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My 6" f8 scope

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#1 Eric Slaghuis

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Posted 20 January 2013 - 01:58 PM

Eventually my life reached the point where i believe I can make some time available to build my first scope.

I joined the friendly guys at ScopeX this Saturday afternoon. In no time I had a 6" blank and a matching glass tool in my hand!

The disks seems to be cast. 1 inch thick. I searched for one with minimal bubbles and started to level a side of both the tool and the blank on a metal disk and some grit. Also did the edges of both.

I was shown how to do a center over center stroke, about 1 third over, issued with some 80 grit and encouraged to start grinding.

I have brought the lot home, built myself a grinding table and am now hogging out my first mirror. Measured it tonight (after 5 hours of grinding) and I think my Sagitta is just over 0.5 mm. I need 1.1 for a f8 scope, thus still quite a bit of glass to remove! My measurement was with a straight edge and some feeler gauges, thus not very accurate.

I think I found myself some good coaches. They have all the tools neede to measure and figure a good mirror. Johan, the guy who is taking me under his wing is busy with a monster disk. Something like 24"!

Many people are in various stages of telescope making. Some were done, others were polishing and others like me were issued with some glass.

I must say, it has only been the first 5 hours of a very long journey, but I am finding it quite addictive!

Eric Slaghuis

#2 StarDusty

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Posted 20 January 2013 - 02:40 PM

Good luck with your new project. Keep us informed on your progress.

#3 John Carruthers

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Posted 22 January 2013 - 04:08 AM

>but I am finding it quite addictive!<

Bwa ha ha, another soul for ATM :lol:

#4 Ed D

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Posted 22 January 2013 - 10:11 AM

Sounds like you're having a lot of fun. Post pics whenever you can so we can see the progress of your scope.

Ed D

#5 coopman

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Posted 22 January 2013 - 10:48 PM

I've never ground a mirror, but I would imagine that there is a great sense of accomplishment from those who see it through to completion and do a good job on it.

#6 StarDusty

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Posted 22 January 2013 - 11:01 PM

Go for it Clay. A 6" mirror is the place to start. Here are two tutorials on how to make a grinding tool and polishing lap.

http://www.clearskyo...le/20-telesc...

#7 Eric Slaghuis

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Posted 23 January 2013 - 03:01 AM

It is actually going quite quick. I am working with 80 grit and have reached 0.9mm Sagitta in only 5 and a half hours. Maybe an hour to go till I reach the target of 1.2mm

#8 Eric Slaghuis

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Posted 23 January 2013 - 02:11 PM

My most (in)accurate measurement shows I reached a Sagitta of 1.2 mm. That is close enough to f8.

I am measuring with a straight edge and a vernier. Will go off to ScopeX on Saturday. There I can get a good measurement.

Hope to move on to 120 grit! Progress!!!

#9 Eric Slaghuis

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Posted 26 January 2013 - 01:00 PM

Went to class today. My measurement was not that accurate, Sagitta was not quite on. I discussed it with Johan and he suggested I start the 120 grit. I can fix the Sagitta whilst starting to smooth out the mirror. Spent three hours on 120. The Sagitta went down to 1.23mm equivalent to f7.7. Still one or two pits so ill spend half an hour on 120 this week before moving onto the finer stuff.

#10 StarDusty

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Posted 26 January 2013 - 02:02 PM

For my 6" mirrors, I did not spend more than an hour to 1 1/2 hours per grit once I was at my desired sag. Maybe you need to add fresh grit more often to make the most of your grinding effort.

#11 Eric Slaghuis

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Posted 28 January 2013 - 01:28 PM

Thank you Allen.
Johan had a theory that I went too quick in rough grinding. He recommended a bit more time on 120 to ensure the rest goes quick.

I am really happy with the surface now. Will start 240 tomorrow. Hopefully the hour and a half per grit s true. I have stock all the way down to 400.

I am so excited about this project.

#12 ccaissie

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Posted 28 January 2013 - 04:21 PM

Using the right amount of grit is important. If too much, the grains interfere with each other and you get a slow acting mud.

Using the right amount of pressure is important. Too little, and it is slow work. Put significant weight on it, and change grit often, reduce pressure towards the end of the grit grade.

If it seems that you're using a lot of grit, scoop it up, wash it, and reuse.

All this will become second nature on your 3rd or 7th mirror. :lol:

#13 StarDusty

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Posted 28 January 2013 - 07:36 PM

You will find that the "wet" on finer grits will last a lot longer.

Remember to keep the outside edge beveled. As you grind your edge will sharpen and you can quickly lose the bevel you made in the beginning. It is worth the time you spend keeping your bevel in shape.

A sharp edge will crack, chip or spall. Fixing that takes a long time.






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