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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
"In lumine tuo videbimus lumen."
Scopes: Stellarvue SV80 Raptor Carbon Fiber ED Doublet / Celestron SCT C8
Williams Optics 66mm APO / DIY 8" f/4 Newtonian astrograph / Nikon 180mm f/2.8
Mounts: Orion Atlas EQ-G / Celestron AVX / DIY mini-equatorial
Cameras: QHY163m / Fujifilm X-A1 (modded) / Fuji X-A2 (not modded) / Orion StarShoot Auto Guider
Filters: ZWO 7nm NB set / ZWO LRGB set / ZWO Dual Band / Astronomics UHC
I will be watching your progress and hope to see the finished scope soon!!
Gordon
Scopes: Explore Scientific ED80CF, Skywatcher 200 Quattro Imaging Newt, SeeStar S50 for EAA.
Mounts: Orion Atlas EQ-g mount & Skywatcher EQ5 Pro.
ZWO mini guider.
Image cameras: ZWO ASI1600 MM Cool, ZWO ASI533mc-Pro, ZWO ASI174mm-C (for use with my Quark chromosphere), ZWO ASI120MC
Filters: LRGB, Ha 7nm, O-III 7nm, S-II 7nm
Eyepieces: a few.
Primary software: Cartes du Ciel, N.I.N.A, StarTools V1.4.
"I am more than a sum of molecules.
I am more than a sum of memories or events.
I do not one day suddenly cease to be.
I am, before memory.
I am, before event.
I am"
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.
Marshall
Sky-Watcher 90mm f/13.8 Maksutov-Cassegrain on motorized Multimount
Orion Astroview 120ST f/5 Refractor on EQ3 mount
Celestron Comet Catcher 140mm f/3.64 Schmidt-Newtonian on alt-az mount
Celestron Omni XLT150R f/5 Refractor on CG4 mount with dual axis drives.
Orion 180mm f/15 Maksutov-Cassegrain on CG5-GT Goto mount.
Orion XT12i 12" f/4.9 Dobsonian Intelliscope.
Kamakura 7x35 Binoculars and Celestron SkyMaster 15x70 Binoculars. ZWO ASI 120MC camera.
>)))))*>
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.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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 ?
scopes :gso/bintel f4 12"truss tube, bresser messier ar127s /skywatcher 10'' dob,meade 12'' f10 lx200 sct
cameras : asi 1600mm-c/asi1600mm-c,asi120mc,prostar lp guidecam, nikkon d60, sony a7,asi 290 mm
mounts : eq6 pro/eq8/mesu 200 v2
filters : 2'' astronomik lp/badder lrgb h-a,sII,oIII,h-b,Baader Solar Continuum, chroma 3nm ha,sii,oiii,nii,rgb,lowglow,uv/ir,Thousand Oaks Solar Filter,1.25'' #47 violet,pro planet 742 ir,pro planet 807 ir,pro planet 642 bp ir.
extras : skywatcher f4 aplanatic cc, Baader MPCC MKIII Coma Corrector,Orion Field Flattener,zwo 1.25''adc.starlight maxi 2" 9x filter wheel,tele vue 2x barlow .
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 .
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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
"In lumine tuo videbimus lumen."
Scopes: Stellarvue SV80 Raptor Carbon Fiber ED Doublet / Celestron SCT C8
Williams Optics 66mm APO / DIY 8" f/4 Newtonian astrograph / Nikon 180mm f/2.8
Mounts: Orion Atlas EQ-G / Celestron AVX / DIY mini-equatorial
Cameras: QHY163m / Fujifilm X-A1 (modded) / Fuji X-A2 (not modded) / Orion StarShoot Auto Guider
Filters: ZWO 7nm NB set / ZWO LRGB set / ZWO Dual Band / Astronomics UHC
I remember grinding my own mirror back in the 80's. It was a fun project. Congrats on your progress BABO!
-Michael Refractors: ES AR152 f/6.5 Achromat on Twilight II, Celestron 102mm XLT f/9.8 on Celestron Heavy Duty Alt Az mount, KOWA 90mm spotting scope Binoculars: Celestron SkyMaster 15x70, Bushnell 10x50 Eyepieces: Various, GSO Superview, 9mm Plossl, Celestron 25mm Plossl Camera: ZWO ASI 120 Naked Eye: Two Eyeballs Latitude: 48.7229° N
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
"In lumine tuo videbimus lumen."
Scopes: Stellarvue SV80 Raptor Carbon Fiber ED Doublet / Celestron SCT C8
Williams Optics 66mm APO / DIY 8" f/4 Newtonian astrograph / Nikon 180mm f/2.8
Mounts: Orion Atlas EQ-G / Celestron AVX / DIY mini-equatorial
Cameras: QHY163m / Fujifilm X-A1 (modded) / Fuji X-A2 (not modded) / Orion StarShoot Auto Guider
Filters: ZWO 7nm NB set / ZWO LRGB set / ZWO Dual Band / Astronomics UHC
Very nice Babo, it looks like you are progressing well with the mirror.
Gabrielle See Far Sticks: Elita 103/1575, AOM FLT 105/1000, Bresser 127/1200 BV, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS 100/1000, Vixen SD115s/885 EQ: TAL MT-1, Vixen SXP, SXP2, AXJ, AXD Az/Alt: AYO Digi II, Stellarvue M2C, Argo Navis encoders on both Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102 Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Tak prism, TAL, Vixen Eyepieces: Antares to Zeiss (1000101) The only culture I have is from yogurt