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  #141  
Old 25-02-2013, 09:49 AM
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Satchmo (Mark)
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David ,

This does sound fishy . Just project the Sun image onto a piece of paper and move until you get focus ( get someone to hold the paper just beside the Sun for you and hold the end of the tape measure) . A sanity check while grinding is to calculate the saggita and then buy a drill bit whose diameter is just shy of the required depth and lay a steel straight edge over the drill bit as it lays in the centre of the mirror and grind until you can sit the edge down on the mirror flush with the drill bit in contact. Your saggitta is about 1.56mm so a standard 1.5mm drill will do the job !

If you are still grinding just spray a light mist of water mixed with Glycerine onto the surface for quick reflectivity. If you have a polish the full Moon can also be used as a FL check. Always use the Sun or Moon - don't complicate things as a beginner by trying to focus anything else.

Mark



Quote:
Originally Posted by racecar View Post
I'd say hello fellow mirror makers, but I am yet to finish my first mirror. I am fine grinding at the moment.

I am making a 150mm F6 Newt (at least I hope it is an F6). While it was wet, I held a 40mm eyepiece up to it at arms length and tried to focus on the neighbour's roof. I figured that as it was around F6, and a sphere is close to the Raleigh Criteria, I would get a reasonable image off axis if my arms were long enough.

It surprised me that I could focus on a near object holding the objective at arm's length. I could not get it to focus at infiinity, which was a relief, because I don't know how easily i could figure an F4 or F3.

It is also amazing how much light is reflected off glass during the daytime. I am considering leaving it uncoated for use as a drag and drop lunar telescope.

Anyway, I am glad that there are still mirror makers out there and this thread has had some good tips.
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  #142  
Old 25-02-2013, 06:04 PM
Rod
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Quote:
Originally Posted by Satchmo View Post
Hi Rod

TK-68 is a very old cerium - I'm not sure if its available any more. I used to use it - don't remeber it being particularly aggressive. if your surface doesn't clean up to a nice sphere quickly maybe your pitch is too hard and you are not pressing enough . I always use polishers on the soft side and get nice smooth zone free surfaces cold pressing fairly regularly. What kind of pitch are you using ?

Mark
Thanks Mark

That doesn't surprise me I got it 16 years ago and I suspect my friend had it for several years before that. Good to know you found it ok. Do you recommend finishing with rouge? I have some I bought from willmann-bell. I found it needs elutriating or else you get lots of sleeks.

I am using gugolz pitch which again I have had for several years. I softened it with castor oil. The lap needs trimming every 3 or 4 hours so I assumed it was soft enough.

I think the zones were caused by too aggressive work by me to reduce a central hill - led to a deep depression. I am used to faster mirrors so have had to learn to be more gentle!

Rod
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  #143  
Old 25-02-2013, 06:12 PM
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Quote:
Originally Posted by dave brock View Post
That's a little worrying because as you focus on closer objects, the focal point moves away from the primary. That is, if the mirror is 36" focal length for infinity then the distance you need to hold the eyepice for a closer object will be, say, 37"-39" for example. Have you done a focal length test?

Dave
Quote:
Originally Posted by Satchmo View Post
David ,

This does sound fishy . Just project the Sun image onto a piece of paper and move until you get focus ( get someone to hold the paper just beside the Sun for you and hold the end of the tape measure) . A sanity check while grinding is to calculate the saggita and then buy a drill bit whose diameter is just shy of the required depth and lay a steel straight edge over the drill bit as it lays in the centre of the mirror and grind until you can sit the edge down on the mirror flush with the drill bit in contact. Your saggitta is about 1.56mm so a standard 1.5mm drill will do the job !

If you are still grinding just spray a light mist of water mixed with Glycerine onto the surface for quick reflectivity. If you have a polish the full Moon can also be used as a FL check. Always use the Sun or Moon - don't complicate things as a beginner by trying to focus anything else.

