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SACK
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How much time do you view at Zenith? new
      #5831287 - 04/29/13 10:52 PM

Well, in building and designing a scope, a big factor is eyepiece height at zenith. I had one, a dob, for a year and am now building another, but can only think of being at zenith a couple of times.
So, just collecting your input so as to place that aspect in its right place for me.

To answer my question, it seemed 20 to 70 degrees above horizon was were I spent alot of time. So, I am thinking about eyepiece height at 70 degrees, if I were to max it.
Jonathan


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SACK
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Re: How much time do you view at Zenith? new [Re: SACK]
      #5831291 - 04/29/13 10:55 PM

If this would fit better in General Observing let me know. I thought it kinda goes with atm'ing as all think of this when building.
Jonathan


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Dave O
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Re: How much time do you view at Zenith? new [Re: SACK]
      #5831316 - 04/29/13 11:12 PM

I suspect a lot depends on 1) the latitude you are observing from; and 2) the objects you like to observe. Living in Sri Lanka (7 deg. North latitude), and preferring to observe planets, I spend a great deal of time observing objects near the zenith ...

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SACK
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Re: How much time do you view at Zenith? new [Re: Dave O]
      #5831385 - 04/30/13 12:01 AM

Great point Dave O! Geography does play into it. Maybe this would be a latitude sensitive question. So, any 30 to 45 degree'ers want to chime in? North or South.

Someone building near your latitude would indeed consider that. Dave O is your scope driven by servo cat or something?
Jonathan


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Jon Isaacs
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Re: How much time do you view at Zenith? new [Re: SACK]
      #5831561 - 04/30/13 03:56 AM

Quote:

Well, in building and designing a scope, a big factor is eyepiece height at zenith. I had one, a dob, for a year and am now building another, but can only think of being at zenith a couple of times.
So, just collecting your input so as to place that aspect in its right place for me.

To answer my question, it seemed 20 to 70 degrees above horizon was were I spent alot of time. So, I am thinking about eyepiece height at 70 degrees, if I were to max it.
Jonathan




Jonathan:

I think if you do the geometry, for an eyepiece height of about 68 inches (about right for a 6 footer), what you will find is that between 65 degree elevation and 90 degrees elevation there is only maybe 4 inches of change in eyepiece height. Between 70 degrees and the zenith it's about 3 inches. Not a lot.

Since the difference is small, designing the scope so that you can view the entire sky without a ladder or stool is worthwhile.

Jon


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GlennLeDrew
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Re: How much time do you view at Zenith? new [Re: Jon Isaacs]
      #5831632 - 04/30/13 06:21 AM

What qualifies as *at* the zenith? Within a degree or two? Ten? Twenty? If one looks at the question statistically, the time spent in any given zone could, to first order, scale as the area of that zone. At least in the case of deep-sky observing, where our targets are scattered all over the sky.

Let's take the 'zenith' as being, say, the 20 degree wide cap overhead, and consider the zone below 20 degrees attitude as being seldom explored (mostly near the southern meridian). The 20 degree cap of the 140 degree near-hemisphere constitutes only 3.4% of the observed sky.

If we expand the zenith zone to a 40 degree wide cap, its area as compared to the 140 degree explored zone is still only 13.2%.

Naturally, to exploit the generally better sky overhead one could wait for objects to rise into this area (declination permitting), and in general spend more time examining them here. Additionally, local obstructions restrict more of the lower sky, and for some locales this can be significant indeed. And so it can well be the case that some observers almost specialize in near-zenith work.

Incidentally, if one wishes to calculate the solid angle, or area of a zone of sky, the formula for solid angle is

S = sin^2 (theta) * 2 * pi

theta is the angular radius of the zone
pi = 3.1415926...

Example. What is the solid angle of a 20 degree wide zone at the zenith? Theta, the radius, is 10 degrees, which you find the sine of and then square it.

S = sin^2 (10) * 2 * pi
S = 0.03 * 2 * pi
S = 0.189 steradians

In our problem here, we did the same calculation for the 140 degree (theta = 70) zone of the observed sky, and divided this result into the zenith area cap to find the latter's fractional area.


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Jon Isaacs
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Re: How much time do you view at Zenith? new [Re: GlennLeDrew]
      #5831916 - 04/30/13 10:12 AM

Glenn et. al:

One thing about viewing within 20 or 30 degrees of the Zenith, it's where the best views are. For viewing the planets and double stars, ideally the object will be at least 45 degrees above the horizon and the higher, the better. The area between above 60 degrees is 13.4% of the area of above 45 degrees and it's the best 13.4%.


Add in the fact that viewing at the maximum eyepiece height is generally somewhat awkward, designing a scope to allow viewing the zenith without using a stool or ladder is reasonable approach... Why make the best part of the sky, the most awkward to view?

Jon

Edited by Jon Isaacs (04/30/13 10:22 AM)


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Tom and Beth
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Re: How much time do you view at Zenith? new [Re: Jon Isaacs]
      #5832438 - 04/30/13 02:12 PM

Only thing I would add to that is that objects at Zenith only remain there for a short amount of time, so if the design is a bit awkward at Zenith, wait a 1/2 hour or so for it to transit.

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SACK
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Re: How much time do you view at Zenith? new [Re: Jon Isaacs]
      #5833178 - 04/30/13 08:38 PM

Thanks, good perspectives to consider. I see your point Jon about a few inches in difference from zenith to 70. What I was curious about was peoples experience at zenith more so than time spent, I will clarify. So if it is very awkward and most people avoid objects slightly before and after than maybe it is not worth making it a high priority in design, for a newt-dob.

