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Re: [ATM] The best time to Foucault?
The loss caused by masking the 8" to a 7.6" mirror is only
5*Log(8/7.6) = 0.11 magnitudes. The area of a 1.5" secondary is not 7 but
only 1.8 square inches, which is only
1.8(/(4^2*pi))/100)= 3.5%. An 8 inch (fully functioning) mirror with a 1.5
inch secondary is equivalent to a 7.86 unobstructed mirror.
The masked mirror with the 1.5 inch secondary is equivalent to a 7.47 inch
unobstructed mirror. The lightloss as compared to a fully functioning,
unobstructed 8" mirror is 5log(8/7.47)= 0.15 magnitudes, which is much less
then the 2.1 magnitudes you calculated (if I understand your calculations
well).
I don't say that it's not worth fixing, but the lightloss of the masking is
very small and barely noticable.
Jan
http://home.wanadoo.nl/jhm.vangastel/Astronomy/
----- Original Message -----
From: "Dave Tabor" <david@tabor.com>
To: <atm@atmlist.net>
Sent: Monday, October 16, 2006 4:21 PM
Subject: Re: [ATM] The best time to Foucault?
> Hi, All.
> Keep in mind that I'm still new-ish myself, so this isn't so obvious to
me,
> but would the light loss really be negligible?
>
> Here's what I'm thinking..
>
> IDEAL (but ignoring the beveled edge loss):
> If I had an 8" mirror the surface area would be 201 square inches.
> Subtracting the loss for a 1.5" secondary (7 square inches), I get 194
> square inches of light gathering.
> Limiting magnitude 7.5 + 5 LOG (20.32 cm aperture) = 14.03
>
> MASKED (and still ignoring the beveled edge loss):
> If I masked 5%, just less than 1/4 inch all the way around, I would now
have
> a 7.6" mirror and the mirror would be 181 square inches. Subtracting the
> same secondary, it's now 174 square inches of light gathering.
> Limiting magnitude 7.5 + 5 LOG (19.30 cm aperture) = 11.90
>
> TOTAL LOSS:
> 20 square inches of light gathering (10.3% of the usable mirror surface)
and
> 2.1 magnitudes of celestial objects.
>
> To me, that seems significant and I'd want to fix it, which I know isn't
> quick, but is possible, instead of putting a band-aid over it.
> - Dave
>
>
>
> ----- Original Message -----
> From: "jamoore47" <JohnMoore47@mail2web.net>
> To: "jamoore47" <JohnMoore47@mail2web.net>; "Roman Toledo"
> <rtoledo2002@yahoo.com>; <atm@atmlist.net>
> Cc: "John Moore" <jamooretx@web.com>
> Sent: Monday, October 16, 2006 5:59 AM
> Subject: Re: [ATM] The best time to Foucault?
>
>
> > In addition to my previous post, I wish to add,
> >
> > In checking for a TDE, with the Foucault-tester, [and a Ronchi-grating
> > is highly recommended for this test], if the diffraction-ring
> > is the same brightness all the way-around, your still ok,,
> > because some people will notice the slight difference-of-brightness,
> > some wont!.[or it could actually be that on some mirrors it
shows-itself,
> > while on other mirrors, it doesn't, or it could be only a very-slight
> > change
> > of knife positioning, makes a difference?].
> > On all the mirrors I have made, I had either;
> > 1.a noticeable dark/bright side,
> > 2.a ring with a slightly noticeable brighter-side.
> >
> > PLUS,,
> > if you do end-up with a visible TDE,[one-side is bright,,the other dark]
> > you can just mask-off the outer 2-5% of the optic, and you will be
> > much better-off, and the light-loss will be negligible.
> >
> > Thnx for your attention,
> >
> > John Moore.
> > atm since 1974.
> >
> > ________________________________
> >
> > From: atm-bounces@atmlist.net on behalf of jamoore47
> > Sent: Mon 10/16/2006 2:20 AM
> > To: Roman Toledo; atm@atmlist.net
> > Cc: John Moore
> > Subject: Re: [ATM] The best time to Foucault?
> >
> >
> >
> > No, Thank You! Roman!,
> >
> > its so-good to hear from a fellow atm, deeply involved
> > with a optical, or a mount project!, we here at the list love-it!.
