RAC

8 hours ago, zfedoran said:

The following is a comparison between a stock 20D and a debayered canon 20D using the same lens, both at the same distance and same settings. The color image has been desaturated, but the levels/curves have not been changed. 

It looks like there is a minor loss in QE and a gain in resolution sensitivity. This would make sense because the micro lenses have been removed.

Well that is very interesting, the color one is horrible to look at compared to the mono one. The color one looks brighter but there is a lot better data in the mono image and way less noise?

Wow looking really nice. I really like the carbon secondary holder, similar to my ones I make.

Have you thought about motorizing the scope. I'm doing that atm with my 20" dob. I'm going down the Onstep road as any other option just costs way too much.

On 19/02/2016 at 16:07, MattC867 said:

Good to know.  I ended up deciding to put the camera back together without the springs.  I just screwed the sensor assembly tight against the body.  I'll keep the screws around in case I need to make any adjustments.  Have you had any issues?

I only even removed the IR filter with my 60D and that worked fine but I ended up selling the camera before really using it. The only camera I ever made mono was the 1000d in the first post of this thread.

My 60d had springs.

That mirror looks really nice. Should be good.

Any updates on the mirror?

What a fantastic thread, which I've been following for some time now. I've had a 350d sitting around for a while, feeling sorry for itself, so I thought I'd treat it to a CFA removal. Thought it'd be helpful to share my experience.

I performed the standard IR/UV removal on the 350 as a start, which went very smoothly (apart from the  pain of desoldering the protection plate). Disassembly of the sensor itself was fairly easy. I didn't find removing the glass in front of the sensor too hard. I placed some insulating tape on the glass itself in case of an 'incident' while removing it to avoid any glass shards dropping on the sensor. I then gently slid an old style razor blade under the edge of the glass and it came off in one piece.

On to the microlens and CFA removal. I'd decided to use the sharpened edge of a 00 paint brush for removal. The microlens layer came off very easily and took about 10 mins. The CFA is a lot tougher though. I had to start off the CFA removal with a flat head screwdriver (exerting very little pressure), just to get a tiny score in the CFA. Once the score was made, the paint brush end did the trick but it took ages (about 1.5hrs) to fully remove the CFA.

This is the final result once I reassembled the camera. The whole operation took approx 5 hrs. I also used a cheapo USB microsope to check the sensor. I found this very useful as it clearly showed me the layers I was removing.

Good job. Now get out and use it! Good to see someone not killing their camera lol.

Ah I see wonder how you were removing the glass.

Obviously as the first part of the thread describes I did it all by hand and have to admit I will never go that way again. It definitely gives you an appreciation of why machines are used.

It was only the 25 that came with (some of) a curve already in it. Both my 20" and 18" were done by hand just like your one although I do have a powered spinning table and I wouldn't want to make a mirror without it. I really don't want to hog out the 30" by hand as it's going to need an 18mm sagitta!

Anyway keep it up as it's awesome viewing with a large mirror you have made yourself !!!!!!!

If I were to even consider making a fast mirror 25", or 30",  the blank would need to have a pre-generated curve  .Far too much glass to hog out from 

a plane surface  .

Ron.

Yep it's a lot of work. The 25" came to me as an unfinished f5 so I only needed to hog it out a few more mm's to get to f3.8. There is no where here in NZ that can pre generat a mirror sadly. For the 30" I'm going to make something using a grinder to get most of the glass out.

Big good flats are never going to make it into my budget sadly. I have no doubt it's a very good way of making a good mirror but it's not going to happen in my shed

The 25" is not for me but the person I have been getting the glass from. He made a deal with me, I make him a fast 25" mirror and he'll give me a 30" blank with a secondary to suit and all the grit needed. The 25 has taken a good amount more time than the my 20" and has been more difficult with every step.

I think your man John was interested in buying 2 of these 30" blanks I now have but the cost of getting them over there was too much.

I do use the Ronchi test all the time. It's great and has it's uses but it's very easy to do the wrong things with it and it's hard to tell the exact shape of the surface. With a 20" f5 it would be about all you would need but not with these fast mirrors and even then I would use Figure XP. I wouldn't want to be figuring a mirror with the need to transport the mirror a long way just for testing as there is a need to test so often.

