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Equipment Discussions >> Reflectors

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alonzo
super member


Reged: 06/06/12

Vic,Jason and Nils I have another problem.
      #6217454 - 11/26/13 03:38 PM

...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

Edited by alonzo (11/26/13 03:41 PM)


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choran
Pooh-Bah
*****

Reged: 12/28/12

Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6217489 - 11/26/13 03:49 PM

How does it do on stars?

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acochran
professor emeritus


Reged: 06/19/08

Loc: So. CA
Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6217552 - 11/26/13 04:19 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



Neither does mine. You must try on a star/the Moon.
Andy

Edited by acochran (11/26/13 04:20 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: acochran]
      #6217706 - 11/26/13 05:39 PM

One easy option is raising or lowering the primary. From your description, it seems to me that raising your primary a little might help; do that by loosening ALL the primary screws as much as is reasonable, and then recollimate.

Bet it will help, perhaps enough to bring the image to focus.


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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: backwoody]
      #6217741 - 11/26/13 05:59 PM

If the primary mirror is supported by springs, though, raising the mirror by loosening the collimation bolts will result in a scope that is not stable, where collimation is concerned.
Better to do one of the following:
1) Drill new primary mirror cell mounting holes 1/2" up-tube from the current ones. Remount the cell in the new holes. If your OTA is mounted on a German EQ mount, you will have to slide it down in the rings to re-balance it. If your OTA is in a dob, you will have to add some counterweights to the lower end of the tube to keep the scope balanced with the current altitude trunnion positions. If you're up to it, and have space in your rocker box to do so, you can remount the altitude trunnions higher on the tube and lower the OTA.
2) If your scope is a truss tubed dob, it becomes easier, since shortening the poles 1/4" (once or twice) is an easy fix.
3) re-mount the mirror cell using Bellville springs. Using these, you can maintain high spring tension with a greater distance between the mirror's cell and its support structure. This is one way you can raise the mirror without making the scope too loose to hold collimation.
4) replace the focuser with a shorter focuser. They come as short as 1".
5) if it's 1.25" eyepieces that don't come to focus, and you have a 2" focuser, obtain an In-Travel adapter for the focuser. The Glatter Parallizer lowers the eyepiece to the level of the top of the focuser. The TeleVue In-Travel adapter lowers it 1/8" more than that, and the Astrosystems and Scopestuff in-travel adapters lower the eyepiece even more--up to 1/2" below the level of the top of the focuser. If it's 2" eyepieces that don't come to focus, see #4.

Remember also that focusing on nearby land objects requires more inward focuser travel than do stars. Try it at night before doing anything.


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johndgaul
member


Reged: 05/19/11

Re: Vic,Jason and Nils I have another problem. new [Re: Starman1]
      #6217797 - 11/26/13 06:21 PM

Quote:

Remember also that focusing on nearby land objects requires more inward focuser travel than do stars. Try it at night before doing anything.




I thought it was the opposite, that focusing on near objects needs outward focuser travel, the focal point being further away from the optic.

This here:
http://stellafane.org/tm/newt-web/newt-help-scope.html#SpareFocuserTravel

and an earlier thread started by me:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6184831/page...

Also, if I imagine the diverging light rays from a nearby object, they will be deflected by the mirror so that they focus at a point further away from the mirror than parallel rays from objects at infinity.

Maybe I've got this all wrong! Someone please set me right


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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]
      #6217907 - 11/26/13 07:15 PM

Quote:

...While attempting to view thru the telescope this morning I realized I cannot focus things thru it...



First, don't panic--this happens all the time--really!

Quote:

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.



You need to attach a picture so we can get a better idea of your mechanicals. But close up objects focus farther away from the primary mirror than objects at infinity, so if you can't focus on close (terrestrial) targets, you won't be able to focus on planets or stars...

Quote:

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.



As others have already suggested, it's usually easier to move the primary mirror forward.

In another week, you'll have the Moon to use as a target for finding the best focal plane position. I suggest you have all of your eyepieces ready so you can try to get all of them to focus. If you have presbyopia, you may want to include a friend with 20/20 vision to verify that the focuser can accommodate different eyeballs too (or vice versa).


