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# Focuser angle

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### #1 Pharquart

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Posted 05 January 2013 - 12:03 AM

For anyone with a solid tube Dob with a focal length somewhere around 48 inches:

What angle does your focuser make relative to the axis of the telescope? In other words, point your tube straight horizontal. What angle is your focuser relative to the ground? Zero degrees would mean the focuser is parallel to the ground, sticking straight out to the side. 90 degrees would mean the focuser is pointing straight up, and -90 degrees means it's pointing straight down.

I spent a couple hours doing some constant observing, and now I have a kink in my back and neck. My focuser is at roughly 45-50 degrees based on my definition above. I can easily rotate my OTA in its mount to put the focuser in a better position if there is one.

Where is yours, and is it comfortable for you?

Brian

### #2 Pinbout

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Posted 05 January 2013 - 12:18 AM

I built a stellafane 8 in dob so I can rotate it and adj balance by push/ pull the tube back/forth. I like rotating it. When I'm low to the horizon the focuser is around 80*. When I'm at zenith the focuser is around 0*.

### #3 panhard

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Posted 05 January 2013 - 01:17 AM

For anyone with a solid tube Dob with a focal length somewhere around 48 inches:

What angle does your focuser make relative to the axis of the telescope? In other words, point your tube straight horizontal. What angle is your focuser relative to the ground? Zero degrees would mean the focuser is parallel to the ground, sticking straight out to the side. 90 degrees would mean the focuser is pointing straight up, and -90 degrees means it's pointing straight down.

I spent a couple hours doing some constant observing, and now I have a kink in my back and neck. My focuser is at roughly 45-50 degrees based on my definition above. I can easily rotate my OTA in its mount to put the focuser in a better position if there is one.

Where is yours, and is it comfortable for you?

Brian

I believe mine is about the same as yours. What make of scope do you have? Do you view standing up if so. You need a viewing chair something like these. link

### #4 Pharquart

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Posted 06 January 2013 - 10:10 AM

I have a homebuilt 10" Dob. The tube is held inside a square box of plywood that has the altitude bearings on the side. I say I can easily rotate it because I can loosen the screws holding the box together to relive the compression, spin, then retighten. It's easy to do, but not something I want to do several times in a viewing session. I may look into redesigning to permit easier rotation. I've noticed when doing outreach that younger (shorter) kids have troubles getting up to the eyepiece.

I have the parts to make an observing chair. That's on the agenda for later this week when it's warm enough outside to work in the (unheated) shop. I'll definitely try the chair, as just about everyone says you need one to observe comfortably. I haven't quite visualized how my body lines up to the telescope while sitting on a chair. Seems like the chair base and telescope base would interfere, or perhaps my knees would knock the tube. That's why I'm asking about focuser angle. I'm wondering if there's a better angle that permits more comfortable body positioning.

Brian

### #5 Jon Isaacs

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Posted 06 January 2013 - 10:20 AM

Brian:

I find that about 45 degrees works best unless the scope requires a ladder.

With a chair, I sit slightly behind the scope and I am positioned alongside but well behind the eyepiece, leaning forward with my hands on my knees, my arms supporting my upper body.

A good chair has narrowly spaced legs on front to allow it to be positioned very close to the scope and widely spaced legs in back to provide stability.

Jon

### #6 Starman1

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Posted 06 January 2013 - 12:27 PM

Well, it depends on factors:
1) Do you do a lot of observing with the telescope pointed below +30 degrees altitude? If so, you'll find a focuser mounted at 30-50 degrees around from horizontal to be comfortable.
2) Do you do a lot of viewing with the telescope pointed between 60 degrees and 90 degrees? Then you will find an angle closer to zero to be more comfortable.
3) How big is the scope? The larger the scope, the more having the focuser above the horizontal becomes uncomfortable at high altitudes. Example: On my 12.5", with a focuser at 30 degrees I had to stand to view when the scope was above 70 degrees or so. With the focuser at zero degrees, I can sit all the way to the zenith, and sitting is a lot less fatiguing when you plan to observe all night. But, that's a 12.5". On a 10", I could have tolerated a little more of an angle on the focuser. And on an 8", I could have tolerated even more angle.

For me, my answer to #2 above is: I try to do ALL my viewing in that circle if possible. I try to wait until the object is in that window to observe it. Of course, that would eliminate the entire southern sky if I stuck to that, so I do a fair amount of viewing down to about 25 degrees off the horizon when viewing objects in the south (for me, that's -30 degrees). And I kneel or sit on the ground for the occasional "dirt diving".

How you view, what you view, and the size of your scope will determine the best answer to the focuser angle. And if your OTA rotates? You can choose based on comfort no matter where the scope points.

