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nemo42
journeyman


Reged: 11/30/12

Re: Recommended secondary size new [Re: Jon Isaacs]
      #5621696 - 01/13/13 09:12 PM

My main mirror cell is 2" deep and the mirror is 1.25" thick Pyrex. Maybe these dimensions make the difference and cause the vignetting with the 1.83" secondary when I run it with .5" spare focuser in travel. My focuser barrel bottomed out though measures about 3.75" to the tube from the top of the barrel. The 1.52" secondary also causes a narrow 100% ray F.O.V. I see that the 2" focuser makes the vignetting problem go away.

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dpwoos
Pooh-Bah
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Reged: 10/18/06

Loc: United States
Re: Recommended secondary size new [Re: nemo42]
      #5621774 - 01/13/13 09:49 PM

Quote:

Here's the deal with the secondary. I purchased the parts as a group and 3 vanes of the spider were damaged in shipment. I have to replace the spider so I figure, maybe I should replace the mirror and holder too to optimize the scope. If the thread tap in the new spider would also accept my current secondary mirror I could possibly switch between the secondaries.




If you model your scope using software or even drawing it out on a long piece of paper, you will see that you can't swap secondaries without also moving the primary mirror. I hope that the secondary size discussion has been useful to you, but I fear that it has caused more concern and confusion than warranted. If you ever get to the point where you can make an informed decision that you want a smaller secondary, then you will have become a very skillful observer who has decided to optimize your scope for a specific purpose. In the meantime, my advice is to get your scope out under the stars and start your journey. I think you will find that the secondary size is nowhere on your list of things that you need to pay attention to.

Edited by dpwoos (01/13/13 09:54 PM)


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Recommended secondary size new [Re: dpwoos]
      #5621777 - 01/13/13 09:52 PM

Quote:

If you model your scope using software or even drawing it out on a long piece of paper, you will see that you can't swap secondaries without also moving the mirror.




???

Jon


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dpwoos
Pooh-Bah
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Reged: 10/18/06

Loc: United States
Re: Recommended secondary size new [Re: Jon Isaacs]
      #5621809 - 01/13/13 10:17 PM

I guess that wasn't very well stated. Of course the two secondaries can be swapped, but if one models it then it is clear that to take full advantage of the secondary size change some of the other dimensions ought to change as well, including the distance between the primary and the secondary and the width of the tube. The same configuration can't be optimal for both secondaries.

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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Recommended secondary size new [Re: dpwoos]
      #5621849 - 01/13/13 10:34 PM

Quote:

I guess that wasn't very well stated. Of course the two secondaries can be swapped, but if one models it then it is clear that to take full advantage of the secondary size change some of the other dimensions ought to change as well, including the distance between the primary and the secondary and the width of the tube. The same configuration can't be optimal for both secondaries.




There is no "optimal solution", any solution is a compromise. I see no need to is no need to change the tube diameter, change the mirror spacing.

If one starts with a design that provides acceptable performance with the smaller secondary, the larger secondary only provides more fully illuminated field of view, there is no downside I can see. If one makes sure that the original design is sufficient for the smaller secondary, then there should be no other changes needed to swap between the two.

That's how it seems to me.

Jon


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dpwoos
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Reged: 10/18/06

Loc: United States
Re: Recommended secondary size new [Re: Jon Isaacs]
      #5621910 - 01/13/13 11:21 PM

If you optimize for a smaller secondary, then that includes optimizing the entrance pupil (i.e. the tube diameter). If you then swap in a larger secondary, then you will vignette at the entrance pupil. On the other hand, if you optimize for a larger secondary, then a smaller secondary will not illuminate as wide a fov as it otherwise would. I guess that a compromise design could be used that was acceptable for both, depending on what "acceptable" means. It seems to me that, in talking about using a smaller secondary in a scope that is optimized for "planetary", that compromising in this way defeats the purpose.