Setting up a good foucault tester isn't hard when you use a spare focuser. I don't find it too subjective to use because I make a point of never remembering the required numbers for each mask zone, this means my brain will never try and target the right numbers even when they may be off. I must have a bad short term memory.

Please please tell me you are going to use Figure XP.

I'm currently figuring a 25" f3.8 and I'm about a 1/4 the way in and with Ronchi only I'm lost so todays plan is to make a mask and start using Figure XP and when I do that I will be unlost and I will know what my next move will need to be.

I have been reading this with interest and I've every admiration for the perseverance and dedication shown on this project, it would have defeated me long ago!

One question I have is how commercial telescope makers overcome mirror issues such as turned edges, central mounds/hollows etc. Is this dealt with on the basis of computer controlled grinding and polishing, where pressure, stroke and speed is governed very carefully to a specific program?

Wishing you all the best for a succesful project completion!

For grinding and polishing the fixed post method would be the most common but I maybe wrong. It's easy and it works great. If I was to put Damian's mirror as it is on my fixed post machine with a 70ish % sized lap I would say you could have it back in good shape right to the edge in about 3-4ish hours so long as the lap was in good shape and playing the game.  When using a fixed post setup the edge never really is a problem if your offset is right, In most cases it will leave a small hill in the middle but normal figuring strokes take care of that anyway.

You can cut through a lot of glass with ceruim some weight a good lap and a fixed post grinding machine.

Figuring is a little different and requires constant testing nearing the end so most would do this by hand.

I would like to here from anyone how has tried one of these with a fast scope(f3-f4) with a Paracorr.

Yep its a wire spider. I've never used one before so thought I'd better try one.

Yeah I just think the square trusses are more easy to mount and work with. No other reason.

Mmmm. With the looks of that I would be making a full or near full sized lap and take it back a few steps.

I'll say it again and it will save you a lot of time, You really need to make couder mask and start using Figure XP! You don't need a camera as you will be getting the numbers visually.

Chasing Ronchi curves on large fast mirrors is NOT going to happen. Making a 10" f6 would be fine but not this mirror.

Once you start using Figure XP you will see the light.

I had my first light for the 20" two nights ago and it is totally awesome, it star tests perfectly and the view is amazing! I could view the moon at 700X and the detail was stunning!

I still need to finish a few things on it.

It's amazing how much work goes into the dob structure of one of these scopes. I'm making mine for my 20" but the work just doesn't seem to be stopping and I haven't even made the base yet.

It's time to start using Figure xp. I wouldn't personally take one step more without using it.

It'll bring a huge amount of knowledge to you about the exact shape of the mirror and where to work next.

Hows it going?

I noticed you haven't pointed the edges of the lap as this helps with blending of the laps action on the mirror.

I've always liked the idea of plaster tools as it is very stable and will never go out of shape. What I do is take a mold of the mirror at the end of rough grinding then take a mold of the that mold so I have a plaster model of the mirror to mold any laps on top of.

One thing to note If you plan on using Figure XP is that apparently it doesn't use the correct math if you select and use the fixed light source option. Only the moving light source setup is correct. Not sure how true that is but the moving source is easy anyway.

My 20" I just did was all done in about 10-12degC with very hard pitch. Never once did I have to re cut the channels or scratch it up apart from scrub it with a brass brush. This was for the polishing lap and the figuring lap. When I did the 18" the channels would only last an hour or two. So there is a lot of give with what works.

The best canon lens I have used for imaging with foucault is an old cheap canon 75-300 zoom. I have a canon 15-85 and also a 200mm f2.8L but those are no good for some reason.

This is how simple it can be. A focuser on an old engine flex plate and a dial gauge touching the focuser tube. The flex plate sits on three points. Two are bolts machined to points and the third(the brass one) is a thread with a hole down the middle so it can rest on a ball bearing, this keeps it very smooth as this adjustment effectively moves the knife edge across the mirror. I have the return beam come back about 10mm below the LED. When using a camera all you do is place it on a tripod behind with the center of the lens looking down the beam. The front lens of the camera is about 10mm behind knife edge. The camera does not need to move with the knife edge, it stays on the tripod.

You need to have the focuser very square to the mirror but this is easy, you know when it's not as when you move it out or in the mirror won't null out evenly with the couder mask.

The radius of Curvature is very easy to find with the foucault test as all you do is null out the center of the mirror and measure from the knife edge to the center of the mirror. Enter that into Figure XP and away you go.