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gatorengineer
Carpal Tunnel
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Reged: 02/28/05

Loc: Hellertown, PA
Re: Secondary Size Revisited - A New Realization? new [Re: Lamb0]
      #6149440 - 10/21/13 08:53 AM

just a comment. In this day and age we also have to work with what is commercially available. You wont find a high end secondary less than 1.30". Also mounting and having adjustment on anything smaller becomes extremely difficult.

Based on the 1.30 commercially available size, i wouldnt go any slower than F7, which gives a 0.50" fully illuminated at a focal distance 6" above the focuser. This size gives you a co of 16.25%, which is refractoresq.... The only advantage of going slower would be a reduction in coma which is proportional to the square of the focal length.


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MitchAlsup
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Reged: 08/31/09

Re: Secondary Size Revisited - A New Realization? new [Re: Jeff Morgan]
      #6149768 - 10/21/13 12:12 PM

Quote:

Then I realized that the field lens diameter on one the most common low power eyepieces (the 31 Nagler) is not 2", but rather 42 mm (1.65").




The focuser baffle on my large DOB has a 1.58" opening, for similar reasons.

Edited by MitchAlsup (10/21/13 12:12 PM)


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MitchAlsup
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Reged: 08/31/09

Re: Secondary Size Revisited - A New Realization? new [Re: Jeff Morgan]
      #6149769 - 10/21/13 12:12 PM

Quote:

Then I realized that the field lens diameter on one the most common low power eyepieces (the 31 Nagler) is not 2", but rather 42 mm (1.65").




To focuser baffle on my large DOB has a 1.58" opening, for similar reasons.


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hottr6
Carpal Tunnel
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Reged: 06/28/09

Loc: 7,500', Magdalena Mtns, NM
Re: Secondary Size Revisited - A New Realization? new [Re: gatorengineer]
      #6150025 - 10/21/13 02:55 PM

Quote:

You wont find a high end secondary less than 1.30".



I have recently bought both 0.75" and 1" quartz secondaries from Protostar. I doubt that there are better commercial secondaries.


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gatorengineer
Carpal Tunnel
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Reged: 02/28/05

Loc: Hellertown, PA
Re: Secondary Size Revisited - A New Realization? new [Re: hottr6]
      #6150061 - 10/21/13 03:29 PM

I stand corrected. Glad to hear Protostar is alive and well.

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hottr6
Carpal Tunnel
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Reged: 06/28/09

Loc: 7,500', Magdalena Mtns, NM
Re: Secondary Size Revisited - A New Realization? new [Re: gatorengineer]
      #6150300 - 10/21/13 05:45 PM

I read the stories about Protostar here at CN, and needed some quality smaller secondaries. You are correct in that other than Protostar, I was unable to find another source.

So despite the naysayers, I placed an order for the secondaries; no email conformation or anything followed. One week later, the secondaries arrived, complete with test certificates.


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Jeff Morgan
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Reged: 09/28/03

Loc: Prescott, AZ
Re: Secondary Size Revisited - A New Realization? new [Re: mark cowan]
      #6150917 - 10/22/13 12:16 AM

Quote:

Not really - most of the mirror is still capable of looking well off axis no matter what the UTA, so long as the UTA is at least the size of the primary.




I should have realized that, but had never thought about it before. My thinking has always been it's worth the extra inch to keep my body warmth away from the tube - but that is my perception, I have never seen it tested. It should be easy enough to do on a cold night.

Quote:

I worked out a lot of the tradeoffs involved while first analyzing geometrically exactly what size the secondaries should be for large ultrafast mirrors, retaining only full illumination for a small central area. And an associated issue - how close should the barrel of a Paracorr 2 come to the edge of the primary light path, vs the larger secondary required to get it completely clear...

Best,
Mark




Back in my binoviewer days I was a bit taken aback at how far the OCS intruded into the telescope tube. Russ at Denkmeier Optical assured me that: 1) it only would happen at low magnifications; and 2) at those magnifications you would never notice the diffraction effect. He was right on both counts. I can't speak to the Paracorr.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Secondary Size Revisited - A New Realization? new [Re: Jeff Morgan]
      #6151303 - 10/22/13 09:32 AM

Quote:

I should have realized that, but had never thought about it before. My thinking has always been it's worth the extra inch to keep my body warmth away from the tube - but that is my perception, I have never seen it tested. It should be easy enough to do on a cold night.




One of the advantages of a refractor is that the light spends most of it's time quite distant from the tube. A tight UTA assembly would seem to compromise the thermal issues.

Jon


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Jeff Morgan
Postmaster
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Reged: 09/28/03

Loc: Prescott, AZ
Re: Secondary Size Revisited - A New Realization? new [Re: Jon Isaacs]
      #6151685 - 10/22/13 01:15 PM

Quote:

Quote:

I should have realized that, but had never thought about it before. My thinking has always been it's worth the extra inch to keep my body warmth away from the tube - but that is my perception, I have never seen it tested. It should be easy enough to do on a cold night.




