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Recommended secondary size

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#1 nemo42

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Posted 11 January 2013 - 06:47 PM

I am making an 8" Newtonian telescope. I would like to use it for planetary viewing and for as many deep sky objects as possible. The primary mirror is an f6 (47.3"). As I see it, my choices are probably between a 1.52" or a 1.83" secondary mirror. Which would you recommend and to what advantage?

I have a 1.25" focuser and don't want an unnecessarily narrow field of view but also want to maximize light gathering and contrast. I am considering a 3 vane spider to reduce visible diffraction.

#2 jg3

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Posted 11 January 2013 - 09:29 PM

Welcome to CN!

A great read on secondary sizing is by Gary Seronik at http://www.garyseronik.com/?q=node/8

It covers the trade-offs involved in the choice. For that size and smaller, there's really no having it both ways (deep-space or planetary optimized).

Try to keep the spider vanes thin. Good luck with the project!

#3 bremms

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Posted 11 January 2013 - 10:30 PM

1.52 is good if it's a planetary scope. For an all rounder 1.83 is a better choice. I have an 8 in f6 mirror in my to build pile. Have 1.83 and 1.52 diagonals too. Haven't decided either. Leaning toward a 1.83 with a low profile focuser. CO is still a lot smaller than a C8

#4 Pinbout

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Posted 12 January 2013 - 12:33 AM

Personally I'd go with the 1.5 but more importantly I would use a 2in focuser. the 2in wide field eyepieces will make your scope totally awesome. don't limit yourself...

2in lowprofile...

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#5 Jon Isaacs

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Posted 12 January 2013 - 12:39 AM

I am making an 8" Newtonian telescope. I would like to use it for planetary viewing and for as many deep sky objects as possible. The primary mirror is an f6 (47.3"). As I see it, my choices are probably between a 1.52" or a 1.83" secondary mirror. Which would you recommend and to what advantage?

I have a 1.25" focuser and don't want an unnecessarily narrow field of view but also want to maximize light gathering and contrast. I am considering a 3 vane spider to reduce visible diffraction.


I would run a program like Newt for the web or Mel Bartels secondary calculator. The right size for the secondary depends on a variety of factors including the focuser height, focal ratio, tube diameter. Without knowing those things, it's hard to make a decision.

I am one who prefers slightly larger rather than slightly smaller secondaries. The effect on the contrast is tiny and a larger secondary is more forgiving.

Jon

#6 mconnelley

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Posted 12 January 2013 - 03:51 AM

Hello:

While I'd normally vote for the larger secondary, I think the 1.52" one is fine in this case if you keep the focuser short. The beam size at the secondary will be around 1.1" to 1.3", so a 1.5" secondary gives you some room for a useful field of view. If you go with a really short focuser, extend the tube out front a bit more for good baffling. I have a Kineoptics helical crayford focuser on my scope, and I really like it. I got it because it's light, but it's also low profile. I think it'd be really hard to tell the difference at the eyepiece between these two choices.

Cheers
Mike

#7 siriusandthepup

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Posted 12 January 2013 - 04:41 AM

+1 for what Pinbout said.

10" diameter tube. 1/2 of that is 5". Add 2" for a low profile focuser. 5" + 2" = 7". Divide 7" by the focal ratio. 7" / 6 = 1.16" point illuminated field diagonal size. Add some additional size for a bit of 100% illuminated field and positioning tolerance. Next standard size up is 1.3". Hardcore planetary stickler only! Very tight on the positioning. For me, the next standard size up 1.5" is pretty much perfect for general observing. 1.83" or 2.1" are good if widefield photography is your top priority.

#8 Jon Isaacs

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Posted 12 January 2013 - 08:59 AM

+1 for what Pinbout said.

10" diameter tube. 1/2 of that is 5". Add 2" for a low profile focuser. 5" + 2" = 7". Divide 7" by the focal ratio. 7" / 6 = 1.16" point illuminated field diagonal size. Add some additional size for a bit of 100% illuminated field and positioning tolerance. Next standard size up is 1.3". Hardcore planetary stickler only! Very tight on the positioning. For me, the next standard size up 1.5" is pretty much perfect for general observing. 1.83" or 2.1" are good if widefield photography is your top priority.


With a true Low profile focuser like the Kineoptics Helical crayford, the 1.5 inch looks very good. With a standard focuser, which would put the focal plane more than 3 inches from the tube, it would be questionable...

I think the 10 inch tube is a good idea, better flow, helps keep the tube currents that that initiate from temperature differences between the tube and the air out of the optical path.

Jon

#9 george golitzin

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Posted 12 January 2013 - 12:20 PM

I would also recommend 1.83 for general use--that said, I have a 1.5 on my 8-inch f/6, but the tube is (IIRC) 9 inches ID and the focuser is a low profile JMI 1.25 mini. As others have said, I would only recommend 1.5 (or smaller) if you're careful to control focuser height above the secondary, or else you'll wind up vinetting the primary.