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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Recommended secondary size new [Re: dpwoos]
      #5621937 - 01/13/13 11:50 PM

Quote:

If you optimize for a smaller secondary, then that includes optimizing the entrance pupil (i.e. the tube diameter). If you then swap in a larger secondary, then you will vignette at the entrance pupil. On the other hand, if you optimize for a larger secondary, then a smaller secondary will not illuminate as wide a fov as it otherwise would. I guess that a compromise design could be used that was acceptable for both, depending on what "acceptable" means. It seems to me that, in talking about using a smaller secondary in a scope that is optimized for "planetary", that compromising in this way defeats the purpose.




Dennis:

We are talking very small changes here. It's worth noting that the 10 inch A-P MAK has a 22% CO, many MAKs have 30% COs and yet provide excellent planetary views, either one of these secondaries is plenty good.

In any event, I believe the tube diameter should be chosen for thermal reasons, not because of vignetting concerns. A planetary scope should have a large tube to help with air flow and keep the light path away from potential tube surface thermal issues.

There was a recent discussion that here of Albert Higne's book where he points out that even a tube that has the same diameter as mirror causes little effective vignetting.

Use a 10 inch tube, a low profile 2 inch focuser and all will be well with either secondary. With the small secondary, the scope will be optimized for planetary.

Jon


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dpwoos
Pooh-Bah
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Reged: 10/18/06

Loc: United States
Re: Recommended secondary size new [Re: Jon Isaacs]
      #5621957 - 01/14/13 12:05 AM

I'm not advocating for the smaller secondary, but rather pointing out that simply swapping secondaries is not going to produce an optimized "planetary" scope. Specialized "planetary" scopes are designed to get as much light into the small secondary as possible, and so they cannot take advantage of a larger secondary. Again, I am not advocating for such a scope, and I am certain that the 1.83" secondary is capable of producing knock your socks off planetary views, so long as everything is working properly.

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Ajohn
sage
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Reged: 12/03/07

Re: Recommended secondary size new [Re: dpwoos]
      #5622132 - 01/14/13 05:07 AM

I would be happy with 1.83in too and would wonder if there would be much gain going smaller as the Texereau book suggests where you might choose a 2ndry to just cover the moon for planetary work. However if you throw in short focusers and maybe just having enough projection for a dslr I'm not so sure. I'm suggesting the dslr aspect because some one might want to fit a larger 2ndry for photography - that is what Texereau is getting at. He suggests limiting to 0.1mm radial fanning of stars for that which probably isn't a bad idea even today and could be improved with a coma corrector. The problems with that large a field is that it gets rather big at longer focal ratio's and can lead to a massive 2ndry.

What bought the obstruction size home to me was switching to a SCT. The optics were obviously good but pointed at Jupiter contrast was obviously lacking. There is a bit of a moral there - if you make a swap like that go bigger. What happens then is that the resolution is higher and as consequence the contrast can hold up above the smaller scope. When you get near the limit of the scopes resolution things are back where a perfect scope would be and as it's a bigger scope the resolution is higher too.

I've found his book very useful. It even helped me pick eyepieces on my 1st scope and I still do that as he suggests. Big scope owners might want to be a bit cautious at the higher mag end - he even explains why big scopes can be a bit of a problem optically in that respect. Some of the comments seem mad but bear in mind he's talking true 1/10 wave optics not p/v or rms and holding that sort of accuracy in a complete telescope does mean weights go up. I also believe the book made plossl eyepieces famous.

Yes I am a bit of a Texereau fan. I reckon it's the best book available on that subject. Some might vary some aspects as usual but it's all sound advice.

John
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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Recommended secondary size new [Re: dpwoos]
      #5622151 - 01/14/13 05:42 AM

Quote:

I'm not advocating for the smaller secondary, but rather pointing out that simply swapping secondaries is not going to produce an optimized "planetary" scope. Specialized "planetary" scopes are designed to get as much light into the small secondary as possible, and so they cannot take advantage of a larger secondary. Again, I am not advocating for such a scope, and I am certain that the 1.83" secondary is capable of producing knock your socks off planetary views, so long as everything is working properly.




