Newbie seeking advice on mirror grinding

Hi,
I am a newbie to telescope making.
Although I have taken a course on the subject (hands-on). I didn't complete
my project, due to circumstances.

I am currently grinding a 10 inch mirror for a Newtonian (supposed to be
F/6).

The focal length is unfortunately now only 49 inches This is an improvement
over what is was since I started grinding (with 120 carbo) and the 8 inch
tool on the bottom.

Can I avoid getting a turned down edge and still get my focal length back to
what it should be (60 inches)?

What advice do you have for me?
I have a couple of textbooks on the matter (Texereau, Howard). And my notes
and handouts.

Troubleshooting is not extensively covered in this material, however.

Thanks for any advice.
--
....Dave Dawson
P.S. My real email address is: beagle7[X]pacificcoast.net
replace the [X] with an @

Hi David,

You will find a lot of very helpful and experienced ATMers on the shore.net
atm list. You can find out how to get on at:
http://home.comcast.net/~cassarole/a...tmsect1.htm#13

Clear Skies

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/

Are you interested in understanding optics?
Try http://groups.yahoo.com/group/ATM_Optics_Software/

************************************

"David Dawson" wrote in message
...
Hi,
I am a newbie to telescope making.
Although I have taken a course on the subject (hands-on). I didn't
complete
my project, due to circumstances.

I am currently grinding a 10 inch mirror for a Newtonian (supposed to be
F/6).

The focal length is unfortunately now only 49 inches This is an
improvement
over what is was since I started grinding (with 120 carbo) and the 8 inch
tool on the bottom.

Can I avoid getting a turned down edge and still get my focal length back
to
what it should be (60 inches)?

What advice do you have for me?
I have a couple of textbooks on the matter (Texereau, Howard). And my
notes
and handouts.

Troubleshooting is not extensively covered in this material, however.

Thanks for any advice.
--
...Dave Dawson
P.S. My real email address is: beagle7[X]pacificcoast.net
replace the [X] with an @

Thank you very much!

CLT wrote:

Hi David,

You will find a lot of very helpful and experienced ATMers on the
shore.net atm list. You can find out how to get on at:
http://home.comcast.net/~cassarole/a...tmsect1.htm#13

Clear Skies

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/

Are you interested in understanding optics?
Try http://groups.yahoo.com/group/ATM_Optics_Software/

************************************

"David Dawson" wrote in message
--SNIP--
--
....Dave Dawson
P.S. My real email address is: beagle7[X]pacificcoast.net
replace the [X] with an @

My suggestion is that you give "got grit" a call, pick up some more #80
and some more #120 and put the focal length where you want it. Just
grind Tool on Top with moderate overhang (chordal but not agressive
chordial) and the FL will be reduced with only a couple more hours of
work.

I went wild on a 12.5 and got it to F/3.5 before I realised my error
(target F/6). It took me a long time to get that one back to F/6. But
there is a good deal of zen in mirror grinding.

Mitch

wrote:

My suggestion is that you give "got grit" a call, pick up some more #80
and some more #120 and put the focal length where you want it. Just
grind Tool on Top with moderate overhang (chordal but not agressive
chordial) and the FL will be reduced with only a couple more hours of
work.

I went wild on a 12.5 and got it to F/3.5 before I realised my error
(target F/6). It took me a long time to get that one back to F/6. But
there is a good deal of zen in mirror grinding.

Mitch
Thanks Mitch.
If I get tired of grinding with the 120, I'll go back to the 80 of which I
still have lots.
--
....Dave Dawson
P.S. My real email address is: beagle7[X]pacificcoast.net
replace the [X] with an @

David Dawson wrote:
Hi,
I am a newbie to telescope making.

Me too!

The focal length is unfortunately now only 49 inches This is an
improvement
over what is was since I started grinding (with 120 carbo) and the 8
inch
tool on the bottom.

The first time through I ground my mirror down too much as well. This
must be a common mistake among newbies. My problem was using feeler
gauges to measure sagitta. Somehow the focal length ended up 6" too
short. I switched to a drill bit combined with the spit test (using a
candle for the source), and I'm now back within 2" of desired ROC.

What advice do you have for me?

To fix my mistake I went TOT and used chordal strokes until the drill
bit just slightly rolled around under the straight edge. Then I
continued with TOT, but swtiched to the normal stroke. This seemed to
work for me.

Good luck with your mirror.

FWIW, you don't worry about a turned down edge until you are polishing the
glass. At the last grit, you just need to insure that you are not trying to
use long strokes (as well as all of the other grits after the 80 grit
hogging stage) to grind. A stroke of about 1/4 to 1/3 of the diameter of
the glass is the right stroke to get the glass surface spherical.
It is quite common for first timers to overshoot the focal length that they
desire as they feel that they need to get all of the curvature in with the
80 grit (you should really just do 80% or so of the curvature with the 80
grit and finish up most of that 20% left with the 120 grit) and they end up
short as a result. Just turn the mirror and tool over and continue with
your grinding that way. Do about 4 wets of 120 grit with a strong chordal
stroke and see where your curvature is and that may get you most of the way
towards the 60" focal length that you want. You will probably have a hole
in the middle of the old curve but working more with the normal 1/3 W stroke
will get that out pretty quick.
I'll note that using a 8" tool on a 10" mirror will tend to make the radius
decrease much more than if you were using a 10" tool.

--
Why isn't there an Ozone Hole at the NORTH Pole?

I'm responding to you three above.
Thanks all.
I have plenty of 120 grit carbo, so I'll do 4 wets with a chordal stroke,
tool on top and then see. I'm assuming that a 'spit' test will show me
something semi-useful, but if not I've got a drill bit to test with too.

I should add that when I did the hogging out with 80 grit and a chordal
stroke with the mirror on top, I got the right depth according to the drill
bit under the straightedge, If my measurement technique was correct, that
is.



--
....Dave Dawson
P.S. My real email address is: beagle7[X]pacificcoast.net
replace the [X] with an @

I should add that when I did the hogging out with 80 grit and a chordal
stroke with the mirror on top, I got the right depth according to the
drill
bit under the straightedge, If my measurement technique was correct, that
is.

I think the problem many folks have with maintaining the fl when moving on
from #80 is that, even though they may measure the correct sagitta, the
mirror is not spheroidal and so the measurement is bogus. If the ground
area is not spheroidal, then bringing it to spheroidal with #120 can
significantly change the fl. This is why it is necessary to get a good
Sharpie test at #80 before moving on to #120.

Dennis

Dennis Woos wrote:

I should add that when I did the hogging out with 80 grit and a chordal
stroke with the mirror on top, I got the right depth according to the
drill
bit under the straightedge, If my measurement technique was correct, that
is.

I think the problem many folks have with maintaining the fl when moving on
from #80 is that, even though they may measure the correct sagitta, the
mirror is not spheroidal and so the measurement is bogus. If the ground
area is not spheroidal, then bringing it to spheroidal with #120 can
significantly change the fl. This is why it is necessary to get a good
Sharpie test at #80 before moving on to #120.

Dennis
Thanks,
I'm measuring the sagitta with a small stack of coins of about the right
height.

I follow what you have said, so is the fact that my fl is changing due to my
technique then?

Texereau advises that one should grind with the mirror altermately on top
and bottom to prevent the fl from changing, which it looks like I should
do.
--
....Dave Dawson

"If you wrestle in the mud with a pig,
you both get dirty, and the pig likes it."