One thing that gets overlooked in these conversations is that the gain in contrast in moving to a smaller secondary can be wiped out by the aberrations of the edge of the secondary--in general, you want to avoid using the extreme edge of the secondary. I found, in my 10-inch f/5, that when I went from a 1.83 Antares secondary to a 2.1 protostar secondary, there was less scatter and a general improvement in planetary images.

So that's why I think you're better off with the 1.83.

-geo

#10 bremms

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Posted 12 January 2013 - 04:38 PM

Yes , the edge of the secondary is not where you want the light cone. 1.83 is not a big CO and you don't have to mash your face against the tube like a really low profile focuser.
1.5 will work fine but a 1.83 won't limit your focuser choices as much. My 6" F8 has a 1.52" diagonal and it is better than any of my other scopes on Jupiter. I have a nice 1.30 inch quartz that needs to be glued to a holder an painted. The 1.5 gives a full +20mm unvingetted field and I'm really happy with the performance. The 1.3 would be better for planets and smaller objects. Don't know if I could tell the difference or not.
I have a nice 8" F6 waiting to be built and will probably use the 1.83. A good friend had an 8" F6(VERY good mirror) with a 1.5 and it gave really good planetary images. Some of the best I've seen of Jupiter.The mirror was about 1/18 wave P-V wavefront. Truly a superb optic. The fully illuminated field was very small. It was noticeable on Deep sky objects if you really looked.

#11 derangedhermit

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Posted 12 January 2013 - 09:50 PM

1.52" for a 1.25" focuser is plenty, as long as the distance from the center of the diagonal to the focal plane is less than 7.25". With a 3" tall 1.25" focuser, you will get some vignetting from the drawtube. If it's 2" tall, you're good. The link to Seronik's page shows in the chart how a 1.83" diagonal is overkill for a 1.25" focuser with the distance diagonal-focal plane of 7".

It takes a 1.83" diagonal to make full use of a 2" focuser.

I have an 8" f/6 mirror, and will build the tube using:
- 9.6" ID Protostar flocked tube
- 1.52 Protostar quartz diagonal with curved spider (high reflectivity, but low scatter is the main appeal to me for the quartz mirror)
- Kineoptics 1.25" focuser (I wish Moonlite made a 1.25" focuser; a 2" Moonlite would also be fine)

#12 nemo42

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Posted 13 January 2013 - 12:30 AM

I currently have a 1.25" University Optics focuser and it is rather tall. From the top of the focuser tube to the top of the 10" scope tube with the focuser racked all the way in would be approximately 3.75". It also has a lot of extra outward travel that is probably not necessary. With the "Newt for the web" program this ends up requiring non standard secondary sizes and the 1.83" one I have would cause vignetting at the 75% ray.

Focal length of the mirror is 47.3", f ratio 5.9125.

I may have to change my focuser to a 2" low profile one to allow a standard secondary size and a smaller secondary size. If so, I would like an economical option and something that will work with a 48" overall tube length. I haven't bought a tube yet but 48" tubes should be available.

Comments, further recommendations?

#13 dpwoos

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Posted 13 January 2013 - 12:56 AM

I have always been able to get any length I want by going to places that specialize in concrete work stuff. Also, the quality of the sonotube is better at this kind of place than it is at a home center. Sonotube is great, but you don't want the cheap stuff.

#14 derangedhermit

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Posted 13 January 2013 - 02:36 AM

I currently have a 1.25" University Optics focuser and it is rather tall. From the top of the focuser tube to the top of the 10" scope tube with the focuser racked all the way in would be approximately 3.75". It also has a lot of extra outward travel that is probably not necessary. With the "Newt for the web" program this ends up requiring non standard secondary sizes and the 1.83" one I have would cause vignetting at the 75% ray.

Focal length of the mirror is 47.3", f ratio 5.9125.

I may have to change my focuser to a 2" low profile one to allow a standard secondary size and a smaller secondary size. If so, I would like an economical option and something that will work with a 48" overall tube length. I haven't bought a tube yet but 48" tubes should be available.

Comments, further recommendations?

If you want to keep the focuser, then you need to keep the 1.83 diagonal to go with it. It's required to illuminate the 1.25" field that far outside the tube.

If you can figure out some way to shorten the focuser and drawtube from 3.75 min to, say 2" min, then you can use a 1.5" diagonal. All you lose is travel, and you don't need more than 1"-1.5" if you are just using eyepieces.

If you go with a Kineoptics 1.25" focuser and a 9" tube, then you can use a 1.3" diagonal. I say Kineoptics since it is very low profile and it is relatively inexpensive.

