Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

6in Dall-Kirkham

  • Please log in to reply
127 replies to this topic

#51 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 02 June 2019 - 10:12 PM

I got the test plate done. A little surface roughness but its a really nice sphere. I wound up grinding with 5 micron again. Then I hot pressed for 30 minutes, did tool on top 1/3 overhang w stroke with a slight 1/6 side to side. It took about five 4 minute sessions and it was polished out far enough. The very edge is still a little dull but it does reflect light and I didn't want to press my luck. On top of that the secondary mirror is oversized by .200" anyway. 


  • MKV likes this

#52 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,779
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 03 June 2019 - 09:25 AM

 How many fringes do you see when the concave test plate is in  contact with the convex surface ?  You  need to see 5 rings or fringes or less to have enough sensitivity to determine the errors on the convex surface and that requires that the radii match pretty closely.  It has been  my experience that when polishing a convex surface on pitch that no matter if the lap is on the bottom or the top, the radius want to length. So what I have to do is make up two laps. One with a star pressed into it and one with the common grid pattern The star lap  just wears the edge of the convex curve and shorten the radius. When they get to a good match I then switch to the  pitch lap with the common  square grid pattern to smooth it out and bring to a sphere.  

   So my advice is start testing the radius match as soon as you get enough polish on the surfaces to test them and then keep watching them so they don't drift too far apart.

 

                         - Dave 


  • Matthew Paul likes this

#53 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 03 June 2019 - 01:00 PM

Thank you. I have not polished the secondary yet. I was just working on getting the test plate to be as spherical as I could get it. I'll do a quick polish on the convex mirror tonight or tomorrow and see how it matches up against the tool. That's a good idea with the star lap, if I find that they are far off Ill make one for sure.



#54 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 07 June 2019 - 08:13 AM

So this is where I am at right now. If I push on the bulls eye side, the bulls eye moves to the center. If I push hard in the center the rings move inward. If I push on any edge, the fringes move away from the pressure point. Prior to this last bout of polishing, the central lines were straighter up until about the 70% zone where the curved hard, like they do here. 

I'm not 100% sure what I am working with but I believe that I polished too much with with the star lap initially, and that I should continue with just the full contact lap to bring the mirror back in even contact with the test plate. 

There are no spacers, as there are too many fringes when using a very thin spacer of saran wrap. 

The next time that I grind a mirror like this I will paint the back of it several times to help avoid the scratches around the edge from rough grinding. you can see the fringes a bit better in person, but it's not ideal. 

 

IMG_9955-2.jpg



#55 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,779
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 07 June 2019 - 08:46 AM

 The fringes always moved towards the air gap that is getting thinner  when pressure is applied ie the glass surfaces are getting closer together, So if you push in the center and the fringes move inward that is saying the convex piece is flatter then the concave piece. 

  Since you have somewhat straight fringes  in the middle , the two radii are close but  the out edge of the convex piece is turned. 

 

                 - Dave 


  • Matthew Paul likes this

#56 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 07 June 2019 - 09:41 AM

So, if I am understanding what I am looking at, and what you are saying. 

The convex glass piece:
The center is close to the test plate, but slightly too flat.
The outer potrion is far too convex, which is evident by the hard curve of the bands, and the fact that when I push on the edge of the glass the fringe number becomes extremely high, which is due to the increased airga across the mirror. 

The only part that still confuses me as that the fringes seem to move away when pushing at the edge of the glass. Should they not move toward the point of pressure? 

So essentially what I am looking at in the above, would be represented below?

When I press on A, the fringes become many, sometimes flipping 180deg. 

When I press on B, the bullseye moves toward the center due to the small air gap created by the slightly flatter center becoming less. 

When I press on C the bullseye moves to the center and the rings move in because of the same air gap becoming smaller.

When I press on D the lines move away and become many due to the larger radius on the edge zone. 

For whatever reason the is more air gap at D than A because of a tilt in the table or whatever the reason may be....

Summed up to say, flatter center, turned down edge. 

Focusing on the ~70% zone where the bands start to curve should help with both of these things...?

Untitled.png

I greatly appreciate all of the help. I'm still trying to wrap my head around a lot of this stuff.... 



#57 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 07 June 2019 - 09:46 AM

If the edge was turned up (Less radius/flatter on the edge of the convex glass), the fringes would hook back the other way in some sort of abomination like this?

Untitled2.png


Edited by Matthew Paul, 07 June 2019 - 09:48 AM.


#58 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,779
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 07 June 2019 - 11:03 AM

 Matt,

  Your picture from what I see of the fringes looks to be correct.  So your going to need to make the convex piece more convex from the 70% toward the center. Once you get that area to match up, wear the whole surface down while holding that radius to push the curve out the edge and get read of the turned edge. 