### #7 Pinbout

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Posted 06 January 2013 - 03:57 PM

but for my rehab'd truss dob its set at 45*

### #8 Cygnus2112

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Posted 07 January 2013 - 10:55 AM

Nice scope Danny!

### #9 Mr Magoo

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Posted 07 January 2013 - 12:16 PM

My homebuilt 6" f/10 is in a cradle that clamps. I can loosen it up and move it where I need it depending on viewing angle and balance issues.

### #10 ThreeD

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Posted 07 January 2013 - 02:11 PM

My experience is that for a dob with a ~48" focal length that the 45 degree orientation is best. For scope with a FL more like 72" it is best to be 0 degrees. It's really more about the eyepiece height versus your height so this assumes a typical base and not one that causes the focuser to be elevated and that you are a typical height adult.

For the smaller scope, rotating the focuser to to 45 degrees allows reasonable viewing at lower altitudes by bending over the tube where as if the focuser is at 0 degrees it becomes awkward unless you like being on your knees.

For a larger scope, rotating the focuser to 45 degrees can cause awkwardness at higher altitudes and unless you are viewing very close to the horizon there is little benefit over having the focuser at 0 degrees.

It sounds to me like you need an observing chair. My experience is using a typical dob your size is a pain in the back without one.

### #11 george golitzin

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Posted 08 January 2013 - 01:04 AM

Well I have two: my 10-inch f/5 and my 8-inch f/6. The 10-inch is set at 45 degrees (it's a truss) and I find this is a good compromise--I can view pretty comfortably at all elevations. However, my 8-inch solid tube rotates on roller bearings to any orientation, and I think that's the cat's meow. Near the horizon I can point the focuser straight up, and at zenith I'll rotate it perpendicular to the plane of the alt bearings. There's nothing like being able to customize your comfort, so if you're going with a solid tube, you might as well make it rotate.

-geo

### #12 george golitzin

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Posted 08 January 2013 - 01:05 AM

FWIW here's the 10

### #13 george golitzin

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Posted 08 January 2013 - 01:10 AM

I also think that the angle of the focuser should decrease as focal length increases. My newly built 12.5-inch f/5 has the focuser at 30 degrees (if you're interested, check out my thread in the ATM forum), which I find just right for that length. My 18-inch f/4.2 has the focuser closer to 20 degrees--and I prefer that very much to my old 16 f/5, on which I foolishly set the angle at 0. Frankly, I don't think the 0 degree setting makes sense at all unless the scope is a lot taller than the observer.

-geo

### #14 Starman1

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Posted 08 January 2013 - 01:38 AM

I also think that the angle of the focuser should decrease as focal length increases. My newly built 12.5-inch f/5 has the focuser at 30 degrees (if you're interested, check out my thread in the ATM forum), which I find just right for that length. My 18-inch f/4.2 has the focuser closer to 20 degrees--and I prefer that very much to my old 16 f/5, on which I foolishly set the angle at 0. Frankly, I don't think the 0 degree setting makes sense at all unless the scope is a lot taller than the observer.

-geo

Or unless you prefer to sit even when the scope is pointed at the zenith. With my 12.5" f/5, the only focuser angle that could accommodate that was 0-5 degrees.

### #15 Jon Isaacs

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Posted 08 January 2013 - 06:59 AM

I also think that the angle of the focuser should decrease as focal length increases. My newly built 12.5-inch f/5 has the focuser at 30 degrees (if you're interested, check out my thread in the ATM forum), which I find just right for that length. My 18-inch f/4.2 has the focuser closer to 20 degrees--and I prefer that very much to my old 16 f/5, on which I foolishly set the angle at 0. Frankly, I don't think the 0 degree setting makes sense at all unless the scope is a lot taller than the observer.

-geo

Or unless you prefer to sit even when the scope is pointed at the zenith. With my 12.5" f/5, the only focuser angle that could accommodate that was 0-5 degrees.

I find that something between 30-45 degrees works well for me with scopes that do not require ladders. My 16 inch has the focuser set at about 30 degrees and I can view seated in my Catsperch chair while viewing at the zenith.

I set the chair alongside and somewhat behind the scope and lean my upper body forward, my hands rest on my knees so my arms can support my upper body.

I am not quite sure why Don was unable to view near the zenith while seated with his 12.5 inch F/5???

With the 25 inch F/5, the focuser is at 0 degrees, I am not sure it's optimal but that's what is it and it works.

