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C8 Schmidt-Newtonian Frakenscope

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#1 Michael Miles

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Posted 13 May 2014 - 04:22 PM

Hi folks:

I'm finally moving ahead on my project to convert a Celestron 8 to a Schmidt-Newtonian. I wanted to poll you folks for ideas on how to build certain things.

I've got related threads at:

http://www.cloudynig...Board=atm&Nu...

http://www.cloudynig...Board=atm&Nu...

http://www.cloudynig...Board=atm&Nu...

http://www.cloudynig...Board=ccd&Nu...

and here's a picture of the components I'm assembling:

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#2 Michael Miles

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Posted 13 May 2014 - 04:24 PM

and with the 4.1" minor axis flat sideways so you can see how big it is:

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#3 TxStars

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Posted 13 May 2014 - 04:52 PM

All I can say is "WHY??"
If I was going to try something like this I would move the corrector out to the radius of curvature of the primary to give a longer tube to work with.
I would also make the primary fixed position so the mirror will not move and remove the baffel tube.

#4 Michael Miles

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Posted 13 May 2014 - 05:05 PM

So, here's what I have to build:

1) secondary mirror holder
2) secondary mirror corrector mount and collimation screws
3) tube or connectors joining corrector mount and main mirror mount.
4) Hyperstar to tube mount
5) Hyperstar to camera adapter

I have a metal lathe (7"x14") and a small milling machine. I'm planning on making most parts on the lathe. I have never done any threading on the lathe, but am willing to learn for this project. Here are my ideas on how to do the above items (your input welcome):

1) secondary mirror holder - I've ordered one of these from Astrosystems. Would have built it myself, but I don't have good skills with sheet metal.

2) secondary mirror corrector mount and collimation screws - I was hoping to use the existing collimation mount in the C8, but it's an older glued in version, and the secondary mirror holder cannot be removed without removing the glued in secondary light baffle. Ideally I'd like a Starizona Hyperstar adapter, but I paid as much for my lathe as one of those costs, so I'm going to build one. Again, ideally, I would like to build a Hyperstar compatible secondary mount so I could swap my diagonal flat for the cassegrain secondary and use the scope both ways. I may or may not make a unit which is full Hyperstar compatible. I only need an adapter which will mount the Newtonian secondary,and may build that first before opting for the more complicated Hyperstar setup. I would really like the Hyperstar setup since this would give me the option of using the scope as a SN, SC, or Hyperstar Schmidt.

3) tube or connectors joining corrector mount and main mirror mount - I would prefer to not modify the old C8 orange tube primarily for resale reasons. That leaves me with choices about what to do instead. Since the prototype phase of the project will be so experimental, I think I'd like to not use a tube and use some sort of tube end connections instead so I have working room to fiddle around in the optical path to try things out. I'm thinking about using 3 inch aluminum channel top and bottom with maybe side truss(es) to experiment with. The Hyperstar mount could easily be mounted to the channel on top. A nice flat mounting suface gives me more options.

4) Hyperstar to tube mount - This could be as simple as a ring mounted to the top channel with a locking screw or as complex as a genuine Hyperstar-like screw mounting. Again, I don't have screw-cutting experience, so this seems daunting at this point.

5) Hyperstar to camera adapter - This is the only part which will absolutely require threads being cut. I've got a Canon T-mount which has the t-threads in a ring which is held into the bayonet mount with set screws. I would be making one with the same OD as the current t-thread unit but with the Hyperstar threads instead. I would also be machining the adapter to put the Hyperstar focal plane coincident with the camera focal plane. I could conceivably buy the Hyperstar C8-T-thread adapter, but the unit backfocus does not work with Canon DSLRs. However, I did measure the unit, and it is possible to build a lower profile Canon adapter which would allow use of the Hyperstar. This was the whole purpose for building the SN-C8 so I could use my DSLR with it.

Would appreciate any brilliant ideas that I've overlooked,

Michael

#5 GlennLeDrew

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Posted 13 May 2014 - 07:14 PM

At f/2 the off-axis correction (coma and field curvature, for starters) is likely to be poor, if not awful. Have you done a ray trace?

#6 Michael Miles

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Posted 13 May 2014 - 09:09 PM

At f/2 the off-axis correction (coma and field curvature, for starters) is likely to be poor, if not awful. Have you done a ray trace?


Hi Glenn:

The Hyperstar is supposed to correct for those.

Michael

#7 Michael Miles

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Posted 13 May 2014 - 09:15 PM

All I can say is "WHY??"
If I was going to try something like this I would move the corrector out to the radius of curvature of the primary to give a longer tube to work with.
I would also make the primary fixed position so the mirror will not move and remove the baffel tube.


Why - fun with optics!

Corrector plate at radius of curvature - I thought the corrector plate at that position needs different curves - I want to use the existing optics along with a Hyperstar.

Remove baffle/fix mirror - these are under consideration. Won't do it if I don't have to, so it depends on how the current configuration works.

