| gaz-in |
| (sage) |
| 07/02/09 04:49 PM |
How Good a Secondary is needed?
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Good Evening All,
Now that I have figured what size secondary to order for my 14.5 inch Telekit project, my question is: How good of a secondary do i need. I realize this is a very subjective question. But I am interested in any input. Has anyone used a $75 secondary, replaced it with a $175 one, and noticed a difference? How about the difference between quartz and pyrex?
Thanks in advance for the input.
Dan
| bbasiaga |
| (professor emeritus) |
| 07/02/09 05:02 PM |
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Re: How Good a Secondary is needed?
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Here's the thing - the optical system is only as good as its weakest link. I'd hate for that to be a major optical component such as the secondary. Stick with a good name brand like Galaxy or Protostar. Others will chime in with those they've had good luck with I'm sure.
I'd be interested to know how the quartz ones perform as well. I don't think they're common in the size I need for my project, and are quite a bit more expensive. Still, I'd be interested to know what their advantages are besides thermal stability.
I hope you're enjoying your telekit project. Have you actually received it yet? I did a 15" f/5 Telekit a few years back. Love it. I also used Randy for the secondary holder and spider for my current DIY. If you've got any questions as you go along with the telekit, let me know I'll be glad to help. Randy is quite accessible too, and his instructions are very good, so you may not need me, but I'm here if you do.
-Brian
| llanitedave |
| (Humble Megalomaniac) |
| 07/02/09 06:40 PM |
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Re: How Good a Secondary is needed?
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It's my understanding that the surface of quartz can be given more micro-smoothness than pyrex. I haven't heard that this is a real issue in any practical sense.
Someone with broader knowledge than me will have to chime in about whether there's a real quality difference. To answer the original question, I'd advise to get the highest quality secondary you can afford. Fortunately, they aren't hard to find. The brands mentioned above have excellent reputations.
| Bird |
| (Post Laureate) |
| 07/02/09 09:41 PM |
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Re: How Good a Secondary is needed?
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I'd suggest spending as much as you can afford... to quote Ripley, "It's the only way to be sure"
Bird
| Mike I. Jones |
| (Carpal Tunnel) |
| 07/02/09 10:13 PM |
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Re: How Good a Secondary is needed?
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Agree - without posting up a bunch of supporting ZEMAX analysis and/or quoting Suiter, I can tell you with certainty that diagonal curvature more than 1/10 wave P-V is enough to generate astigmatism at high powers. 1/20 wave P-V in fused silica so it keeps it shape in changing temps is the way to go.
Mike
| walt r |
| (Post Laureate) |
| 07/02/09 11:55 PM |
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Re: How Good a Secondary is needed?
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Jay (look for his posts on Green Goblin scope) had two GSO secondaries that were astigmatic. Once he bought a high quality secondary the scope gave good images. Don't skimp.
| Houdini |
| (professor emeritus) |
| 07/03/09 06:19 AM |
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Re: How Good a Secondary is needed?
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Quote:
Agree - without posting up a bunch of supporting ZEMAX analysis and/or quoting Suiter, I can tell you with certainty that diagonal curvature more than 1/10 wave P-V is enough to generate astigmatism at high powers.
Please do post a couple of ZEMAX analyses, I don't accept your certainty without proof
.Robert
| Mark Harry |
| (Post Laureate) |
| 07/03/09 07:28 AM |
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Re: How Good a Secondary is needed?
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My basic rule of thumb, a good smooth 1/8-1/10th surface error; no edge hook. General viewing, it'll kick butt on 99% of what you want to do. Try to keep any variance in the outer 1/4" of the secondary's aperture.
Planetary or other hi-rez/hi-x stuff, get the best you can possibly afford. (big apertures will put you several hundred $ in the hole.)
I tested one of Jay's secondaries when he was having trouble. Pretty sad!
Mark
| Mark Harry |
| (Post Laureate) |
| 07/03/09 07:34 AM |
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Re: How Good a Secondary is needed?
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Might bring this point up to keep things on the same page.
**************
Flats are tested to accuracies expressed as SURFACE ERROR, PV. (because they are basically just transmission elements, and the angles of incidence, and final systems error varies with application.)
M.
| Mike I. Jones |
| (Carpal Tunnel) |
| 07/03/09 08:14 AM |
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Re: How Good a Secondary is needed?
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OK Rob you got it - a fair swap for your very fine deflection analysis of my 24" tapered blank a while back. Which one of your scopes do you want analyzed? How bout that 43" megascope?
.... How far inside focus is your diagonal, and what diameter fully illuminated field?
I have initally assumed a 43" mirror radius of 344" (exactly f/4) and placed your diagonal 29" inside focus. For a 1" diameter fully illuminated FOV the diagonal is 8.25" x 11.668", in-plane offset 0.715", lateral offset 0.506" away from the focuser and toward the primary. Optical analysis next.
Mike
| Houdini |
| (professor emeritus) |
| 07/03/09 08:42 AM |
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Re: How Good a Secondary is needed?
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Mike,
Try a 16" f/5 with a 3.1" diagonal 12.6" from focus (my current planetary "grab & go" scope).
Assuming a perfect primary and a 1/10 wave P-V diagonal curvature, how much astigmatism does one get?
I'm asking the question because this page claims that you can have 4 or 5 waves of curvature on the diagonal without problem.
Robert
| jayscheuerle |
| (Post Laureate) |
| 07/03/09 10:52 AM |
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Re: How Good a Secondary is needed?
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Yah... 2 mentions of my problems...
Secondaries get overlooked because they're smaller and they're flat, so it looks like no big deal, but when they're off, they can ruin all the good work your primary has done.
I've been very happy with my Protostar secondary. If you start off with a bad one and your experience is not great, it can be very frustrating trying to hunt down what your problem is. If you figure it out and replace it with yet another inexpensive one because you're pinching pennies and THAT one is astigmatic as well... THAT is maddening and throws your sleuthing confidence way off!
Buy a good one and forget about it. - j
| Mike I. Jones |
| (Carpal Tunnel) |
| 07/03/09 11:00 AM |
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Re: How Good a Secondary is needed?
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Let's respect the OP first. Dan, what's your mirror EFL, diagonal distance inside focus, and desired FOV diameter? I'll ZEMAX up your system then do Rob's.
I looked at that website and am thoroughly unconvinced. His very first point is bogus - it's a lot easier to make flat parts very flat on a continuous ring polisher than to make spheres and conics. Been done there that - a guy that knows how to set, tool and weight his CP can pull 1/10 to 1/20 wave parts off all day, right to the edges. I ain't buyin' no 3 wave diagonal! Back in a while.
Mike
| gaz-in |
| (sage) |
| 07/03/09 12:01 PM |
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Re: How Good a Secondary is needed?
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Cool Mike her are the stats:
14 inch mirror
70.6 inch focal length
10.85 inch distance from the center of the secondary to the focus point.
.5 100% illuminated immage size.
From my (admittedly crude) calculations I think I need a 2.6 inch secondary.
| Mark Harry |
| (Post Laureate) |
| 07/03/09 01:09 PM |
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Re: How Good a Secondary is needed?
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Do 'em one at a time, with conventional polisher!!!!!!!
Mark