are still a little beyond me atm. But I like reading...


Thanks again, your comments have been appreciated,
Steve
Newt for the web has my secondary at 31mm with a ID of 9.01inches, J Texereau seems to suggest closer to 25mm for the secondary at this focal length.
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Old 23-08-2021, 08:49 AM
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Hi Steve

I think you are confusing spider spikes with the obstruction caused by the secondary itself.

People argue about how much secondary obstruction causes a noticeable impact on performance. Most would agree that an obstruction of less than 20% - eg 40 mm secondary for a 200mm primary causes very little impact.

You’ll notice Texereau suggests an optical window for a specialist planetary cassegrain with a small secondary. The focal ratio that he suggests for a classical cassegrain is F20 or higher. Part of his reason for the window is to remove the spikes that are always visible independent of focal ratio.

Yes affordable ronchi gratings are hard to get now. You could make a rough one by printing one on an overhead transparency. I know there is a file on Stellafane you can use to do that. There may be other sites with files too. I might have something you can use. I’ll have a look and get back to you.

You can do most of the testing with just a ronchi screen and a suitable light source. Much more convenient than a star test. I think most people use the star test for final figuring and confirmation of bench test results.

Rod
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  #11  
Old 23-08-2021, 12:51 PM
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Hi Rod,

"I think you are confusing spider spikes with the obstruction caused by the secondary itself. " More than likely...

"I think most people use the star test for final figuring and confirmation of bench test results."

Yes, from what I had read that is true, I was hoping for such a shallow mirror to be fairly close after the 1st polish though. I did take the stellafane ronchi file to Office works and got 10 copies, all were fairly rubbish. I will have to try and track down a better printer. I have glass slides as well.



Steve
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Old 14-09-2021, 05:03 PM
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I made much better progress with figuring my big mirror once I stopped trying to use the ronchi test for figuring analysis and built a stepper motor-moved foucault test rig from an old scanner bed and a long-throw (50mm) 0.01mm dial gauge. then I was able to get repeatable test numbers to plug in to the figurexp software which is fantastic.



Quote:
Originally Posted by mura_gadi View Post
Hello,

I have been looking at what is needed for both Ronchi and Foucault tester. The Ronchi eyepiece is the cheapest option and use with a star test. I need the dial indicator (Foucault) or a decent Ronchi screen, both are seen as expensive within my budget. (i have no budget atm tbh, so, its galvanized pipes and elbow bends if I wanted to setup a EQ pier out the back sort of attitude.)

The comment about the diffraction spikes comes from J Texereau again and tied in well with the plans for the F9 8" using a small secondary.

"At any rate the effect in the Newtonian reflector almost disappears if we make the focal ratio 8 or 10 and the diagonal mirror only large enough to accommodate the incident light cone"

Mind you he goes on to quote a secondary of about 1/8th of the primary, so, I need to get my head around the science of that statement. The effects of the obstruction on altering the diffraction figure are still a little beyond me atm. But I like reading...


Thanks again, your comments have been appreciated,
Steve
Newt for the web has my secondary at 31mm with a ID of 9.01inches, J Texereau seems to suggest closer to 25mm for the secondary at this focal length.
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Old 15-09-2021, 08:44 AM
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Sounds interesting Hamish.
Do you have an image of your test rig.?

I have some scannner stepper motors and belts lying around.

Quote:
Originally Posted by hamishbarker View Post
I made much better progress with figuring my big mirror once I stopped trying to use the ronchi test for figuring analysis and built a stepper motor-moved foucault test rig from an old scanner bed and a long-throw (50mm) 0.01mm dial gauge. then I was able to get repeatable test numbers to plug in to the figurexp software which is fantastic.
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Old 15-09-2021, 08:57 AM
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Hello,

The Ronchi was only the first test, I was going to move to foucault and figureXP later.

I see scanners are as cheap as chips, a little more info on how you went about your setup would be great. At this moment I was going for a dial indicator at 0.001 and $130 posted. Main reason being is that the figurexp goes to three decimal places.

The scanners are from $30 though.



Steve
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Old 05-11-2021, 07:05 AM
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Quote:
Originally Posted by Sunfish View Post
Sounds interesting Hamish.
Do you have an image of your test rig.?

I have some scannner stepper motors and belts lying around.
sorry for the slow response. it's under the house but will be dug out soon as my big mirror needs recoating so I want to improve the figure a bit first and will need the tester.
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