One of the advantages of a refractor is that the light spends most of it's time quite distant from the tube. A tight UTA assembly would seem to compromise the thermal issues.

Jon




And the refractor objective is above the sharp temperature gradient near the ground. Tough one for us reflector owners to beat.


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Secondary Size Revisited - A New Realization? new [Re: Jon Isaacs]
      #6151688 - 10/22/13 01:16 PM

Well, who says you need to illuminate your lowest power eyepiece to 70% at the edge?
It's not that that isn't a nice goal, since it would be a very unusual person who could or would notice the roughly 0.3 magnitude drop-off in brightness.

But I'm looking at it this way these days: if i use my lowest power eyepiece 5% of the time (or less) when observing, why use a secondary that illuminates that eyepiece to 70% at the edge? You could probably tolerate a slightly higher vignetting on that eyepiece since you use it so seldom. Just don't make the UTA small enough to vignette the field--why ADD the vignetting of the UTA to the vignetting of the secondary if you're going to skirt the thin ice of "too small" on the secondary.

But, on the flip side, the objects being viewed in that eyepiece are likely to be large and fill the field, so looking at the edge is more common. Plus, the large exit pupil makes the whole image brighter, and vignetting is (I speak from experience) easier to see at large exit pupils than it is at higher powers and smaller exit pupils where the image is darker anyway. And, the edges of a secondary are the poorest places, so a smaller secondary needs to have better edges (or be mounted on a stalk*).

I recently decided that the field of view of a 21 Ethos was of sufficient size for me to use that as my lowest-power eyepiece. So now I have a 36.2mm field stop to illuminate instead of the 31 Nagler's 42mm. You might think about that one. I found the 31 was beginning to display astigmatism from my eye because the exit pupil was as large (or larger) than my pupil, while the 21 showed no astigmatism at all and better contrast.

But, let's say, for comparison, that I'd kept the 31 Nagler. I didn't use it much anyway, preferring the 21 Ethos. If I chose my secondary size to illuminate the 21 Ethos and just let the illumination in the 31 Nagler slide a bit, that might allow me to use a smaller secondary. You could do the same--illuminate your next eyepiece up and let your lowest power be what it is, illumination-wise.

*The typical secondary holder covers some of the secondary mirror. My secondary holder is 2.7" in diameter, houses a 2.6" mirror, but exposes only a 2.4" reflective surface to reflect light. In a calculator, I use 2.4" as the size, even though the obstruction is really 2.7". If you choose to skirt the "too small" end of things on secondary size, you will need to mount it in such a way you use its entire surface and the obstruction equals the mirror's diameter. Otherwise, you will shrink the mirror but actually get a larger obstruction than the mirror diameter.


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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Secondary Size Revisited - A New Realization? new [Re: Starman1]
      #6153029 - 10/23/13 06:36 AM

Too bad visual guys just can't take a flat with the eye and have the brain do corrections. Perhaps there's a way to model the eye with an imager in front of the eyepiece and then take a flat. At least one would have quantitative values instead of subjective options. I do really like that vacuum tube sound ;-)


Don


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Secondary Size Revisited - A New Realization? new [Re: Starman1]
      #6153042 - 10/23/13 06:56 AM

Quote:

Well, who says you need to illuminate your lowest power eyepiece to 70% at the edge?




I don't worry about the edge illumination, I worry about the size of the fully illuminated field of view. In a fast scope, the edge illumination pretty much takes care of itself, it's the size of the fully illuminated field that is my concern.

How big is big enough? Do you want a 100% illuminated field of view with your Ethos at 200x? That's requires a 1/2 a degree to be fully illuminated. In my mind, 0.5 degrees is big enough, 0.1 is too small, probably 0.35 is enough.

Jon


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dvb
different Syndrome.
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Reged: 06/18/05

Loc: Vancouver, Canada
Re: Secondary Size Revisited - A New Realization? new [Re: Jon Isaacs]
      #6159731 - 10/26/13 07:53 PM

A nice thing about the 1800 Destiny curved spiders is you can try different diagonal sizes - the 2.6" secondary I'm using with my f/4.5 10" is based on the "sec" program and seeking the 70% edge illumination.

I am going to order a separate "Quick Change" secondary holder from 1800 Destiny, so that I can try a smaller secondary for mainly planetary use, assuming smaller field stop eyepieces, and 40% edge illumination.

But, for the nice, wide field, low illumination objects, like M33 in an Ethos - I think I'll stick with the max illumination.

Because, the other side of the argument is that the contrast lost with the >20% obstruction is probably as irrelevant as the illumination lost with the <70% illumination.

(Vacuum Tubes are the way to go - I've just ordered some Shuguang 2A3-Z Treasures. Downstairs, I'm still enjoying the Mullard (New Sensor EL34).