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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: johndgaul]
      #6217984 - 11/26/13 08:02 PM

Quote:

Quote:

Remember also that focusing on nearby land objects requires more inward focuser travel than do stars. Try it at night before doing anything.




I thought it was the opposite, that focusing on near objects needs outward focuser travel, the focal point being further away from the optic.

This here:
http://stellafane.org/tm/newt-web/newt-help-scope.html#SpareFocuserTravel

and an earlier thread started by me:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6184831/page...

Also, if I imagine the diverging light rays from a nearby object, they will be deflected by the mirror so that they focus at a point further away from the mirror than parallel rays from objects at infinity.

Maybe I've got this all wrong! Someone please set me right



You're correct. I had it reversed.
However, if you don't have enough in-travel for land objects, you'll definitely not have enough for celestial ones. You definitely need to raise the primary.

Edited by Starman1 (11/26/13 08:04 PM)


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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: Starman1]
      #6218051 - 11/26/13 08:34 PM

... I have been trying to see what I can do to correct this and I found an old book by Peter Francis called "Newtonian Notes",its a wonderfull book that I have had for years on telescope making and is geared towards Richest Field telescopes. In his book he has formulas on telescope layout and boy am I off. According to the figures I punched in my focuser needs to go towards the primary by almost an inch along with the secondary package . This I sure as heck don't want to do. I hope I am understanding everyone regarding moving the primary. If I move it up by the amount I need would it not constitute the same procedure but just in reverse.

Note: My telescope is not mounted but laying on a table in my garage. I cannot at this stage view moon or stars yet. It
weighs a ton and needs to be counter balanced before I place it on my fork mounting I built for it.

Alonzo

Edited by alonzo (11/26/13 08:42 PM)


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johndgaul
member


Reged: 05/19/11

Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6218087 - 11/26/13 08:50 PM

alonzo,

Different eyepieces will bring the image to focus with the focuser at different positions.

More things you could try to help you work it out:
- a different eyepiece, which might come to focus further away from the mirror than the one you've tried
- adding a barlow lens, if you have one, before the eyepiece, which will make the eyepiece come to focus further away from the mirror than when without the barlow
- starting out by focusing on something even closer, just to check you can get a clear image


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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]
      #6218144 - 11/26/13 09:21 PM

Quote:

...I have been trying to see what I can do to correct this and I found an old book by Peter Francis called "Newtonian Notes"...



I have a copy somewhere (and have referenced his secondary mirror advice on many occasions).

Quote:

...According to the figures I punched in my focuser needs to go towards the primary by almost an inch along with the secondary package. This I sure as heck don't want to do. I hope I am understanding everyone regarding moving the primary. If I move it up by the amount I need would it not constitute the same procedure but just in reverse.



Yup.

Quote:

Note: My telescope is not mounted but laying on a table in my garage. I cannot at this stage view moon or stars yet. It weighs a ton and needs to be counter balanced before I place it on my fork mounting I built for it.



Choose your poison. You can place the unbalanced scope on your fork mounting and hold on tight while you find focus, or balance, check, rebalance, check, rebalance... Remember that friend with 20/20 vision (or presbyopia) I suggested you invite over to verify focus--maybe he could be a weightlifter too!

When I was a kid just getting started in this hobby, I had a homebuilt 10-inch f/8 in a sonotube that I leaned against the fence to find focus. IIRC, it was pretty heavy too, but I was into weightlifting back then...


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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: Vic Menard]
      #6218147 - 11/26/13 09:21 PM

Starman1 (Don) is correct, the springs on your primary screws must be long enough and firm enough to allow loosening. I have seen several scopes that have been collimated over time by only tightening these screws, never loosening. Thus, they became overtightened after several iterations. In these cases, and perhaps in yours, it was possible to significantly adjust/loosen them and thereby raise the primary, moving it closer to the secondary and focuser.