Michael

#8 GlennLeDrew

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Posted 13 May 2014 - 10:38 PM

Michael,
Ah, I see. I misread, thinking you were striving for the usual Hyperstar configuration AND a Schmidt-Newt, to be interchangeable. In your case, then, the moniker Schmidt-Newtonian does not apply, strictly speaking, due to the presence of the Hyperstar corrector.

#9 Michael Miles

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Posted 13 May 2014 - 11:35 PM

Hi Glenn:

Well, there is limited room in the post title, so I had to pick something. I mainly wanted to use my Canon DSLR with an 8 inch C8, but the normal Hyperstar configuration wouldn't work. If I build the proper secondary mount, I could use the scope in the normal C8 configuration, the C8 with Hyperstar configuration (for one of their supported astrocams), and the Schmidt-Newtonian-Hyperstar-DSLR configuration. Only the C8 configuration would be usable visually.

Michael

#10 DocFinance

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Posted 14 May 2014 - 09:07 AM

Sorry, man, but I guess I just don't see the vision. If you've gat an Orangetube C8, why would you want to screw it up? If you want a Schmidt camera, sell the C8 to someone who will use it and buy a Schmidt camera.

#11 tim53

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Posted 14 May 2014 - 09:54 AM

You can nearly accomplish the same end with a Meade f3.3 FR and a Mogg 0.5x FR threaded into the nosepiece of a camera with a 1 1/4" adapter. I've not done this with an f/3.3, but have with the common f/6.3, but I'm told it works well because the Mogg is so close to the ccd.

And I'm with other folks on the orange tube. They are still common, though. And it is YOURS, so enjoy!

Tim

#12 dmcnally

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Posted 14 May 2014 - 10:44 AM

Hi Michael,
I think what you're doing is very interesting. It is definitely out of the box thinking. If you already had the hyperstar and C8 and wanted to be able to use larger cameras (maybe a CCD with filter wheel) I can see the attraction, especially since you already have the expensive parts.

I'm a noob, but the secondary mirror (4.1" minor axis) seems huge and I wonder how much it will effect the light gathering power of the finished scope.

Good luck and please update the thread with your progress.

Dave

#13 GlennLeDrew

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Posted 14 May 2014 - 01:21 PM

The 4.1" secondary blocks 26% of the light, which makes the f/2 geometric ratio into a T-stop of f/2.33. For fast prime focus imaging systems, such larger--and larger--obstructions are common, and do not impair resolving power.

Retaining the original configuration and attaching a 0.33X reducer at the rear provides a useable image circle hardly larger than 8mm. That's fine for so-called 1/2" format chips. Adding another reducer in the train only further reduces the useable image circle.

#14 Michael Miles

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Posted 14 May 2014 - 03:27 PM

Hi folks:

Well, lots of posts so far about why I SHOULDN'T do it, but they're moot because I'm GONNA do it, so enjoy the ride.

I heard from Randy at Astrosystems, and he's thinking he'll ship the secondary holder in about a week - much sooner than I'd hoped.

We need as many crazy ideas as we can here on the CN DIY. I always enjoyed S&T's Gleanings the most when they had crazy ideas like polar scopes and Crayford focusers. This project isn't outside the skill set of most of us, so I thought it would be fun to share.

As to the central obstruction, Rutten & Van Venrooij' book Telescope optics says that a central obstruction of up to 40% is ok for photography.

I think this scope will meet my criteria of a fast optical large aperture (larger than a camera lens) system which allows me to use my DSLR and avoid carting a laptop and paraphernalia around (required by other astrocameras).

;), Michael

#15 mattyfatz

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Posted 14 May 2014 - 04:24 PM

Michael,

I am excited to see the results. I wonder about the goal of this scope though. Are you hoping to get better results from this scope, than you would from an RC, Dall-Kirkham, or schmidt camera optical arrangement? Those designs are suitable for AP work with large illuminated fields (and less concern for the CO). A project like that might be a worthy one as well!
Since the Hyperstar is made to attach to the front cell of an SCT, how will you mount it now?

Thanks,

#16 jzeiders

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Posted 14 May 2014 - 05:17 PM

Have you looked into Wright-Schmidts? The one I used was a fast f/3 Newtonian with a large true flat field and was used mostly as an astrograph.

This scope was a home made back in the '70s. As I recall the primary was a hyperbaloid and the corrector had steeper curves than a SCT corrector. I am unsure if this was because the builder may have put all the correction on one surface rather than splitting it front and back. He did his own optics.

Jack

#17 Michael Miles

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Posted 14 May 2014 - 10:22 PM

Michael,

I am excited to see the results. I wonder about the goal of this scope though...
Since the Hyperstar is made to attach to the front cell of an SCT, how will you mount it now?

Thanks,


Hi Matt:

Optically, the Hyperstar should work the same with this configuration (side mounted facing down at the diagonal mirror) as if it was in the front except for the larger central obstruction.