I do view near the horizon, there are interesting things to be seen within 10 degrees of the horizon and while the views may not be optimal, they are enjoyable. Omega Centauri is always worth a look and there are a variety of galaxies and clusters low on the horizon. If I restricted my viewing views that were optimal, I would not spend time viewing DSOs from my light polluted backyard or turn a telescope towards a planet or a double star when the seeing was not near perfect and the target well above the horizon.

Maybe if I didn't get out 10-20 evenings a month with 6 or more being from relatively dark skies, I would concentrate more on the optimal views. But in my mind, each and every view, near perfect or highly flawed, is worthwhile, enjoyable and offers something to be learned.

Jon

### #16 george golitzin

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Posted 08 January 2013 - 06:19 PM

Yes, as I mentioned, my 12.5 f/5 has the focuser at 30 degrees, and I view every elevation seated in that scope--no problem with zenith, because the scope is not very tall. As for larger scopes, while I agree that 0 degrees is better for zenith and near-zenith viewing, I find in my 18 that 20 degrees is a good compromise across the range of elevations. A chacun son gout!

### #17 Starman1

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Posted 08 January 2013 - 06:50 PM

Well, focuser height is 5'6".
My eye height, when seated on the 29" seat height of my chair: 5"6'.
Eyepiece and Paracorr stick at least 8" out of the UTA's side.
I have to sit up very straight to look through. Jon is probably taller than I am and has a taller chair.
When the focuser was racked around the scope 30 degrees on my Discovery, the eyepiece height was slightly above that and angled a little differently and that required I stand to observe (but bend over the scope to look through). After that experiment, I went back to 0 degrees.
However, 5 degrees would have been OK and resulted in a slightly higher eyepiece height when pointed low.
My current scope has stops to prevent the scope from going below ~10 degrees off the horizon. That *just* gets me to Omega Centauri where I observe, though I try not to go that low.

### #18 george golitzin

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Posted 09 January 2013 - 02:19 AM

That makes sense. A scope is a bit like a suit of clothes--one tailors both to fit.

-g

### #19 Jon Isaacs

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Posted 09 January 2013 - 06:09 AM

I have to sit up very straight to look through. Jon is probably taller than I am and has a taller chair.

Don:

My experiences are a composite of several scopes but primarily two.

In terms of the scope in the photo, I maybe be a bit taller, I am 6 foot even, the chair maybe a bit taller, it's a Starbound and the maximum seat height is 32 inches. But most importantly, yes, it's a 12.5 inch Discovery that is now in a JStar structure but it's F/4.06 rather than F/5 so the scope itself is about 12 inches shorter... Big difference, I was told that it was made for the Japanese market... So it's very similar to a 10 inch in terms of eyepiece height. The comparison between your scope and this scope is not very instructive.

But I have a Catsperch chair for my 16 inch F/4.42. The Catsperch chair has a maximum seat height of 42 inches, the eyepiece height of the scope is about 68 inches, the optimal seat height at the zenith is about 38 inches or less... The only problem is climbing up the chair, the focuser is somewhere around 30-40 degrees and I find it quite comfortable.

I think leaning forward somewhat is more comfortable, more stable... Since I do often observe near the horizon with both these scopes, the site where they live looks out over a valley to the south, the 30-45 degree focuser angle makes that comfortable.

With the 25 inch, the focuser is horizontal but eyepiece height when the scope is low on the horizon is higher because of the larger diameter mirror and base/bearings...

Jon

### #20 coopman

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Posted 09 January 2013 - 07:53 AM

I re-located the side bearings on my AD10 so that the focuser angle is only about 20 degrees, because when viewing objects at near vertical, I felt like I was having to straddle the scope and I wasn't comfortable with that awkward position.

### #21 Jon Isaacs

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Posted 09 January 2013 - 11:44 AM

I re-located the side bearings on my AD10 so that the focuser angle is only about 20 degrees, because when viewing objects at near vertical, I felt like I was having to straddle the scope and I wasn't comfortable with that awkward position.

Clay:

What chair are you using?

Jon

### #22 coopman

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Posted 10 January 2013 - 07:55 AM

The Stardust Observing Chair is what I have

### #23 Sarkikos

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Posted 10 January 2013 - 09:57 PM

Brian,

My focuser is angled up about 45 degrees. But I sit on an adjustable stool and I directly face the focuser. I don't sit sideways to the telescope. I've set up my 10" f/4.8 Dob so that I can remain seated while directly facing the telescope, and am able to look in the focuser at all positions of the telescope while I continue to remain seated. If you recall movies of submariners looking through a periscope, that is basically how I'm looking through the focuser of my telescope. IME, this is the most comfortable way to observe. I can sit for hours with no backaches and no neckaches.

Mike

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