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Secondary Size Revisited - A New Realization? new [Re: hottr6]
      #6166032 - 10/30/13 09:46 AM

Quote:

You are correct in that other than Protostar, I was unable to find another source.

... complete with test certificates.




I bought a 1/10 wave .75" secondary from Newport. No cert sheet, tho'. I guess that's what we're all looking for.

C


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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: Secondary Size Revisited - A New Realization? new [Re: Starman1]
      #6167834 - 10/31/13 07:01 AM

Quote:

Just don't make the UTA small enough to vignette the field--why ADD the vignetting of the UTA to the vignetting of the secondary if you're going to skirt the thin ice of "too small" on the secondary.





Simple ADDition does not reflect () what is happening. A smaller UTA provides, overall, LESS vignetting and LESS light loss. A smaller UTA reduces secondary vignetting by more than the UTA adds.

When UTA and secondary vignetting are combined, the sample numbers I've tried show it is always beneficial to add UTA vignetting to reduce secondary vignetting, at least up to the point where the UTA vignetting is the limiting factor for 100% illumination. No matter what the size of the secondary is.

Reducing the UTA diameter allows a focal plane closer to the secondary, just like a shorter focuser does. The (beneficial) effect on secondary vignetting is the same.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Secondary Size Revisited - A New Realization? new [Re: derangedhermit]
      #6167851 - 10/31/13 07:19 AM

Quote:


Reducing the UTA diameter allows a focal plane closer to the secondary, just like a shorter focuser does. The (beneficial) effect on secondary vignetting is the same.




The thermal effect is something else... I have to think that a UTA that provides little clearance for the on-axis light column increases the sensitivity to tube currents.

Another issue is eyepiece height. The closer the focuser is to the secondary, the taller the scope. In some situations, a larger secondary can turn a scope that requires a stool into a flat foot scope.

Jon


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Jeff Morgan
Postmaster
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Reged: 09/28/03

Loc: Prescott, AZ
Re: Secondary Size Revisited - A New Realization? new [Re: Jon Isaacs]
      #6168218 - 10/31/13 11:06 AM

Quote:

Another issue is eyepiece height. The closer the focuser is to the secondary, the taller the scope. In some situations, a larger secondary can turn a scope that requires a stool into a flat foot scope.

Jon




Last night I came across an interesting article by the late Ernie Pfanneschmidt where he uses relay lenses to take the secondary mirror of a 8" f/6 Newtonian down to 0.75" But as you point out, any shortening does raise the focuser height (which for that scope would still be quite manageable, and perhaps even improved).

For those interested, the article was in the October 2001 issue of Sky & Tel.


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dvb
different Syndrome.
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Reged: 06/18/05

Loc: Vancouver, Canada
Re: Secondary Size Revisited - A New Realization? new [Re: Jeff Morgan]
      #6168377 - 10/31/13 12:23 PM

I would not reduce the tube diameter to make decrease the secondary size - at least 1" is needed in addition to the radius of the primary to ensure proper air flow to deal with thermal currents in the tube.

In my view, the biggest failing in commercial solid tube Newts is tubes that are too narrow.


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Jeff Morgan
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Reged: 09/28/03

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Re: Secondary Size Revisited - A New Realization? new [Re: dvb]
      #6168963 - 10/31/13 05:38 PM

Quote:

I would not reduce the tube diameter to make decrease the secondary size - at least 1" is needed in addition to the radius of the primary to ensure proper air flow to deal with thermal currents in the tube.

In my view, the biggest failing in commercial solid tube Newts is tubes that are too narrow.




I'm not really wild about the idea either. I just haven't put a lot of thought into trading off some vignetting of the primary for the smaller diagonal.

But for those that wanted to pursue it - who says the tube has to be a uniform diameter throughout it's length? By fabricating a compound tube and using louvers and fans, a builder could get the heat plumes from the primary mirror effectively, and still shrink the diagonal intercept distance on the upper end.


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davidpitre
Post Laureate
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Reged: 05/10/05

Loc: Central Texas
Re: Secondary Size Revisited - A New Realization? new [Re: Jon Isaacs]
      #6169121 - 10/31/13 07:29 PM

Quote:


Another issue is eyepiece height. The closer the focuser is to the secondary, the taller the scope. In some situations, a larger secondary can turn a scope that requires a stool into a flat foot scope.

Jon



This is exactly what happened to me very recently. I had never really taken into account the lip of my secondary holder reducing my secondary diameter when I originally made my 18". The scope just barley allowed my to stand flat footed at zenith. When I took the actual effective diameter of the secondary into account, I realized my 100% illuminated field was tiny. .1" radius max, perhaps less. I was not excited about moving from a 3.5" to a 4" diagonal. I was able to move my focuser board in enough to substantially enlarge my 100% illuminated field, but I really need a step stool now at zenith. My focuser tube does intrude somewhat into the light path. I've thought of hacking off a bit of the Feathertouch tube, but haven't taken a look at it to see if it is practical.


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