If you already have the focuser and 1.83 diagonal, and were to change one, I'd keep the diagonal and change the focuser to a 2" Moonlite (single stage, 1.6" min focuser height w/ curved adapter, order with 1.5" travel). The JMI RCF is cheaper, but I think the Moonlite is easily worth the extra price. There may be a cheaper import similar low-profile Crayford, but if there is, I don't know about it.

That focuser would also work if you can find a way to be able to swap diagonals from 1.83 to 1.3 for planetary viewing and back again - as long as the tube OD is around 9.5" or less.

Stock concrete form tube may be 48" or 45", I forget. But if it is 45", it may be all is not lost. As long as it is long enough to use for attaching the spider, you're OK. You probably want to add a removeable light shield out beyond 48" anyway to reduce reflected and scattered light.

#15 Jon Isaacs

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Posted 13 January 2013 - 05:58 AM

Nemo:

I ran Newt for the Web with the basic specifications for your scope with the 1.83inch secondary, a 10 inch tube ID, a 3.75 inch focuser height with an added 0.5 inches for inward travel.

It looks good to me. The vignetting of the 75% ray is not a problem, it takes a while to understand why it is not an issue but you can ignore it. A 2 inch focuser will make this flag disappear...

Still, I like 2 inch focusers, I think you ought to get yourself a 2 inch focuser so you can use 2 inch eyepieces. You need to decide on the focuser now because if you go to a lower profile focuser, you will need to change things around, move the primary back or move the focuser and secondary forward.

Since you have the 1.83 inch secondary, try it... the secondary can be changed later without needing to reconfigure the scope.

Jon

#16 Ajohn

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Posted 13 January 2013 - 01:44 PM

There is a free downloadable book on the internet archive called "how to make a telescope" by texereau. All of the sums and suggestions on 2ndry sizes based on the level of aberrations you are prepared to accept. There is no reason why you shouldn't go larger but it's a fact that planetary contrast will suffer. He suggest having 2 different flats if that aspect is important. Not too difficult if you are making the holder yourself. Then there are all of the suggestions for reducing the size of it that people have posted.

John
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#17 dpwoos

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Posted 13 January 2013 - 05:17 PM

There is no reason why you shouldn't go larger but it's a fact that planetary contrast will suffer.


All kinds of stuff will degrade the image, including taking the scope outside and using it (where the coatings will degrade). The question is by how much, and in this case I think that the difference is negligible. If I had a 1.83" then I would use it. In fact, when I built my 10" f/6 dob I did have a 1.83" secondary in hand, and I did use it - resulting in some of the best planetary views of any scope in the field.

#18 Ajohn

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Posted 13 January 2013 - 07:42 PM

It would be very good on a 10in as it's less than 1/5th of the dia of the scope. That's not a bad idea for planetary work and similar. What's degrading the image in this area is pure simple physics. It wouldn't matter if the scope was swinging from a tree.

I didn't suggest that the 1.83 shouldn't be used either. At 22% on an 8in scope that is fairly good too. For deep sky work he might like to go bigger. There is a pretty good explanation of what goes on some way down this page. Not theory fact.

http://www.handprint.../ASTRO/ae3.html

It's generally reckoned that a 10% obstruction has a truly negligible effect. 20% an acceptable level of effect and that 30% in real terms is a bad idea but unavoidable on some designs. The manufacturers often quote percentage area which is missleading. The important part is the percentage of the diameter of the scope.

The book I mentioned that can be downloaded as it's well past copyright goes a little bit further. It gives mirror sizes for a field size set by how much star images are allowed to smear out radially due to the way newtonians work. That can be taken further with a coma corrector but on many scopes even the smear limit can cause a rather large obstruction due to the 2ndry mirror which in turn knocks contrast on the head making it a poor choice for planetary work. Often commercial newtonians come out at 25% (or did). That's a sort of in between limit. It results in slightly poorer contrast levels than a 1/4 wave mirror
would give. Pity the graph on the page I just linked to doesn't show that.

John
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#19 nemo42

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Posted 13 January 2013 - 07:56 PM

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.

#20 Jon Isaacs

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Posted 13 January 2013 - 09:10 PM

It would be very good on a 10in as it's less than 1/5th of the dia of the scope. That's not a bad idea for planetary work and similar. What's degrading the image in this area is pure simple physics. It wouldn't matter if the scope was swinging from a tree.



However.... the effect of the secondary size is small. Some people obsess over the size of the secondary. I suggest obsessing over the thermal equilibrium, the collimation and the seeing.

Jon

#21 nemo42

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Posted 13 January 2013 - 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.

#22 dpwoos

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Posted 13 January 2013 - 09:49 PM

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.

#23 Jon Isaacs

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Posted 13 January 2013 - 09:52 PM

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

#24 dpwoos

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Posted 13 January 2013 - 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.

#25 Jon Isaacs

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Posted 13 January 2013 - 10:34 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.


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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