Well dear reader, whats your take on dobsons hole?

No problem, dislike it? And is your scope automated or not?
I am sure many will find your experience very helpful.
take care,
Jonathan


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Dave O
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Re: How much time do you view at Zenith? new [Re: SACK]
      #5833198 - 04/30/13 08:54 PM

Quote:

Dave O is your scope driven by servo cat or something?





Now that you've brought up "Dobson's Hole" ...

My scope uses an equatorial fork. The polar axis is just 7 deg above the horizon, so it pretty much 'hangs' out there.


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Pinbout
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Re: How much time do you view at Zenith? new [Re: SACK]
      #5833199 - 04/30/13 08:55 PM

I always felt the dobsonian telescope was the most comfortable to view zenith. the worst to scrub the horizon.

my 5in newt on a dscv1 mount with a geared center column tripod, the most comfortable to view the entire sky.

so with either design i can spend a lot of time above 70°. its easy to get lost up there. what time is it anyway...


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Jon Isaacs
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Re: How much time do you view at Zenith? new [Re: SACK]
      #5833233 - 04/30/13 09:20 PM

Quote:

Thanks, good perspectives to consider. I see your point Jon about a few inches in difference from zenith to 70. What I was curious about was peoples experience at zenith more so than time spent, I will clarify. So if it is very awkward and most people avoid objects slightly before and after than maybe it is not worth making it a high priority in design, for a newt-dob.

Well dear reader, whats your take on dobsons hole?

No problem, dislike it? And is your scope automated or not?
I am sure many will find your experience very helpful.
take care,
Jonathan




Johathan:

I have 4 Dobs, a 10 inch F/5, a 12.5 inch F/4.06, I observe with these two seated 100% of the time. I also have a 16 inch F/4.42 which is probably most relevant to this discussion since the eyepiece height such that to observe at the zenith, I have to stretch myself and maybe even tippy toe. And then the 25 inch F/5 is a ladder scope.

No drives, all hand tracking. The biggest challenge is that near the zenith, it requires tracking both the altitude and azimuth axes simultaneous, it does take practice and is a challenge at very high magnifications but I don't shy away, I have hand tracked double stars that come with in 7 or 8 degrees of the zenith right across the top at 800x...

I really do think that the zenith needs to be included in the equation, if only because the eyepiece height changes very little until you are down around 60 degrees. I am 72 inches tall, a 68 inch eyepiece height is about the maximum for me, a few inches less would be better.

Jon


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Bill Kocken
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Re: How much time do you view at Zenith? new [Re: Jon Isaacs]
      #5833932 - 05/01/13 09:34 AM

My 16"F4.5 is just a little too tall for me at zenith. I don't mind the height since I can sit on my tall chair, or stand on something to give me just just a few of inches height.

Dobson's hole is the bigger problem. It is a nuisance to move the scope around. Another issue is that digital setting circles become less and less helpful. As you get closer to 90 degrees, the math requires a tangent function, which approaches infinity. I find it best to wait an hour or so.

If I really want to see something at zenith, I lay a chunk of 2x4 under one of my dob's feet. This moves Dobson's hole away from the zenith.


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obin robinson
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Re: How much time do you view at Zenith? new [Re: SACK]
      #5834413 - 05/01/13 01:31 PM

How much time am I at Zenith? Probably 5 minutes or less per night and that's only if the satellites happen to cross the zenith. If none of the ones I am observing go that high then I am not going to be looking up there.

obin


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StarStuff1
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Re: How much time do you view at Zenith? new [Re: obin robinson]
      #5835778 - 05/02/13 08:20 AM

Same here. I have two driven scopes and several other alt-az push-to scopes. Unless something unusual is happening I generally avoid the zenith other than with hand held binoculars.

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derangedhermit
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Re: How much time do you view at Zenith? new [Re: StarStuff1]
      #5853034 - 05/10/13 11:44 PM

Trying to view M3 with my Dob at TSP at SkyTools 3's "optimum observing time" was an exercise in frustration: altitude (elevation) 87.75 degrees, azimuth 179.8 degrees. OK, airmass is ideal at 1.0 (zenith), but 15 degrees (an hour) before or after only makes the airmass 1.04. 30 degrees from zenith, airmass is still only 1.15, or an increase of only 15% compared to directly overhead. That's a trade I'll make so I can get on and comfortably manually track an object. 45 degrees, halfway to the horizon, is about 1.4, and the ST3 filter of "above 2x airmass" means about 30 degrees above the horizon.

Using Glenn's method, and staying 15 degrees (worst case one hour before / after) away from the zenith for the sake of convenient telescope usage, and 30 degrees above the horizon for viewing quality reasons, I have to adjust my viewing time for less than 7% of objects to avoid Dobson's hole. The 2x airmass limit of 30 degrees puts 25% of the sky off-limits, if my calculations are correct.

Viewing only a doughnut between 30 and 75, or even 60, degrees elevation as a general rule is fine with me. Exceptions (e.g. Omega Centauri at 12 degrees elevation) can always be made.


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Dave O
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Re: How much time do you view at Zenith? [Re: derangedhermit]
      #5853119 - 05/11/13 12:42 AM

Giving up the best part of the sky is not something I'd consider lightly without exploring other options first.

Bill Kocken mentioned placing a block under one of the dob's feet, tilting the AZ axis and moving Dobson's Hole to another part of the sky when viewing objects near the zenith ... I think that is a wonderful idea; is it practical with a typical Dobsonian mount (say ~15 deg of tilt)?


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