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Equipment Discussions >> Reflectors

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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6223389 - 11/29/13 01:17 PM

....Don if I go a little more towards the moving my focal plane to incorporate barlows etc. would I not introduce a little vignetting with the secondary ( I think thats the word ). Peter Francis book "Newtoinian Notes" give absolute measurements as were the focal plane should be. The measurent is ALMOST an inch and I think I will round it off at that.

Alonzo


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6223446 - 11/29/13 01:41 PM

Quote:

....Don if I go a little more towards the moving my focal plane to incorporate barlows etc. would I not introduce a little vignetting with the secondary ( I think thats the word ). Peter Francis book "Newtonian Notes" give absolute measurements as where the focal plane should be. The measurement is ALMOST an inch and I think I will round it off at that.

Alonzo



Correct. The shorter the distance from the primary to the secondary, the larger the secondary has to be.
We normally size a secondary to provide 70% illumination at the edge of the field of our largest field stop eyepiece. Imagers use an even larger secondary, aiming for 100% illumination at the edge of the largest chip camera.
In truth, though, there is little utility in using a secondary large enough to illuminate the lowest power eyepiece ideally if that eyepiece is seldom used.
You can accept a larger drop off at the edge of the lowest power eyepiece (say, half a magnitude) and still not notice it.
Daytime viewing of the sky reveals vignetting easily, both photographically and visually. If you don't notice it visually, it isn't of much consequence if you use your instrument visually.

How far out from the focuser the focal plane has to be is dependent on many factors: camera? Paracorr? Barlow? eyepiece with recessed focal plane? myopia? I use a Paracorr, which parfocalizes all my eyepieces, and i use a PowerMate, which focuses in the same place as the Paracorr when inserted. I could get buy with <1/2" of focuser travel. Having a shorter focuser drawtube means the front end of that tube will vignette less (it's one of the places vignetting occurs in a scope), but since I don't notice vignetting now, there's no reason to change.
If you're placing the focal plane yourself, though, it is necessary to take all the options into account.


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mayidunk
Don't Ask...
*****

Reged: 02/17/10

Loc: Betwixt & Between...
Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6223527 - 11/29/13 02:21 PM

Quote:

...While attempting to view thru the telescope this morning I realized I cannot focus things thru it. The scope is laying on a table in my garage. Trying to focus something about an eight of a mile does not quite come to a focus. I need to focus inward a little farther but I am restricted by my focusers movement. My focuser is flush to the inside of the tube and has only a half inch upward travel.I need to change my focal plane position but what would be the procedure. Do I move the secondary forward or do I move the focuser forward or both.

Alonzo



Do you plan on using this scope for terrestrial viewing? If not then why not just wait until you can get it pointed at the sky, and then just try focusing on a star?


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Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: Vic,Jason and Nils I have another problem. new [Re: mayidunk]
      #6223649 - 11/29/13 03:40 PM

Quote:

Do you plan on using this scope for terrestrial viewing? If not then why not just wait until you can get it pointed at the sky, and then just try focusing on a star?




I agree. Until Alonzo gets at least one eyepiece to focus on a star, I wouldn't be in too big of a hurry to drill new primary mirror mounting holes. I would even consider holding an eyepiece inside the OTA while looking through the empty focuser to get a better idea where the (infinite focus) focal plane is currently located. If necessary, the focuser can be removed to better visualize where the tops of the various eyepieces are located relative to the outside of the OTA. If a Paracorr or Barlow will be used, it may be possible to hold the assembled Paracorr/eyepiece, Barlow/eyepiece, Paracorr/Barlow/eyepiece in the empty mounting hole to find the optimal focus position. And hopefully the accumulated focus positions will fit in your 1/2-inch focuser travel.

I would also suggest finding the "midpoint" of your mirror cell collimation screws and setting the cell there before you start looking for the best focusing point--that way you'll still have some wiggle room when you drill the new mounting holes.

And don't worry too much if you need to drill another set of mounting holes if your second round turns out to need additional adjustment. In fact, you may want to rotate your next three mounting screw holes 60-degrees so your final mounting holes can be placed in the position you want them (this would allow smaller adjustments that would otherwise put the mounting holes too close to each other). Empty holes are easily filled with short stainless steel screws which can be glued in place or secured with hex nuts for a "finished" look.

And if you're still trying to figure how your 49-inch focal length "fits" relative to the actual focal plane, you need to remember to include the sagitta (about 0.2-inch for your 12.5-inch f/4--although you should gain that back in offset assuming your secondary mirror is mechanically centered in the OTA), and finally, use a small piece of tracing paper (if you want to look "through" the paper) or card stock (if you can look "under" the card) and point the scope at the Moon to find where it reaches focus (the focal plane).

