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Vic,Jason and Nils I have another problem.

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#1 alonzo

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Posted 26 November 2013 - 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

#2 choran

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Posted 26 November 2013 - 03:49 PM

How does it do on stars?

#3 acochran

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Posted 26 November 2013 - 04:19 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

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

#4 backwoody

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Posted 26 November 2013 - 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.

#5 Starman1

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Posted 26 November 2013 - 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.

#6 johndgaul

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Posted 26 November 2013 - 06:21 PM

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.or...reFocuserTravel

and an earlier thread started by me:
http://www.cloudynig...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 :)

#7 Vic Menard

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Posted 26 November 2013 - 07:15 PM

...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!

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...

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).

#8 Starman1

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Posted 26 November 2013 - 08:02 PM

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.or...reFocuserTravel

and an earlier thread started by me:
http://www.cloudynig...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.

#9 alonzo

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Posted 26 November 2013 - 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

#10 johndgaul

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Posted 26 November 2013 - 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

#11 Vic Menard

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Posted 26 November 2013 - 09:21 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"...

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

...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.

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! :lol:

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...

#12 backwoody

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Posted 26 November 2013 - 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,

#13 alonzo

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Posted 27 November 2013 - 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

#14 alonzo

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Posted 27 November 2013 - 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

#15 Starman1

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Posted 27 November 2013 - 05:04 PM

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.

#16 alonzo

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Posted 27 November 2013 - 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

#17 Starman1

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Posted 27 November 2013 - 06:32 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 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?

#18 alonzo

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Posted 27 November 2013 - 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

#19 Starman1

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Posted 27 November 2013 - 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.

#20 alonzo

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Posted 27 November 2013 - 09:38 PM

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

Alonzo

#21 alonzo

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Posted 29 November 2013 - 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

#22 Starman1

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Posted 29 November 2013 - 01:41 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 "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.

#23 mayidunk

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Posted 29 November 2013 - 02:21 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

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? :)

#24 Vic Menard

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Posted 29 November 2013 - 03:40 PM

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? :)

:waytogo:
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.

#25 alonzo

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Posted 29 November 2013 - 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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