The goal is simple - I wanted to use my Canon DSLR with the Hyperstar on a C8, and that configuration is not supported by the manufacturer. So, I'm making my own solution to the problem. The C11 or C9.25 would mount the DSLR on the front, but that would be much bigger and too expensive.

The next step is to cut the shelves in my bedroom closet to the proper length so I can clear the junk away from in front of my lathe. I'm also mounting my lathe and milling machine on a new table that is more stable. If I motivate myself, I should be able to get that done before the new secondary holder arrives :).

No project is just the project...

Michael

#18 freestar8n

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Posted 15 May 2014 - 02:29 AM

Hi-

I think this is a fun idea and I think it's great you are trying it. I would call it a corrected Schmidt Newtonian astrograph - or a Newtonian Hyperstar.

There is definitely some work involved, but you are leveraging commodity components well - including both the optics and the tilt/collimation adjustments already present on the hyperstar.

I don't know how you plan to attach the secondary - but if you can avoid using a spider and attach it directly to the corrector - it would retain the spiderless images while also guaranteeing a clean looking diffraction pattern - which hyperstar images usually lack due to the wires. But if you do need to use a spider, which I expect, it will still result in clean diffraction spikes reliably - and that will have impact on the image.

The use of a large secondary isn't really a downside when using a dslr with hyperstar, since they already block a lot of light.

Finally - if you can use eyepieces with this, then it might be the first visual use of hyperstar that I'm aware of - and that would be interesting by itself.

So - nice work and a fun project - and I look forward to seeing how it goes. I assume no one has done this before with hyperstar - so it is pioneering.

Frank

#19 Michael Miles

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Posted 15 May 2014 - 04:19 AM

Hi freestar8n:

Well, you've got me thinking some more...

I think I will see what it looks like to use an eyepiece with this rig. It will make it even weirder looking because the eyepiece would have to be further out than the focal plane of the camera. I don't know what advantage it would offer though, but it won't hurt to look :).

Michael

#20 Michael Miles

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Posted 15 May 2014 - 04:20 AM

Just an FYI, the guy I bought the C8 from is a DIY builder too, but he builds airplanes instead of telescopes...

We builders have to stick together.

#21 freestar8n

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Posted 15 May 2014 - 04:39 AM

Hi-

The visual aspect is more of a novelty because there is no inherent benefit in using a fast system for visual. It's just kind of cool to use an eyepiece with a well corrected 8" f/2 system - and actually "look into" a hyperstar.

But for imaging, having a well corrected field over a decent sized detector at f/2 represents very high optical throughput - and is very different from f/3, even with the 50% obscuration - which actually isn't unusual for a fast astrograph.

My main concern would be collimation, which will have many degrees of freedom with the secondary tilt included. I don't know if you plan to focus with the primary focuser - which makes sense - but you might want to make that system tighter for this application. You won't be using it at f/10, so you are less limited by diffraction and instead I think you can be more aggressive about clamping the mirror without affecting the images.

Frank

#22 Gil V

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Posted 15 May 2014 - 06:08 AM

At Criterion, one of my co-workers and I made one of these - without the corrector!

As you can imagine, it was a horrorshow.

#23 Michael Miles

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Posted 15 May 2014 - 03:48 PM

...My main concern would be collimation...Frank


Thanks Frank:

I'm concerned about the collimation also. Astrosystems has a web page about the challenges of fast Newtonians that highlight the collimation tolerance issues (but only down to f/3):

https://www.astrosys...ShortFRatio.htm

I was thinking that I could make the secondary holder collimation screw compatible with the cassegrain secondary holder, but this would be more difficult to collimate with the diagonal flat. I've built a mirror mount before that echos the theodolites we used to use for surveying that have a 4-point X-Y axis tilt adjustment instead of the 3-point system (which is more difficult to control precisely). McMaster-Carr might be getting some money from me for fine thread stainless steel bolts for the adjusters.

Actually, I could combine the 3-point and 4-point hols in the secondary corrector plate plug so I could use both the diagonal flat and the cassegrain secondary.

Thanks for bring up the issue,

Michael

#24 Michael Miles

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Posted 15 May 2014 - 04:02 PM

...I don't know how you plan to attach the secondary - but if you can avoid using a spider and attach it directly to the corrector...Frank


Hi Frank:

Well, the good folks at Starizona think that the C8 corrector is strong enough to hold the Hyperstar and an astrocamera, and it is noticeably heavier that the Newtonian secondary will be (but with a shorter moment arm). I'm willing to give it a try and cross any burning bridges as I come to them.

I was actually planning on using a temporary spider to test things before expending the cost/effort in building the secondary corrector plug unit.

Michael

#25 TxStars

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Posted 15 May 2014 - 07:41 PM

Before cutting any tube you can use straight aluminum bars to hold the parts in place and get everything laid out.






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