Mark
OK Thanks for that. I will try the moon tonight.
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  #144  
Old 26-02-2013, 09:29 PM
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racecar (David)
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Yup. You were right. It is around F2.5. I ground out a scratch in it. I guess I went a little deep.
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  #145  
Old 04-06-2013, 05:17 PM
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Home silvered 25" mirror.

Been awhile since I last posted, but would like to show my first attempt at home silvering. Took this route as aluminizing is much too expensive in NZ. Also it seems that those who do aluminize here dont offer any protective SiO overcoatings, so longevity of the coating probably not that great. Cost of materials to silver 22" and 25" together around NZD $50 on this first go. Pic of 25" in scope.
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  #146  
Old 01-07-2013, 08:21 PM
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Hello Stephan, could you tell us what chemicals you use to silver your mirror. I got mine aluminized and as I now have another two mirrors in the pipeline, I might save some money using your method. Incidently, we had a still night in Sydney in May and I could see Saturn at 850x with near perfect clarity. I never thought my mirror would be capable of that, but it taught me the futility of trying to do any testing outside in Sydney, as nearly all evenings must be sky limited, as Saturn normally looks much blurrier. Anyway your coating looks great in the pic, so how is the view..
Frank
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  #147  
Old 03-07-2013, 06:38 PM
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Smile Home silvering

Hi Frank,

Google up "Angel gilding" for supplies of ready to use "pour on" or "spray silver" chemicals. AG have easy to follow video instructions on their web site. Anyone can silver mirror/mirrors at home with minimal equipment. And the ready to use chemicals are relatively inexpensive.

The views provided by the 25" silvered primary are incredible. Omega Centauri almost completely filled the FOV when using my ES82 30mm ocular (100x). To my eye resolution to the center, dust lanes and all. A considerably greater spectacle than seen through my 14" dob. The milky way such as I have not seen it before.

I expected some extra "light scatter" coated v uncoated, but I was pleasantly surprised by the absence of such. Definitely recommend home silvering to anyone who wants a 95%+ reflective coating at a bargain price. And "easy as".
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  #148  
Old 08-07-2013, 08:05 AM
Me2 (Mark)
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Hi Stephen,

Tell us more. Did you spray or pour? How did you get the coating even, did you spin it? Did you use an overcoat or are you happy to redo it regularly? You seem happy with the results, how would you say it compares to a professional Aluminium coating?

Thanks.
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  #149  
Old 12-07-2013, 06:57 PM
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Cool Silvering

Hi Mark,

sorry to be a little slow in answering,but currently involved in shifting house. Re silvering: decided on the spray method as it seemed the mechanics of application were more suited to large surface mirrors. I didnt use any spin or other in applying the silver. Simply stood the mirror(s) on edge and sprayed on the chemicals as per instructions. Coatings must be reasonably uniform in thickness as in scope images of similar quality to pre coating images, but much much brighter. Objects like Omega Centauri and the like amazing. I thought my 14" dob was great, but now such objects seen go to a new level.

Regarding coating longivity: Its about 6 weeks since the coatings were applied and at this stage I see absolutely no deterioration of the reflective surfaces so am hoping 6 months to a year of use before recoating,(time will tell). The coating procedure is so simple and inexpensive that I have no reservations about recoating when it becomes essential.
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  #150  
Old 01-09-2013, 09:38 PM
Me2 (Mark)
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Mirror repolish

I posted this in an ongoing thread in the Equipment section but got no response so I'll ask here where it's probably more relevant. Basically I sent my 12.5" F6 Parks mirror to Wayne Sainty for recoating as I haven't used it for nearly 10 years and it was badly corroded.

