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the mirror is coming along


crashtestdummy

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Well ive not posted for a few weeks as ive had lots to do work and personal life wise so not had that much time with the mirror but I finally finished getting the back flat which was my main aim so I could finally move onto the front.As I said in an early blog post,my mirror was nowhere near uniform when I got it so I had to take lots of glass out of the middle to get it flat as it was convex.i wasn't 100% sure I had got it right so I travelled up to see mirror maker supremo John Nichol for a second opinion.If it hadn't have been for John I very much doubt I would have had the confidence to even think about a project like this but after attending his mirror making seminar last year and email support in the mean time it really did fill me with confidence to have a bash.Was a great eye opener seeing his mirror making facility,all those machines and all that glass!!!! I had travelled to see John to make sure I was going right and I had explained how my mirror had been and how i had tried to grind it flat and that I though it was slightly turned at the edge so he had actually set his machine up ready to correct this as we had both assumed the worse.Well I put my mirror down on the table and unpacked it for John to check out to see if I had got the mirror flat enough and uniform and out came the straight edges and the mirror makers eye and the first comment was "well Rich I think you have had a wasted journey".Well to mee that had made the journey anything but wasted as It was massive relief for me to know it was bang on and ready to go ontot the front after a quick half an hour blast with some 220.i was left with a really fine casting mark on one edge which I may polish out later but on the back to me it isn't important.

after a quick bit of 220.decent bevel too :)

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finally the mirror is turned over to show the high ridges on the front ready to grind.its not 100% clean so not as shiny as it could be

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grinding the higher thin casting marks on the front to me is a bit of a nervy experience as you feel as though they are going to snap off and its a noisy experience with much vibration.rather than doing long strokes I shortened mine down to lessen this vibration and after about half an hour they were much lower

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after another hour I was left with a few higher ridges near the edges which I used a smaller 6" sub diameter tool locally to grind these lower which seemed to work quite well.after about 2 hours the front is almost fully clear of casting marks and has a sagittal although the flipside of having a raised part on the back is that I have a slightly depressed part on the front which John tells me will more than likely grind out as I remove 10mm of glass to get my focal length.one thing I did notice when using the 60grit,its amazing how quick you can grind down a 3mm bevel when the surface is flat!!!I ended up with no bevel at all so have been really luckly not to get lots of shells on the edge.i did get one slight shell but it will bevel out when I next redo it so very very lucky escape!!

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just a few casting marks left now,one on each side and they will grind down slowly as I keep pounding away with the 80grit.

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making decent progress and the next few weeks will be spent doing the boring part of hogging out the curve and topping up the bevel from time to time so I wont likely bore you with regular updates other than when the curve becomes apparent.

just some notes to add:

grinding he back I used 3.5kg of 80grit,2g of 60grit and 250g of 220 grit

so far on the front I have used 2kg of 60grit and about 1kg of 80 so far.

myself and Damien(master) purchased a 25kg sack of 80grit between us and shared it out so hopefully that will get me somewhere towards my curve before I have to order anymore.80grit grinds much much slower than 60 so expecting it to take me well into the back end of February to get to my curve at the minimum!!

in all time wise it took me 14hours to get the back flat!!!!

so far ive spent 4 1/2 s on the front with an hour and a half being on the bevel so 18 1/2 hours in total so far.

Im not managing to put ion the time I had hoped when I planned the grind but with the weather as it is,observing time hasn't been eating into grinding time!!!!

cheers for looking Rich

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Looking good and hasnt taken as long as I thougtnit would have taken.

hope you dont mind me asking but is there any chance you could give a running total of the spend so far as you go along ?

Keep up the good work !

Cheers

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As a rough guide so far:

22" blank £475

10kg weight disc £20

3kg 60grit £30

3.5kg 80grit £15

1kg 400grit £13

1.75kg 220grit £12

Spirit level £8

Gorrilla bucket £12

Belling stones(6!!) £10-15

12.5kg 80grit £50

Still have 10kg of 80,all the 400 and 1kg of 220 left

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Not bad, so by the time its aluminised youll have a 22inch mirror for about a grand assuming coating costs are a few hundred ?

Thats really good value .....

by the way how heavy is that disk , it looks huge, and is it a 2 inch thick one ????

cheers

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Enhanced coatings for the primary and secondary are £338 which isn't too bad but I need to include the price of attatching the secondary holder to the secondary.also the price of the secondary too.I'm hoping the whole scope shouldn't cost more than £2500 including a new featherlite focuser with sips coma corrector.

With smallers scopes you make them for your own enjoyment/achievement but with big scopes the savings are big too.

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Really good job Rich, can't remember if I've asked before, but what focal length are you going for? Presumably less than 1.8m?

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My current 16" f4.5 is 1829mm focal length,the new 22" is going to be f3.6 and 2016mm focal length.likely one small step at zenith which is quite manageable for my height.Any less than 3.6 to me is just more coma and grinding.for an 1800mm 22" would be about f3.2 which is a bit too much to me.

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Somehow I missed your reply.

 

Well, one step isn't too bad - I was considering getting a 24" F/3 as my final scope. However, I'm not going to be doing the grinding ;).

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I think everyone should have a go at grinding at some point,it a really interesting experience but hard work.I know that when I look through that eyepiece for the first time with a mirror that I've ground will be a great experience.

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Im gonna opt for 18inch f4, below that will be punishing on eyepieces and the low f ratios would give too big an exit pupil for me. At f4 I could get away with a 22mm eyepiece and still have a 5.5mm exit pupil, with an 82 degree eyepiece this would give me just under a one degree field so would fit the orion nebula in one field.....only just !

mind you, if I see you with the 22 at a star party id be first in the que.

cheers

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I initially planned an 18" F3.9 but the 22" blank was only about £50 more so I couldn't help myself.obviously the 18" would be much less grinding.are you grinding yours yourself or buying one?

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Im tempted to grind one myself eventually, but then again Im also tempted to just buy one from John Nichol. I must admit though that the idea of first light with my own mirror is tempting.

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It's a slow process,more so the bigger you go but I really wanted to do one and there's no time like the present.

John will do a great mirror for you,that's for sure but he will also give you advice about making your own.I'm sure he will run another of his mirror making seminars some time soon

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I think everyone should have a go at grinding at some point,it a really interesting experience but hard work.I know that when I look through that eyepiece for the first time with a mirror that I've ground will be a great experience.

 

Might have a think about it

 

 

Oh and to add,my next one will be a 26" f3.something :)

 

How much will the mirror weigh?!!?!

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22kg isn't at all bad. Much lighter than I thought it would be considering the C14 is 25kg.

 

I initially ruled out anything above a 24" mirror due to weight - but apparently it doesn't need to be as heavy as I thought :D - I was thinking more like 40-50kg for a 30" mirror :rolleyes:.

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An SCT is much more glass though as it has the primary,secondary and the corrector plate hence the massive weight.mine is a 43mm thick blank,you could always use a 25mm plate glass blank and have it slumped.that would be almost half the weight.John Nichol showed me one and it's probably the way I will go for my 26".

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