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8kids

What size secondary mirror?

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I've amassed all the necessary bits and bobs for my truss dob build, the final piece is the secondary mirror. The primary is a 12" f6.5 so I'm asking those in the know what would be a practical size for planetary and Dso viewing. I've had a look at various programs but want to have a real world view from those who build and use scopes, I know it will be a compromise but does anyone have any "views".......

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The secondary on my 12" F/5.3 is 63mm which is 21% of the primary diameter. You might get away with a slightly smaller one as your scope is slower than mine. Mine is the standard size that Orion Optics supply with their 12" F/5.3 primaries so I assume is suited for general viewing use - it certainly works very well at both low and high magnifications :smiley:

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Mel Bartels published calculations in Sky & Telescope, some years ago. These are now available online at a couple of sites.

http://www.bbastrodesigns.com/diagonal.htm

http://www2.arnes.si/~gljsentvid10/diagonal.htm

The former includes the light lost due to the obstruction and generates a graph. The latter does not make this allowance and gives numerical output.

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A slightly oversize diagonal mirror is far less detrimental to performance than a slightly undersize one. :smiley:

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A slightly oversize diagonal mirror is far less detrimental to performance than a slightly undersize one. :smiley:

Oddly, perhaps not. In the past secondaries were often deliberately undersized to mask off the outer edge of the primary which was typically turned. In this scenario a smaller secondary improves the view. You don't notice the light loss because it's going to be on the order of 10% or so.

When I got my 18" I realised the secondary was under-sized because I couldn't see all of the primary. After some messing about, I calculated that I was losing 16% of the light. I got a larger secondary, but I can't say I noticed the difference. In the process of doing this, I called Rick Singmaster from Starmaster telescopes and, IIRC, he told me that his personal scope still has an undersized secondary.

So long as the secondary is under 25% (ideally, under about 20%) of primary diameter, then you're good. I used the Bartels' calculator: http://www2.arnes.si/~gljsentvid10/diagonal.htm

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I have a 300mm f6 and its fitted with a 57mm secondary. I guess the easiest thing to do would be for you to see what standard sizes are available and pick the closest to your desired spec. If you only want to use it for visual a low profile focuser helps to keep the secondary size down. :smiley:

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Thanks for all the comments. Laurie61 your scope is the nearest f ratio to mine and 56mm is available as a stock item, I'll probably go with that, I can always go larger if needs be.

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Assuming the following:

Secondary cage/tube diameter 350mm, focal plane 100mm outside of the cage, then a 56mm flat will give you a fully illuminated field of 16mm and an obstruction of 19%.

Oddly, perhaps not. In the past secondaries were often deliberately undersized to mask off the outer edge of the primary which was typically turned. In this scenario a smaller secondary improves the view. You don't notice the light loss because it's going to be on the order of 10% or so.

When I got my 18" I realised the secondary was under-sized because I couldn't see all of the primary. After some messing about, I calculated that I was losing 16% of the light. I got a larger secondary, but I can't say I noticed the difference. In the process of doing this, I called Rick Singmaster from Starmaster telescopes and, IIRC, he told me that his personal scope still has an undersized secondary.

So long as the secondary is under 25% (ideally, under about 20%) of primary diameter, then you're good. I used the Bartels' calculator: http://www2.arnes.si...10/diagonal.htm

Not sure that you are correct in qualifying as "in the past". There have been comments here in SGL to the effect that some telescopes are still supplied with undersized flats. While you are correct that an undersized flat will mask the edge errors on the primary, the edges of the flat are now important and if the telescope makers are not providing quality primaries, can we be confident on the quality of the secondaries which are somewhat more difficult to test?

Personally, if I purchased a 12" telescope I would expect the performance of a 12", not of an 11".

Nigel

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Not sure that you are correct in qualifying as "in the past". There have been comments here in SGL to the effect that some telescopes are still supplied with undersized flats. While you are correct that an undersized flat will mask the edge errors on the primary, the edges of the flat are now important and if the telescope makers are not providing quality primaries, can we be confident on the quality of the secondaries which are somewhat more difficult to test?

