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Unable to align secondary mirror to focuser tube

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#101 Vic Menard

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Posted 08 June 2022 - 10:23 AM

...I do have one reservation about seeing the entire primary reflection. As I recall, one image you posted with the focuser all the way down seems to show a rather large primary reflection relative to the apparent size of the diagonal. So, the diagonal may be optimized for minimal obstruction. I am hopeful a test fit at 8.3" will show whether that's the case. 

The central obstruction appears to be around 25-percent (I'm not too sure about the secondary mirror's clear aperture). A 5-inch f/10 with a 1.25-inch focuser should work pretty well with a 1.25-inch secondary mirror (0.5 magnitude drop at the edge of the field of view--a 21mm secondary is the smallest possible for a 24mm max field diameter using Bartles' calculator).


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#102 patrickt

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Posted 08 June 2022 - 09:25 PM

ASBYTEC -

                    I just measured my tube I.D. Its 22.9mm.  Using the f ratio x tube I.D. = tube length :

 

10 x 22.9mm = 229mm ( 9inches ). This would place the peephole/pupil ABOVE the Focal Plane of 210mm ( 8.2 inches ). pardon the capitalized Above, just wanted to emphasize...  

 

                I calculate that i need a 21.5mm I.D. to bring down tube length to a reaonable 8.5 inches.(cut shorter as needed of course). Even if i found a tube with the 21.5mm I.D., the O.D. would be correspondingly too small and loose in the focuser. 

 

                 Short of having a machinist do a correct enough I.D. and O.D., i am of course trying to minimize axial misaligbment of the sight tube, relative to the secondary mirror. hmm...

 

VIC MENARD - 

                          Thanks for your reply. I need to read your response again though, to properly absorb it. I will do it later and get back.


Edited by patrickt, 08 June 2022 - 09:27 PM.


#103 Asbytec

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Posted 08 June 2022 - 10:50 PM

ASBYTEC -
I just measured my tube I.D. Its 22.9mm. Using the f ratio x tube I.D. = tube length :

10 x 22.9mm = 229mm ( 9inches ). This would place the peephole/pupil ABOVE the Focal Plane of 210mm ( 8.2 inches ). pardon the capitalized Above, just wanted to emphasize...

Patrick, then I'd recommend cutting it at 9" to start. That length would include the distance to the cap with a 2mm pupil drilled at the center. Then go from there.

Remember Nils mentioned "or a little shorter". If it's cut a little shorter, you'll simply have a bit larger "white paper" annulus around the diagonal and bottom of the site tube. That's okay. So you may be able to cut it shorter and get on or inside the focal plane.

I presume you're using PVC that is readily available and low cost. Of course all the PVC pieces will fit the full 1" diameter tube, so don't trim 0.5mm at the very top so the cap fits. If you need to trim the length, take it off the bottom of the site tube (before you install the cross hair).

Go ahead and install the cap/peep hole and trim the rest of tube to fit the 0.96" focuser. The wider cap will rest on the focuser and prevent the tube from sliding down into the focuser. So, that's good. Test fit and cut to length that allows you to see all three collimation signatures.

Yes, trimming 0.5mm is probably not a job for sand paper. There may be a lathe where someone might be able to accurately turn the site tube for you. I've taken small projects to local fabricators before. They were happy to help. Depends on what's available in your area. I don't know how to do it easily by hand, but there may be a way.

Edited by Asbytec, 08 June 2022 - 10:55 PM.


#104 patrickt

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Posted 09 June 2022 - 08:36 AM

   Firstly, i remembered that the  focuser has a removable eyepiece holder ( 0.965 inch / 24.5mm), which gives me an opening bigger than 1.25 inches / 31.7mm, after screwing it off. This removable eyepiece holder is 31mm long.

 

   I have a thin walled (1.5mm ) stainless steel tube with O.D. = 25mm. / I.D. = 22.9mm.

 

    1) I will cut the tube to 8.5 inches. 

    2) I will have the fabricator reduce the O.D. of the first 4 inches of the top  to 0.965 inches / 24.5mm ( 4 inches is the longest his lathe can grind off ).   

    3) To attain the 21mm hole at the lower end of the tube, i  will ask the machinist to fabricate a brass insert (to be press fitted to to the bottom of the tube), about 1 inch long, so i can cut the tube + brass insert shorter if need be.  Then i will try it...

 

     I only need to remove the eyepiece holder, slip the larger end of the tube inside the larger focuser, and return the eyepiece holder over the top end of the tube, then lockscrew the sight tube securely.

 

     

 

   

 

 

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Edited by patrickt, 09 June 2022 - 08:38 AM.


#105 Asbytec

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Posted 09 June 2022 - 08:55 AM

Okay, great. Let us know how it turns out. :) 



#106 patrickt

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Posted 09 June 2022 - 06:00 PM

I sure will ! 


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#107 patrickt

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Posted 22 June 2022 - 09:22 AM

Hello Again Everyone !

      Its been over a week, and it was just about five days ago that i finally got to try my diy sight tube. I decided to stick with the present I.D. of 22.9mm (.90 inches) x the f 10 focal ratio = 9 inches length.

 

       This of course makes the Sight Tube about  0.8 inches longer, but i wanted to try it anyway. Its at least more accurate than a mere collimation cap.

 

       So this is what i have so far from about 5 days ago. I apologize that i cant make those nice circles of Vic Menard 🙂    This pic shows the Sight Tube view with focuser racked out all the way. The crudely drawn yellow arc describes a portion of the secondary mirror, while the drawn black arc is part of the secondary holder. I would have wanted to improve the centering of the secondary mirror under the focuser, but was rushing to take advantage of a possible window to view the planets for two days in a row (since it has been mostly cloudy due to the arrival of the "rainy season"). So i then proceeded to try to center rhe primary mirror image in the secondary. Note the primary's off center doughnut.

 

       I hope to have some time this Sunday, and go back to centering the secondary under the focuser. Thats all for now guys. I'll let you know what happens after Sunday. Thanks again !

 

 

     

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