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Re: [ATM] The best time to Foucault?
Also... Don't forget that the resolution gains are supplied by the larger diameter.
Ken Hunter
Dave Tabor <david@tabor.com> wrote:
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"
To: "jamoore47" ; "Roman Toledo"
;
Cc: "John Moore"
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 ;)
> _______________________________________________
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>
>
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>
>
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