Sorry, you're right. I forgot about the GSO and Synta scopes that have been coming out with under-sized secondaries. I don't know what their reasons are for doing that, though. Maybe there is no good reason?

The edges on the flat are always important, however. They're important whether or not the flat is undersized. That's because the full field isn't illuminated (compromise with secondary size) and as you move away from the axis you're starting to use the edges of the flat. If I remember correctly, though, turned edges are not a problem with flats. I think a flat can be curved (and so induce astigmatism) but it can't have a turned edge in the way that primary can. The turned edge will scatter light throughout the whole field and create a nasty, contrast reducing, veiling glare.

I'd say GSO and Synta are, generally speaking, producing good quality primaries. They're not the best primaries you can buy, but they're great for the money and some of them are very good indeed. I would think an optical shop that's set up for it could test a primary and a flat equally easily, no?

Personally, if I purchased a 12" telescope I would expect the performance of a 12", not of an 11".

I agree, although it's also true that you'd be hard-pressed to tell the difference.

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Cheers Nigel and Umadog. It's a John Nichol mirror and I want to lose as little as possible a 19% obstruction is better than 21%. Hopefully I'll be able to tell if it's a problem :smiley:. I was meant to start the build weeks ago but work and everything else as got in the way. I want to take it with me to Scotland on the 20th of this month, so I better get my skates on.

Edited by 8kids

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http://www.telescope-optics.net/obstruction.htm

A very nice article on said subject, thankfully pointed out to me by Umadog at one point, and even without going into the detail of the math in that article, the implications are clearly expressed with a lot of handy graphs. I found it to be very educational anyway.

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Cheers Nigel and Umadog. It's a John Nichol mirror and I want to lose as little as possible a 19% obstruction is better than 21%. Hopefully I'll be able to tell if it's a problem :smiley:. I was meant to start the build weeks ago but work and everything else as got in the way. I want to take it with me to Scotland on the 20th of this month, so I better get my skates on.

If you have an option, think about curved secondary support vanes as well. I have them on my 12" OO dob and, hey presto, no diffraction spikes even on bright objects. Oh, I know there is still diffraction going on but it seems to be reduced overall from conventional 4 vane scopes I've owned prior to this. The 12" OO is the 1st scope that I've been able to see Sirus B with so it's doing something right :smiley:

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If you have an option, think about curved secondary support vanes as well. I have them on my 12" OO dob and, hey presto, no diffraction spikes even on bright objects. Oh, I know there is still diffraction going on but it seems to be reduced overall from conventional 4 vane scopes I've owned prior to this. The 12" OO is the 1st scope that I've been able to see Sirus B with so it's doing something right :smiley:

I've been pondering this very thing John, does yours have three vanes or four? I have a spider I can use but I've had a play at bending some thin steel and aluminium and I like the look of it I also like the steel cable spiders but that looks more complicated.

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Heres a picture of mine, below. I read quite a lot of mixed views on the difference that these can make and was rather skeptical before I used a scope equipped with them but I feel they do make a difference. To be entirely scientific I guess I'd have to have a set of conventional 4 vane ones fitted to the same scope for a while and then compare the views but thats not really practical !. David Lukehurst uses a single curved vane on his dobs smaller than 16" I think so that might be an even better option.

post-118-0-20666600-1372691729_thumb.jpg

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I've not seen that type before. I had a go with a single curved vane and it seemed to work well. As with most things there's too much choice.

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The edges on the flat are always important, however. They're important whether or not the flat is undersized. That's because the full field isn't illuminated (compromise with secondary size) and as you move away from the axis you're starting to use the edges of the flat. If I remember correctly, though, turned edges are not a problem with flats. I think a flat can be curved (and so induce astigmatism) but it can't have a turned edge in the way that primary can. The turned edge will scatter light throughout the whole field and create a nasty, contrast reducing, veiling glare.