  In my experience in making convex surface if I polish with  a standard grid shaped pitch lap they always want to go longer on the radius even when I polish with the lap on the top. So what I have had do to is make a couple of other laps. What has works has been to press a star in a lap so just the edge of the lap is making contact with the glass. That making the piece too convex that I switch back to the standard square grid lap to length the radius again and get them to match  up.  I usually have to keep going back and forth between the two different lap as the curve works it way out toward the edge. 

  But as I have said many time "ask 3 mirror makers how to make a mirror and you get 9 answers, all of them right !" So I'm sure others have different ways that will also work.

 

                - Dave 


  • Matthew Paul likes this

#59 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 07 June 2019 - 11:27 AM

Perfect. That is exactly what I have.  A start lap that will focus on the 70% out, and a standard grid lap. I'll just keep working on them then. It's very helpful to know that my assumptions on shape are correct. I'm somewhat flying blind her as I'm still learning exactly what I'm looking at, but we're making progress and I do have a better understanding. 



#60 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 07 June 2019 - 07:35 PM

Getting there.. I went at it with a star lap which had most of the contact in the outer 70-80% area. Then went back to the normal lap. About 2 hours of polishing. There are still pits evenly throughout the glass, but its getting smooth and the figure is starting to become closer to what I want it to be, and less what it was. 

 

It's looking like I might need to go at it with the star lap again. But at least the edge fringes don't wrap back around anymore. 

 

20190607_202824_resize_81.jpg


Edited by Matthew Paul, 07 June 2019 - 07:38 PM.


#61 MKV

MKV

    Cosmos

  • *****
  • Posts: 9,065
  • Joined: 20 Jan 2011
  • Loc: Southeast, USA

Posted 07 June 2019 - 10:32 PM

That's too many fringes. Three to five fringes is what you want to properly assess irregularity.  Looks like you need to clean your optics better too.


  • Matthew Paul likes this

#62 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 08 June 2019 - 05:11 AM

That's too many fringes. Three to five fringes is what you want to properly assess irregularity.  Looks like you need to clean your optics better too.

 

Thanks for the help.

 

The glass is scratched on the backside from rough grinding. The next time I do this I'll paint the backs so that they do not get as beat up. 

 

The center is low, as is outside the 70% zone. 

 

Question: why do you suggest 3 to 5 fringes for this stage over what is shown above? 


Edited by Matthew Paul, 08 June 2019 - 05:24 AM.


#63 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,779
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 08 June 2019 - 10:36 AM

 Matt,

   Good progress. Until your fully polished out and showing straight fringes with a bunch showing, you don't have to worry about testing down to 3 to 5 fringes.   As I said I had to keep going back and forth between the star lap and the grid lap also.

 

                 - Dave  



#64 MKV

MKV

    Cosmos

  • *****
  • Posts: 9,065
  • Joined: 20 Jan 2011
  • Loc: Southeast, USA

Posted 08 June 2019 - 11:02 AM

Thanks for the explanation. Sorry for confusion. I thought "getting there" was conclusive. Personally, I always prefer fewer fringes, and a bulls-eye pattern. Any asymmetry indicates trouble. To me, this is easier to spot and estimate errors.

 

SAM_4901 (3).JPG    

 

 



#65 Arjan

Arjan

    Apollo

  • -----
  • Posts: 1,263
  • Joined: 21 Jan 2009
  • Loc: Netherlands

Posted 08 June 2019 - 11:19 AM

Paul, maybe you can grind the back matte, down to #360?

#66 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 09 June 2019 - 03:32 PM

Thanks fellas. For the life of me I can not get this to become convex or flat, relative to the test piece. Im going radical and ging to try 40-50% overhang, tool on top. See what happens.... Ive been at it all day today, since 8am. 



#67 dave brock

dave brock

    Vanguard

  • -----
  • Posts: 2,225
  • Joined: 06 Jun 2008
  • Loc: Hamilton, New Zealand

Posted 09 June 2019 - 04:17 PM

There are still pits evenly throughout the glass, but its getting smooth and the figure is starting to become closer to what I want it to be, and less what it was.
20190607_202824_resize_81.jpg


While it's good to experiment with strokes, I wouldn't be too concerned with the figure until the pits are gone. There's a danger of getting the figure right before they're gone, then what?

#68 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 09 June 2019 - 04:23 PM

While it's good to experiment with strokes, I wouldn't be too concerned with the figure until the pits are gone. There's a danger of getting the figure right before they're gone, then what?

They are gone now. Still have not reached the figure yet.

I believe that the idea is to keep them spherical from the start and work to maintain the ROC of the test plate as to not flatten the center by overzealous polishing.


Edited by Matthew Paul, 09 June 2019 - 04:25 PM.