> > Welcome Back!, and Please keep us up-to-date!, we would
> > Love to hear how your doing!
> > PS; your now not considered a beginner,,because ounce you get
> > that grit under your finger-nails!, we know you've been bitten by the
bug,
> > and you are part of our "ATM-Frat" in good standing!", You are now among
> > the
> > "Brotherhood!"
> > [but of course anyone is welcome here, whether you
> > have made a telescope or not!].
> >
> > I too, owned several commercial-scopes, before I started
"pushing-glass",
> > and
> > I was glad i did, because I was able to take my time, and create the
best
> > Optics I could, and boy!, was the time spent worth-it!!,, man, I was
> > ecstatic!,
> > the views were Breathtaking!
> >
> > Here is my recommendation at this point to be used at your discretion,,,
> >
> > [form what you've said about your current state with the optic, my
current
> > tip
> > would be to, go ahead and do a 60/40 attack of MOT/TOT. if you have
> > 30-35 sec.-pitch temper.(semi-hard). If you usually use a pitch-temper
> > of 20-30sec.(semi-soft), try not to use more that slight "hand Pressure"
> > and a work-cycle of about
> > 20/20min.(20minutes/polish)&(20minutes/cold-pressing).
> > I myself prefer to use semi-hard pitch, for 5-10lbs of pressure to
protect
> > the edge.
> > because Im allot better-off with a slightly TUE.,[easier for me to
> > correct].
> > Now Remember, that a "parabola" does have an"Apparent-TDE", that's not
the
> > same
> > as a "True-TDE!", The apparent-TDE can be Easily seen as the slight
> > "Dimness" of the diffraction-ring" on the "left-side of the
> > Doughnut",,,depending
> > of course on whether your light-source is on the "right or Left"
> > [any good atm-book will explain the reason]
> > in other-words a "Well-corrected parabola" will have a ring-of Light
that
> > is Slightly brighter on one side than the other!(depending on which side
> > the light-source emanates-from!)
> > NOW,,if you see one-side lit-up, and the other-side is dark!, then you
> > absolutely have a "degree-of-TDE".
> >
> > My Best wishes to you and your Optics!
> >
> > God Bless,,
> >
> > John Moore
> > atm since 1974.
> > ________________________________
> >
> > From: atm-bounces@atmlist.net on behalf of Roman Toledo
> > Sent: Mon 10/16/2006 12:37 AM
> > To: atm@atmlist.net
> > Subject: [ATM] The best time to Foucault?
> >
> >
> >
> > *****************************************************
> >
> > John thanks for the encouragement, I'm still in stage one,
> > working on 2 mirrors. I've learned a lot so far and know there's more
> > to be learned.
> >
> > Right now I'm working exclusively on my second mirror since I
> > bought a Celestron NGPS 9.25 f/10 so I want to get a fast mirror
> > done. It seems ;) I've changed my mind again and I'm back in love with
> > a fast mirror to supplement my viewing habits. not that I'm complaining
> > about the NGPS I love it.
> >
> > I've only looked once at my second mirror and I'm not worring
> > about errors at this time since I'm using a full size pitch doing COC.
> > I've learned that I like to polish with weights on my 3"thick tool TOT,
> > it allows me to push nice and slowly and make a smother pass, so far
> > the edge has begun to polish out and is currently about 50% in from the
> > edge polished to where there is little light defraction on the edge of
> > the glass.
> >
> > I've been working a lot of hours and seem tired, so I'm putting 15
> > minutes here and there, no real hurry. My first mirror (and my own
> > creation ;) ) sits there 90% polished but since it's a f/7 it's going
> > to wait till the fast one is done.
> >
> > I just also ordered a couple of secondaries from JC Mulherin and once
> > those arrive I'll work on those in the house and try make progress on
> > those too, I'll come back and ask about those at that time, specially
> > how to get them tested, but that will have to wait for now.
> >
> > I was going to take advantage of all the clouds and polish, but I just
> > don't seem to muster the energy. the TV won tonight.
> >
> > thanks again from one beginner and as a update to those wondering what
> > happened to me ;)
> > _______________________________________________
> > ATM mailing list http://www.atmlist.net/
> >
> >
> > _______________________________________________
> > ATM mailing list http://www.atmlist.net/
> >
> >
> > _______________________________________________
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>
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