Take a look. It may be possible to do a simple fix to your focusing problem. Good luck, and c/s,


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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: backwoody]
      #6219222 - 11/27/13 12:30 PM

....Guys and gal and all the ships at sea I need to shut things down for the next few days because of the THANKSGIVING holidays. I will return with more info in a few days.

Thanks all
Alonzo


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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: alonzo]
      #6219774 - 11/27/13 04:31 PM

Today I fooled around a little with telescope layout and it seems I am missing something. My focal lenght is 49",the tube diameter is 8-3/16ths,my focuser height is 1-7/8ths and I have a 1/2" travel and this places the focuser at 38.43 which is just about where it is now. I was thinking I need to go towards the focuser a little more than what this indicates

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]
      #6219839 - 11/27/13 05:04 PM

Quote:

Today I fooled around a little with telescope layout and it seems I am missing something. My focal length is 49",the tube diameter is 8-3/16ths,my focuser height is 1-7/8ths and I have a 1/2" travel and this places the focuser at 38.43 which is just about where it is now. I was thinking I need to go towards the focuser a little more than what this indicates

Alonzo



49" - 4-3/32" (tube radius) - 1-7/8" (focuser height) - tube thickness+adapter plate thickness - 1/4" (half of the focuser travel) = (guess) 42.25"
from primary mirror to secondary mirror. I estimate 6.75" from secondary to focal plane.
That doesn't give you enough in-travel for a barlow or a camera. You will want to have the focal plane farther out if imaging is desired.


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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: Starman1]
      #6219990 - 11/27/13 06:13 PM

....Don you would shoot me when I tell you I made a mistake posting my last post. Things just didn't seem right so I checked it again. I found out my grandson was not holding my tape measure on the mark I had showed him,he was on another mark on my tube. I have gained nearly an inch in the direction I need to go. After the hoilidays I am going to take accurate measurements and move the mirror forward that distance. It will be easier to move the primary rather than moving focuser and secondary towards the primary.

BTW I think you got my tube radius wrong,the tube diameter with tube thickness included is 16" plus tube thickness of 3/16ths and my radius would be 8-3/16ths not 4-3/32nds.

Alonzo

Edited by alonzo (11/27/13 06:14 PM)


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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]
      #6220031 - 11/27/13 06:32 PM

Quote:

....Don you would shoot me when I tell you I made a mistake posting my last post. Things just didn't seem right so I checked it again. I found out my grandson was not holding my tape measure on the mark I had showed him,he was on another mark on my tube. I have gained nearly an inch in the direction I need to go. After the holidays I am going to take accurate measurements and move the mirror forward that distance. It will be easier to move the primary rather than moving focuser and secondary towards the primary.

BTW I think you got my tube radius wrong,the tube diameter with tube thickness included is 16" plus tube thickness of 3/16ths and my radius would be 8-3/16ths not 4-3/32nds.

Alonzo



Sorry. You said tube diameter in your post. I assumed inside diameter. You obviously know how to measure.
Figure, though, that you need the focal plane 1/2" above the racked-in focuser to focus most eyepieces, 3/4" if you use a barlow or Paracorr, and >1" if a CCD camera is used, and >1.5-2" if a DSLR is used.

1/2" of focuser travel? How did you get such a short focuser drawtube?


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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: Starman1]
      #6220127 - 11/27/13 07:22 PM

...Don your right I did say tube diameter so it was me misleading you. The focuser is from moonlite and I had it special made that way. I had done some weird math once upon a time and that was the criteria it indicated. I am beginning to think I used an abacus for the calculations.

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]
      #6220178 - 11/27/13 07:52 PM

Alonzo,
The good news is that changing a drawtube on a Moonlite focuser is a cinch.
I'm even thinking about getting a shorter one since all my eyepieces are parfocal in the Paracorr.


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alonzo
super member


Reged: 06/06/12

Re: Vic,Jason and Nils I have another problem. new [Re: Starman1]
      #6220357 - 11/27/13 09:38 PM

....Vic,Jason,Nils,Don,Choran,Acochran,Backwoody,Johndgaul and all "HAPPY THANKSGIVING" talk atcha again Friday.

Alonzo

Edited by alonzo (11/27/13 09:43 PM)


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