#69 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 10 June 2019 - 12:14 PM

So this is what  have, and I don't know what to do to correct it to be honest. The convex mirror is hollow or flat compared to the test piece, so it has a larger ROC. I think.

I know that it is hollow and not in contact as when push in on the center, the fringes flow toward the center pressure point, and when I push on the edge the bullseye moves away from the pressure point.

Both photos are of the same stage with different positioning. Is the center flat with a turned edge? is the whole thing too long of a ROC? Do I want to wear the whole mirror to come down and meet the edge? Is the whole thing too flat and do I need to wear away the edge to meet the center? Do I need to just wear the 70% zone and have that meet the edge and center? Doing 1/2 length strokes with a normal lap, and doing 1/3 strokes with an edge focused star lap seemed to make things worse on the edge. 

I spent about 10 hours on this yesterday and it is worse than when I started. A good indicator that I have no idea what I am doing. 
 

 

IMG_0030.jpg IMG_0031.jpg

I have a 3rd lap that focuses on the 70% area. Should I try very short strokes with that for 20 minutes and see what happens?
The lap looks close to this:
lap3.png

 

And matches the opposite of what Fringe XP says that the figure currently is... with 1/3 x 1/3 W stroke, and almost exactly matches with a shorter 1/4x1/4 w stroke. 
maybe.png


Edited by Matthew Paul, 10 June 2019 - 12:16 PM.


#70 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,779
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 10 June 2019 - 12:26 PM

Matt,

   Try  making  a standard grid pattern lap but make the channels wider in the center and then taper down in width toward the edge. This will make a facets bigger toward the edge, hence more contact and more wear. So not as aggressive as a star lap. Polish with the lap on top. 

    - Dave 



#71 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 10 June 2019 - 12:36 PM

Matt,

   Try  making  a standard grid pattern lap but make the channels wider in the center and then taper down in width toward the edge. This will make a facets bigger toward the edge, hence more contact and more wear. So not as aggressive as a star lap. Polish with the lap on top. 

    - Dave 

Thanks Dave. Ill give it a go later today and see. Should I do normal 1/3 center over center?



#72 MKV

MKV

    Cosmos

  • *****
  • Posts: 9,065
  • Joined: 20 Jan 2011
  • Loc: Southeast, USA

Posted 10 June 2019 - 02:31 PM

Both photos are of the same stage with different positioning. Is the center flat with a turned edge?

Use the "thumb test" by pressing your thumb on the center on the surface of the mirror and see what happens with the fringes. The warmth of the thumb will create a bump and that should help you decide what is high and what is low.

 

Your bulls-eye suggests 4 fringes (or 2 waves) of power. You have to determine if it's concave or convex.

 

Just make sure the "thumb bump" disappears before you go back to figuring. Otherwise you'll create a whole. 



#73 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 10 June 2019 - 02:36 PM

Use the "thumb test" by pressing your thumb on the center on the surface of the mirror and see what happens with the fringes. The warmth of the thumb will create a bump and that should help you decide what is high and what is low.

 

Your bulls-eye suggests 4 fringes (or 2 waves) of power. You have to determine if it's concave or convex.

 

Just make sure the "thumb bump" disappears before you go back to figuring. Otherwise you'll create a whole. 

Thank you. Ill give that a try. The mirror is concave in relation to the test plate.



#74 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 10 June 2019 - 09:02 PM

Here we go... I used the following lap, to get the following results. Each image is marked with the work that was done. I think that tomorrow I'll start with the same lap and a 1/3 stroke and see what that does..  and then maybe I can go back to the grid lap to even things back out. If not, Im going to make a lap to focus on the outer 90%. 

That is if things continue to work as they are. 

62343689_1590030997796336_2964850498915532800_n.jpg 01.jpg 02.jpg 03.jpg 04.jpg


Edited by Matthew Paul, 10 June 2019 - 09:03 PM.


#75 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 17 June 2019 - 03:09 PM

I'm very close with the secondary, test plate is astigmatic. I think that I'm going to make sure that the convex mirror edges are good, and as smooth overall as I can get it. Check it through the back with a ronchi and knife edge to be sure that it has a smooth figure. Then attempt to fix the test plate.
 

If you look at the last set of photos that I posted you can actually see it.  It's especially evident now that the two pieces are close. I know the mirror is not the one with astig. as I overhung and rotated the mirror and test plate. When I was using the ronchi on the test plate I noticed that the lines changed angle from left to right but assumed that it was the alignment of the tester and stand as I just set it up really quick. I was wrong. Ether way I'm making some good progress. It's amazing what knowing what you want to do, does for a project :)

I ended up making a series of 5 different laps to focus on different areas. Once I came to the realization that what I was tying to do to the convex mirror was the opposite of how you would figure a concave mirror, things took off. 




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics