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Focal length reduced

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#1 Wolfdale



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Posted 17 January 2013 - 05:48 AM

I really dont know what to say, I was in making 6" F/8 but after completing all carborundum series it was decreased to F/4.5
My Focal length is 754mm, As i know there will be less magnification, so can i use 2X Barlow lens to view to increase its magnification with its increase in FL. :)

#2 Bill Whitehead

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Posted 17 January 2013 - 07:12 AM

I used one of those thingy's that measure the spark plug distance to make sure my sagitta was correct. Then I kept flipping the tool from bottom to top to keep it at the depth I wanted.

Once it's gone, I guess you can't add glass to it. It you wanted to work even harder, you could try to reduce the edge.

Ah, the joys of creating your own mirror.

Good luck, Bill

#3 dan_h



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Posted 17 January 2013 - 08:34 AM

Is it possible that you made an error when you originally calculated the sag? Once a mirror is hogged out to the correct radius it takes a great deal grinding with the finer grits to move the radius very much.

If you have kept a log of the grinding progress you should review it to determine what went wrong for you on this mirror.


#4 benula


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Posted 17 January 2013 - 09:09 AM

Maybe you confused radius of curvature and focal length??

#5 kfrederick



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Posted 17 January 2013 - 09:09 AM

Put the mirror on the bottom and work the center of the tool on the edge of the mirror . Then go to regular strokes to get it spherical With a 6inch not much glass needs removed Made that error on a 20 .Learn on the small . If you have the grit and you wont longer You are the master of the glass it does not master you . HA HA just have fun it it is f4 no big deal . still work good

#6 Wolfdale



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Posted 18 January 2013 - 04:33 AM

can this be useable or not? what range of eyepiece do i need?

#7 ccaissie


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Posted 18 January 2013 - 08:02 AM

Sure it's useable. You sway you went through all the carborundum grades. What are you finished with now? 320, 500? Or do you mean you're ready to polish?

What's the sagitta now? Don't panic, and don't do anything yet without thinking it over.

Do you have enough grit to regrind? If so, you can reduce the curve a lot, maybe all the way. It may be a matter of just doing it.

Whaddya say? First mirror? I recommend making it a longer focus, as f/4 will take a long time to figure right, much longer than regrinding it and doing an f/6,7,8.

Don't ask how I know all this.

#8 Wolfdale



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Posted 19 January 2013 - 06:25 AM

Thanks i hope it works

#9 mark cowan

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Posted 19 January 2013 - 10:21 PM

How did you measure the FL? And are you ready to polish or just out of grit?

FWIW I've made those kind of mistakes too. Although not recently. :lol:


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