Dennis:

I see no reason why a telescope designed to use a 1.5 inch secondary cannot provide a large diameter illuminated field of view with a 1.83 inch secondary. Maximizing the diameter of the fully illuminated field for a small secondary means minimizing the distance from the focal plane to the center of the secondary. If the secondary is larger, the fully illuminated field of view is larger in diameter, the 75% illuminated field of view is larger in diameter.

I ran Newtweb using a 2 inch focuser with the focal plane 2.25 inches from the wall of a 10 inch diameter, 0.125 inch wall tube:

1.5 inch secondary:

100% illumination: 0.38 degrees
75% illumination: 1.15 degrees.

This looks good for planetary, an even smaller 1.3 inch secondary would provide a 0.10 degree 100% field.

1.83 inch secondary:

100% Illumination: 0.85 degrees
75% Illumination: 1.58 degrees

This looks good for Low Power Deepsky...

I agree that a 1.83 inch secondary should provide very good planetary views but there is nothing inherent about the design of a scope with a 1.5 inch secondary that limits it from taking advantage of a larger secondary.

This is getting back to the statement that changing the geometry of the mirror-focuser relationship would be necessary if you swap secondaries.

Jon


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Ajohn
sage
*****

Reged: 12/03/07

Re: Recommended secondary size new [Re: Ajohn]
      #5622152 - 01/14/13 05:45 AM

One thing not mentioned is the 3 vein spider. Those give 6 diffraction spikes. This is why 4 is often used. That gives 4. The reason is that a vein causes one opposite it as well. Not something to worry too much about really.

I'm not convinced there is much point worrying about how thin they are within reason. As I understand it's the edges that cause the problem no matter how thin they are. The book also goes through something Coudre came up with to get rid of the spikes and from memory discusses it's effects.

Pass in a way on thickness. I once built a dob and used a single 3/16 dia steel bar across the tube and adjusted it by moving one end around. Adjustment wasn't easy but worked. I recently sold a dob some one else made. It used an oval shaped tube to hold the 2ndry that went across the diameter of the scope to just clear the mirror holder - no spikes. It was an open tube one. Biggest problem was the oval wasn't long enough to keep stray light out. 2 landing net sticks from a fishing tackle shop fitted into the mirror box and supported the oval. I had it for several years but it wasn't my sort of thing really.

Texereau's telescope design is interesting too. Easier to make in some ways and maybe better than a dob. I do wonder about that.

John
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dpwoos
Pooh-Bah
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Reged: 10/18/06

Loc: United States
Re: Recommended secondary size new [Re: Jon Isaacs]
      #5622265 - 01/14/13 08:38 AM

Quote:

Maximizing the diameter of the fully illuminated field for a small secondary means minimizing the distance from the focal plane to the center of the secondary.




It sure does, and the only simple way to do that (assuming the focuser height is already minimized) is to shrink the entrance pupil/tube diameter. Your design holding the tube diameter constant at 10" means that the resulting scope is not an optimized planetary scope. In fact, you might reject this entrance pupil optimization as there is a cost (there always is), but if you do then you have rejected what I think most folks would consider one of the defining characteristics of a newtonian optimized for planetary observing - trading entrance pupil for small secondary obstruction.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Recommended secondary size new [Re: dpwoos]
      #5622407 - 01/14/13 10:17 AM

Quote:

Quote:

Maximizing the diameter of the fully illuminated field for a small secondary means minimizing the distance from the focal plane to the center of the secondary.




It sure does, and the only simple way to do that (assuming the focuser height is already minimized) is to shrink the entrance pupil/tube diameter. Your design holding the tube diameter constant at 10" means that the resulting scope is not an optimized planetary scope. In fact, you might reject this entrance pupil optimization as there is a cost (there always is), but if you do then you have rejected what I think most folks would consider one of the defining characteristics of a newtonian optimized for planetary observing - trading entrance pupil for small secondary obstruction.




