TheSkySearchers

I was home visiting my Mom in DC before Christmas. One of my sisters asked what I do for a hobby. I mentioned making telescope mirrors. She pulled out her phone and bought an 8in PYREX blank for me.

I was not intending to make a mirror this year... but... I got the mirror blank for free ... so, here I am doing another mirror.

I want this one to be for an astrograph ... so BIG secondary and f/4 primary.

Pictures show (in the following order): 1) the blank // 2) the backside // 3) the thickness // 4) the mirror after finishing the rough grinding






BABO

I've been slow but steady progress.

Not quite polished out yet. Here's some pictures of Ronchigrams (outside RoC) and K-E.

BABO

Early on in the polishing -- basic spherical shape


Knife Edge shot:

AS IT STANDS NOW:

Congratulations on the 'gifted' mirror!

I will be watching your progress and hope to see the finished scope soon!!

Proceeding nicely,

Latest Ronchigram (outside RoC) side by side with the computer generated ideal:

.............
BABO

Wow. This is an interesting thread.
Thank you for posting with iimages of your grinding progression.

What John said!

I missed this one when you first posted in February. Looking forward to seeing how the project progresses.

Regards

Graeme

Hi Joe. I remember back in the 1960's when several of us went to the Adler Planetarium near downtown Chicago to grind and polish 6" and 8" f/8 Newtonian parabolic mirrors. At the time, we used red rouge (iron oxide), but the preferred optical polish is now cerium oxide. Your Ronchi patterns look quite good Joe, and thanks for keeping an otherwise "lost art" alive.

here is a silly question , do you grind a mirror as such to be spherical or parabolic to deal with coma for a fast mirror ?

yobbo89 wrote: ↑Sat Mar 13, 2021 5:41 pm here is a silly question , do you grind a mirror as such to be spherical or parabolic to deal with coma for a fast mirror ?

You can't get rid of coma on a mirror. If you grind to a sphere you get spherical aberration and fuzz. If you grind to a parabola you get coma but sharp images on axis. You need to add a lens system to get rid of the coma.

notFritzArgelander wrote: ↑Sat Mar 13, 2021 10:13 pm

yobbo89 wrote: ↑Sat Mar 13, 2021 5:41 pm here is a silly question , do you grind a mirror as such to be spherical or parabolic to deal with coma for a fast mirror ?

You can't get rid of coma on a mirror. If you grind to a sphere you get spherical aberration and fuzz. If you grind to a parabola you get coma but sharp images on axis. You need to add a lens system to get rid of the coma.

I don't want to distract too much from babo,s post, i'll keep it short, so we need to make sure to have a parabola mirror? i kind of know what sphereical aberration is, been doing a little read up on hubbels wonky untested mirror ..

can we use spherical mirrors or lenses if that's the case for main optics at all ?
how does one determine the curve for the parabola grinding process,i assume it's little flatter on the edges of the mirror, does the focult test show the parabola curve on the test ?

yobbo89 wrote: ↑Sun Mar 14, 2021 8:30 am

notFritzArgelander wrote: ↑Sat Mar 13, 2021 10:13 pm

yobbo89 wrote: ↑Sat Mar 13, 2021 5:41 pm here is a silly question , do you grind a mirror as such to be spherical or parabolic to deal with coma for a fast mirror ?

You can't get rid of coma on a mirror. If you grind to a sphere you get spherical aberration and fuzz. If you grind to a parabola you get coma but sharp images on axis. You need to add a lens system to get rid of the coma.

I don't want to distract too much from babo,s post, i'll keep it short, so we need to make sure to have a parabola mirror? i kind of know what sphereical aberration is, been doing a little read up on hubbels wonky untested mirror ..

can we use spherical mirrors or lenses if that's the case for main optics at all ?
how does one determine the curve for the parabola grinding process,i assume it's little flatter on the edges of the mirror, does the focult test show the parabola curve on the test ?

For a given aperture, if you make the focal length long enough the difference between a sphere and a parabola becomes negligible. IIRC for an 8" that's f9 or slower. Foucault testing can show the difference .

Special thanks to nFA for the explanations. I would add to what he said, in case it's not obvious, that I working on a parabolic mirror for a Newtonian reflector. (There are different kinds of reflectors, e.g., SCTs, RCs, etc. Instead of using a parabolic primary and flat secondary, they have different primary and secondary mirrors.)

Anyway, that said, I am reporting on my current status.

I have only had time to do 6 figuring sessions total. But things are going smoothly. Here is where I stand now. (I started doing interferometry testing, now that I am getting close to parabolic).

RONCHIGRAM (outside RoC):

Foucault test




Wavefront from interferometer test:


BABO

This is looking very good so far!

I remember grinding my own mirror back in the 80's. It was a fun project. Congrats on your progress BABO!

Nice work Babo. Looking good from what I remember of test patterns.
Joe

After some more work on this mirror, I finally finished it. Not as smooth as I would have liked it. But I think it will be ok. I sent it for the coatings (with Majestic in the US). It will probably take a couple of months before I get it back in Nairobi.

The mirror is an f/4 8" Pyrex (diameter = 195mm and FL = 777)

Here's the Ronchigram (outside of RoC) and the results from taking a series of igrams (with my DIY Bath interferometer & processed with DFTFringe).

I also include a shot of the Moon that I did with the mirror mounted in the OTA (using my Fuji X-A1)

...
...

BABO

Good to see that you are making excellent progress toward a happy conclusion!

Very nice Babo, it looks like you are progressing well with the mirror.