'Wayne Sainty coated my mirror but noticed some marks so cleaned it off and coated again. Marks were still there so he didn't overcoat it. He kindly sent it back no charge despite all his work. The marks are a bit strange as they don't look like roughness. When I sent the mirror it was quite badly corroded so I assume this had affected the glass somehow. I am rebuilding my telescope so can't test it till it is finished.
Question: If the mirror needs polishing, what would be the best way to go about it? I'd like to do it myself as paying someone doesn't seem worthwhile considering a new mirror can be had for $650 these days.'

I remember it as being a very good mirror so would like to save it.

Cheers,
Mark
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  #151  
Old 01-09-2013, 09:45 PM
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Quote:
Originally Posted by Me2 View Post
'Wayne Sainty coated my mirror but noticed some marks so cleaned it off and coated again. Marks were still there so he didn't overcoat it. He kindly sent it back no charge despite all his work.
That was exceedingly generous of him. I'm sure he will toughen up in time...The etched marks would have had no effect on the image.
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  #152  
Old 01-09-2013, 10:04 PM
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I agree, obviously a man of great integrity. I will certainly use and recommend his services in the future.

Can you explain further what the marks are and why they will not effect the image?

Thanks,
Mark
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  #153  
Old 02-09-2013, 08:15 AM
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Mark- can you post some pictures of your blemishes? I was referring to the fact that there was no reason for him to do free coatings - blemishes are not his fault and compared to the diffraction effects of a spider and secondary mirror are usually of absolutely minimal impact- I would have just run with the overcoated mirror - the straight AL surface will oxidise pretty quickly and look duller after 6 months . If they were not readily visible after the coating was stripped and before coating they are probably pretty negligable.

There would be no way to remove them without professional polishing services. Telescope mirrors need to be accurate to better 1/60,000 of a millimeter-it is not possible to polish of the blems without having to check the surface for change and correct if necessary. A custom mirror of this size is worth at least $2000 today - and Parks had a very good reputation.

Mark S.
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  #154  
Old 05-09-2013, 04:12 PM
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I will try to get a decent picture of the blemishes.
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  #155  
Old 06-09-2013, 07:03 AM
Me2 (Mark)
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Here are some shots of the mirror before and after coating.

When I referred to re polishing the mirror myself I was thinking along the lines of the final stages of mirror making ie making a pitch lap and polishing as if I had made the mirror myself. I have been considering making a larger mirror so am interested in the process.

Thanks for your interest.

Mark
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  #156  
Old 06-09-2013, 07:27 AM
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Hi Mark - yes thats quite bad staining - how does the image look ?

If you haven't done any optical polishing before you would need to start with something easier like an 8" before polishing the 12" back to a sphere to remove the stains and then refiguring.If you are interested in larger mirror making I would recommend making an 8" from scratch, then repolishing the 12" and going for something larger like a 16" . If the mirror is the old 2" thick Pyrex its really better to replace with something thinner. 2" glass in any size can be quite a challenge cooling wise. 1,5" works much better under 24" .
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  #157  
Old 11-03-2014, 05:41 PM
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Some bigger blanks.

Hi Grinders,

Recently had the good fortune, to acquire a quantity of larger plate/float glass blanks, (ex-large public aquarium), in thicknesses 34.1mm (1.342") to 37.1mm (1.462").

Previously I have been working with 19mm float which really is too thin for mirrors of larger diameters. Currently my largest working mirror is 25.2" by 19mm thick, and though it performs ok at lower powers, I know a more stable figure can only be supported with thicker stiffer glass (its said that stiffness increases with the cube of thickness ie 2x as thick= 8x the stiffness), so intend to grind a new one same diameter in the 37mm thickness.

A picture (or two) of some of the blanks from 1.2 ton of ex-aquarium viewing panels. Diameters 12.2" to 33".
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  #158  
Old 11-03-2014, 06:13 PM
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OMG - I wouldn't like to hog one of those out with 120
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  #159  
Old 11-03-2014, 06:19 PM
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Wow
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  #160  
Old 12-03-2014, 11:20 AM
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What a motherload. That should keep you busy for a while.
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