I'd say GSO and Synta are, generally speaking, producing good quality primaries. They're not the best primaries you can buy, but they're great for the money and some of them are very good indeed. I would think an optical shop that's set up for it could test a primary and a flat equally easily, no?

I would like to see some optical proof that a turned edge on a secondary only produces overall scatter before I accept your argument. To me, and simplifying, I believe that a slightly turned edge on the secondary will behave as a negative element giving a slightly longer focus ( + astigmatism ) to the edge rays and the sharp central rays will then be surrounded by an out-of-focus image thus reducing the resolution ( and fine detail contrast ). With an oversized secondary this will only occur away from the optical axis as you state and will only be seen at low powers. As soon as high powers are used then only the central portion of the secondary will be in use and the edge will have no effect.

Certainly an optical shop can test mirrors and flats equally easily, as I can, but many owners of telescopes will have problems testing flats while the primary can be tested with a pinhole and knife edge in any garage.

Nigel

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my once owned 12" f7 had a 2" flat. you could see all the primary as per umadogs post, fov ulimately limited by 11/4" focuser.

Assuming it's for visual use only I think a well placed smaller flat would arguably be better than a larger badly placed one.

By that i mean the closer you can get the ep to the flat the larger the fully illuminated field you'll end up with.

So with some careful forethought you could use a smaller flat than if the ep comes to focus say 7" past the outside of the tube.

Some careful measuring etc before you fix the primary to secondary distance~ you want all your ep's to come to focus, & design accordingly, hope thats of some use.

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I would like to see some optical proof that a turned edge on a secondary only produces overall scatter before I accept your argument. To me, and simplifying, I believe that a slightly turned edge on the secondary will behave as a negative element giving a slightly longer focus ( + astigmatism ) to the edge rays and the sharp central rays will then be surrounded by an out-of-focus image thus reducing the resolution ( and fine detail contrast ). With an oversized secondary this will only occur away from the optical axis as you state and will only be seen at low powers. As soon as high powers are used then only the central portion of the secondary will be in use and the edge will have no effect.

Certainly an optical shop can test mirrors and flats equally easily, as I can, but many owners of telescopes will have problems testing flats while the primary can be tested with a pinhole and knife edge in any garage.

I obviously wasn't writing clearly. I meant what you're saying: that a curved secondary (should it occur) will cause astigmatism, not veiling glare. I was also speculating that, because a secondary is manufactured differently to a primary, it is unlikely to have a turned edge of the sort seen on a primary. Thus, the whole mirror might be curved and so astigmatic, but I think you're unlikely to have a scenario where the centre of the flat is good but the edge is turned. Can you comment on that? All I could find was this page: http://www.loptics.com/articles/ellipticals/ellipticals.html

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Well, you can get all sorts of figures on a flat ( I know, I've produced flats with most of the errors at one time or another ) and the images on the web site you link to show that. There are smooth curves, rather rough surfaces and turned edge ( called here a rolloff edge ) and a turned up edge which is just as bad as a turned down edge. As Mike Lockwood mentions, he no longer trusts that supplied flats really are good enough. He also makes the point that a turned edge secondary will not materially affect the high power image provided it is large enough.

Thanks for that Link, Umadog, nice useful site.

Nigel

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I'm certainly more comfortable with the Antares explanation. Thanks for the link Chris C.

It strikes me that, like a number of other questions, theory and computer programs are all very well but to nail the truth on a subject it needs a practical test. Something along the lines of the Ceravolo test of primary mirrors where he made four mirrors with different figuring errors and used them in telescopes side by side. Unfortunately I won't be able to do this for some time yet as I have a long list of other TO-DO items, not the least of which is item 1 'to make a new workshop' so that I can get on with item 2 on my list.

Nigel

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I'm not an optician so I can't offer any practical argument, but as a retired professional telescope maker I worked "cheek to jowl" with several top class opticians. On the topic of flatness of flats, the general opinion was that smoothness was paramount, a very slight convex or concave surface caused only a minute change to the overall focal length, being as these were figures of revolution there was no mention of induced astigmatism, :smiley:

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