If the focal plane is about 1/4-inch above the focuser, your intercept distance (8-3/16 + 1-7/8 + 1/4 is about 10.3-inches. Divide that by the focal ratio, and you get 2.6 for the minimum secondary mirror minor axis, which means your 3.1 (if that's the clear aperture minor axis) will give you about a 0.5-inch fully illuminated image--which is a good fit for typical visual use.

Edited by Vic Menard (11/29/13 04:15 PM)


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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: Vic Menard]
      #6223816 - 11/29/13 05:24 PM

....Vic when I run the NEWT program with my data I get .51 for my 100% illuminated area and .60 degrees. It also gives 1.9 inches of diameter and 2.25 degrees of field. This corresponds to what you said in your last paragraph of .o5 illuminated image.

Alonzo. I


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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6225154 - 11/30/13 12:58 PM

...Today I am going to move my primary the amount indicated by the formulae thats in by Peter Francis book "NEWTONIA NOTES".
It says my focuser should be 38.45 inches from the front of my primary. I think I will round it off to 38.375" as I have nothing to measue in thousnds. All this will do is raise my focal plane just a hair more. I don't think it will cause any problems. I sure hope I am not moving my mirror clips out of view as that would be bad.

Alonzo

Edited by alonzo (11/30/13 01:00 PM)


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backwoody
Pooh-Bah


Reged: 01/08/07

Loc: Idaho USA
Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6225987 - 11/30/13 09:46 PM

Alonzo, we will await your report on results! Good luck with the project,

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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: backwoody]
      #6226136 - 11/30/13 11:13 PM

...Thanks Woody I'll keep everyone posted.

Alonzo


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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6227715 - 12/01/13 06:47 PM

...Didn't get to move mirror today but played with a little math. The "NEWT" program and formulae in the Peter Francis book come out to be exactly the same. Front of primary mirror to focuser hole should be 38.8125". Since they show this with no variation I suppose this is what it is for my system. Since with my crude garage tools I think I will round it off to 38.675". This will raise my focal plane by
.1375" which isn't very much to be concerned with. Vic I know I am not following your suggestion on bringing the ota outside and leaning it up on something. The telescope unfortunatly is too heavy for a move like that. Its a sonotube painted with several coats of fibremesh and resin incased in a coopered wooden tube. I have many light baffles I cut with a router that also adds to the weight. I made a ringed slipon housing to accomadate the tube with a harness.
When I get the mirror moved I will mount it on my fork but not before I see where to mount some counter weights. I don't know if you remember how heavy the Coulter mirrors were back in the day but the were big hernia things. I will be using a comealong to lift it a little to find the CG. I will keep everyone posted as I proceed.

Thanks
Alonzo

Edited by alonzo (12/01/13 06:50 PM)


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Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6227817 - 12/01/13 07:51 PM



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backwoody
Pooh-Bah


Reged: 01/08/07

Loc: Idaho USA
Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6227831 - 12/01/13 07:55 PM

Alonzo, hmmm....sounds like you need about 1/8 inch upward movement of your primary. Take a close look at those collimating screws and springs to see if there is 1/8 inch available before doing anything more. Hope the fix is easy.

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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: backwoody]
      #6228191 - 12/01/13 11:30 PM

...Woody how are you coming up with this. I don't think 1/8th of an inch will get me here.

Alonzo


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backwoody
Pooh-Bah


Reged: 01/08/07

Loc: Idaho USA
Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6229754 - 12/02/13 05:33 PM

Alonzo, I might have misunderstood a previous post; I think you calculated raising the focal plane by .1375". That would be just a bit over 1/8 " (.125").

Sorry for any confusion!


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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: backwoody]
      #6234069 - 12/04/13 07:27 PM

...Having finding the primary was far more difficult to move than I thought I elected to move the secondary and focuser. Trying to line up holes of the primary mirror cell with new holes in the tube just could not be done especially with the weight of the mirror. I did the best I could in transfering my measurements and I think I did O.K.,only time will tell.
Tommorrow I will try starting the process centering my secondary under the focuser. I did a preliminary check and it seems I am going to have a little lateral problem. It appears I have more space on the bottom between the secondary and focuser tube than I do at the top. Maybe shimming my focuser would help I will just to look at it closer tommorrow.

Alonzo

Edited by alonzo (12/04/13 07:29 PM)


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