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dob "in-focus" moving primary mirror


sunshine185

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i decided to move my primary mirror further up the tube on my 250px so my binoviewers could achieve focus without the 1.6x corrector or a barlow. This method could also be used to achieve focus with a camera. With help from the brother-in-law who is an engineer i removed the old collimation bolts, these were stuck in using epoxy or screwlock, they were so stuck we had to drill them out and re-tap the holes, i then used some m6 countersunk head bolts 100mm long, some m6 penny washers and some m6 lock nuts, i also bought some new springs but these were not used as they were too narrow, the springs need to have an inside diameter of 10mm to allow the collimation nut to fit in, so i used the old springs. we also drilled out and re-tapped the top of the collimation nuts to allow full adjustment of the primary mirror distance. got it all set up this morning and tested on the moon out front, i achieved focus with the drawtube slightly out. special mention goes to moonshane, who`s help and advice made this mod possible, u r a legend shane!!!

some before and after pics

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post-7368-0-23490200-1349515509_thumb.jp

post-7368-0-53031800-1349515577_thumb.jp

post-7368-0-42081800-1349515615_thumb.jp

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Can anyone give me a guide to do this? Step by step and parts list etc. Im sure every binoviewer owner probably wants to achieve this same result and needs a guide. I daren't take the scope apart without having a full and complete understanding of what is required. Also does this mod render the scope useless to single eyepieces once done? Does It lower the scope f number to 4.x? Thanks

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Can anyone give me a guide to do this? Step by step and parts list etc. Im sure every binoviewer owner probably wants to achieve this same result and needs a guide. I daren't take the scope apart without having a full and complete understanding of what is required. Also does this mod render the scope useless to single eyepieces once done? Does It lower the scope f number to 4.x? Thanks

The f number won't change appreciably. Instead of being 1200mm you might end up at 1125mm, so 1125mm/203mm (mirror size) = f/5.5 instead of f/5.9. I did have to use an extension tube to get my EPs to focus afterwards, however.

Aside from some washers I had lying around that seemed to fit, here's what I needed to move my mirror:

- Three 10-32 3″ countersunk slotted bolts. Mine were not countersunk but they worked fine too.

- Six 10-32 nuts

- A Philips head screw driver

- A small crescent wrench or channel locks

Might not want to use my blog as a guide since I redid it after I blogged about it. Surely there's a post on SGL on what to do.

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The f number won't change appreciably.

Okay, but as the primary moves up the tube, surely the light cone grows in size at the secondary, so it would need increasing in size to intercept all of it? Or am I missing something? Does the advantage of using both eyes outweight the loss of the outer light cone?

Russell

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Okay, but as the primary moves up the tube, surely the light cone grows in size at the secondary, so it would need increasing in size to intercept all of it? Or am I missing something? Does the advantage of using both eyes outweight the loss of the outer light cone?

Russell

You're right but honestly I don't see any difference personally. To my eyes, my scope gives me the same views as it did before the mirror move.

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Thanks for the replies. How do you lock the primary mirror in place once it's set? I see you only extended the mounting bolts and not the lock bolts as well. Does this mean that the primary could come loose if tipped past horizontal with it at the top of the pivot point?

thanks, im definitely going to do this as I desperatey want to see DSOs with both eyes. The loss of outer light cone (read: aperture) in my opnion is worth it. And if it turns out not to be that great then i'll just reset it back. :)

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the cell has threaded holes so no locking is needed at the cell end. I doubt you'll lose much aperture or reduce your focal ratio either as there will be likely to be enough capacity in the slightly bigger than needed secondary to account for both and still provide a fully illuminated field. it's a great mod for bino (and probably DSLR) users especially as it is not permanent - an advantage if you ever sell.

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Okay, but as the primary moves up the tube, surely the light cone grows in size at the secondary, so it would need increasing in size to intercept all of it? Or am I missing something? Does the advantage of using both eyes outweight the loss of the outer light cone?

Russell

Correct. First, hats off sunshine for implementing this mod, binoviewing is cool, wide field is cool, so this combines both. It is likely some of the light cone is clipped leading to very small loss of light gathering power, there is a formula relating "intercept distance" (=centre of spider to focal plane) to secondary mirror minor axis and f/ ratio. Likelihood the scope is now breaking the rule slightly buy the proof of the pudding is at the eyepiece :-)

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Indeed. I have spent much of this evening reading Gary Seronik's website regarding a number of Newt construction issues. It would seem likely that the mirror is likely to be slightly oversized anyway, due to the finite number of blank sizes available. There is a great calculator available on that same site that combined with a ruler, will let you know how much latitude your scope has - One for the bookmarks, me thinks. :)

Russell

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The f number won't change appreciably.

The f number of the primary mirror will not change at all! The only change in effective f no of the whole system will be if additional components have positive or negative powers such as a barlow lens or focal reducer. Removing the 1.6x nosepiece from the Binoviewer will change the effective f no at the Binoviewer eyepiece from 7.68 to 4.8 ( for the SW 250 dob ).

I suspect that the 1 1/4" fitting of the binoviewer will be more of a limitation than the diameter of the secondary. Check with a pinhole eyepiece in the binoviewer and see if you can see the whole of the secondary mirror.

Nigel

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The f number of the primary mirror will not change at all! The only change in effective f no of the whole system will be if additional components have positive or negative powers such as a barlow lens or focal reducer. Removing the 1.6x nosepiece from the Binoviewer will change the effective f no at the Binoviewer eyepiece from 7.68 to 4.8 ( for the SW 250 dob ).

I suspect that the 1 1/4" fitting of the binoviewer will be more of a limitation than the diameter of the secondary. Check with a pinhole eyepiece in the binoviewer and see if you can see the whole of the secondary mirror.

Nigel

You're right about the primary mirror but isn't the f-stop calculation focal length / mirror size? If I move the mirror up, the focal length changes right? Or do I have this all wrong? :D

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The f number of the primary mirror will not change at all!

True, but only if the entire primary can be seen in the secondary. If (and it is a big if, but not unknown) the secondary size is tight to the standard light cone and moving the primary causes some of the light cone to fall outside of the secondary, then the effective diametre of the primary has been reduced, whilst the focal length remains the same. It would have to a pretty extreme example for this to have significant effect, I'll grant you. :)

Russell

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