Dennis:

This is fun though I suspect the knowledgeable members of the group think we are making fools of ourselves arguing over something so simple. After all, you really can swap secondaries back and forth and gain the advantages/disadvantages of the change without having to change the diameter of the tube or reposition the primary mirror.

In any event, I think most would agree that a 0.38 degree 100% illuminated field of view is more than sufficient for a "planetary scope." If not, one could use a smaller tube or a even lower profile focuser. In either situation, swapping the 1.5 inch secondary for a 1.83 inch would still provide a larger illuminated field of view.

As far as reducing the tube diameter being a wise thing to do for a planetary telescope, I have to disagree. I think just the opposite. With a Newtonian, the light first passes the entire length of the tube before reflecting off the mirror. I believe it is an advantage in keeping the tube as far away from the light path as is possible to minimize the thermal effects of the tube. It also helps provide clean air flow around the mirror cell. This is one reason why refractors have fewer thermal issues, the light only near the tube for a short while.

If one still decided to go for a smaller tube and an even smaller secondary than the 1.5 inch (19%), say a 1.3 inch (16%), then one can still regain the benefits of the larger illuminated field diameter by swapping back to the 1.83 inch secondary.

Jon


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Ajohn
sage
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Reged: 12/03/07

Re: Recommended secondary size new [Re: Jon Isaacs]
      #5622694 - 01/14/13 01:31 PM

I'd guess that a 2in would be best for wide field giving a fairly evenly illuminated field and it sounded like cardboard tubes where mentioned which is pretty good for thermal problems anyway.

If any one looked at the book they would see that an 8in F6 scope gives just over a 1 1/2 degree 1 1/4in dia field to the 0.1mm radial fanning limit he uses. So 2in eyepieces wouldn't be a bad idea for that size. What size flat is needed for that can be calculated from the sums that are listed afterwards. Some prefer as small a tube as possible to allow the flat to be kept to an minimum. With cardboard or even wood that might not be too bad.

John
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bremms
Carpal Tunnel
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Reged: 08/31/12

Loc: SC
Re: Recommended secondary size new [Re: Ajohn]
      #5622822 - 01/14/13 02:54 PM

Just pick something... Either will do. My preference is for a 1.83. You get a decent tube to focus distance and don't have to worry about field illumination. Still a lot better than an SCT for CO!

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bremms
Carpal Tunnel
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Reged: 08/31/12

Loc: SC
Re: Recommended secondary size new [Re: bremms]
      #5622833 - 01/14/13 03:00 PM

You can swap secondaries any time you want. The plane of the secondary just has to be in the same location. No need to move the primary unless you want to shift the focus point.

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nemo42
journeyman


Reged: 11/30/12

Re: Recommended secondary size new [Re: Ajohn]
      #5623197 - 01/14/13 06:30 PM

Protostar indicates that the diffraction spikes for a 3 vane should be much less bright and thus less noticeable for viewing. It will also be structurally stiffer than an arched spider. I was going to check with them what the thread pitch and size are for their secondary mirror holders for possibly changing them out.

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nemo42
journeyman


Reged: 11/30/12

Re: Recommended secondary size new [Re: Jon Isaacs]
      #5623205 - 01/14/13 06:36 PM

That is the advantage I see of being able to switch secondary mirrors. The scope could be optimized for say planetary viewing and for deep sky it could be improved even if not "optimized" with a wider illuminated F.O.V. at 100%. The guy who couldn't decide which secondary to use for his scope to be built with existing parts could do this too.

Edited by nemo42 (01/14/13 06:42 PM)


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bremms
Carpal Tunnel
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Reged: 08/31/12

Loc: SC
Re: Recommended secondary size new [Re: nemo42]
      #5623250 - 01/14/13 07:10 PM

Where is the emoticon of beating the dead horse? We need one.

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