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Messages - meisporbiopop

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What's the comparison between silicone adhesion and perforated center bolt attachment? Advantages/disadvantages?http://www.rfroyce.c...l/mount CFB.htm


The center bolt attachment requires a conical mirror so that it is adequately stiff with only the center support. Since it is supported only in the center, it is free to expand and contract without constraint. The difficulty with multiple supports is that the shear forces and even compression forces can be applied to the mirror if the cell is not properly designed and implimented. Those forces can result in tiny, but optically significant, elastic deformations.

Jon

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General Astronomy & Observing / Re: Pupil gauge to measure exit pupil
« on: February 08, 2018, 08:48:27 PM »
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[/quote]
If you can show me some actual math that supports your point, then I might reconsider. However until I see that I don't accept it since common sense says it isn't the case and also actual experimentation shows it isn't the case. The link you provided actually works against your claim by stating, "While a 7-mm exit pupil, by matching that of the eye, does give the brightest views of deep-sky objects, it does not necessarily give the best ones. Higher magnifications, despite their smaller exit pupils, will reveal more details, maintain contrast, show fainter stars, and help bypass defects in the eye itself." It states that matching exit pupil to the eye you get the brightest views. The next section about wasted light doesn't support your position either, as it states "With reflectors, however, larger pupils do waste light, but primarily because the black spot in the pupil caused by the secondary obstruction becomes larger. Both light loss and field shadowing occur with reflectors, but as with refractors there is no resolution loss because of the low power." That really talks about hitting the theoretical limit of the telescope itself, where the secondary becomes visible, and doesn't discuss light loss due to exit pupil mismatch. With refractors and reflectors both it talks about resolution, not light loss or dimming. Resolution and dimming aren't interchangeable.

You have x amount of light entering a reflector tube that actually reaches the primary mirror, bounces to the secondary, then exits the secondary.
There is some loss in the mirrors, plus there is some loss in the eyepiece, but since those losses aren't caused by exit pupil they can be ignored here.
So... for the purpose of this discussion x amount of light bounces through the mirrors, x amount of light enters the eyepiece, and x amount of light leaves the eyepiece in the form of the exit pupil.

If you have a 5.6mm eye pupil and the lenses provide a 5.6mm exit pupil, then x amount of light enters your eye.
i.e. not counting losses in the lenses, 100% of the light that comes off the secondary mirror enters your eye.

If you have a 5.6mm eye pupil and the lenses provide an 8.5mm exit pupil, then only 0.43x amount of light enters your eye.
i.e. not counting losses inherent to the telescope itself, 57% of the light that comes off the secondary is wasted, and 43% of the light that comes off the secondary enters your eye.This is also supported by experimentation, where I can actually see that stars and deep sky objects are a lot dimmer through that lens providing an 8.5mm exit pupil (not just the effects of decreased magnification, but actually dimming).

It can also be seen through observation in other similar experiments.

For example, you can take a flashlight with a focusable beam and point that beam at a circle on the wall to represent an exit pupil.
If the beam is the same diameter as the circle on the wall, then 100% of the light from the flashlight is inside that 'exit pupil', and with a bright light can provide a bright enough reflection to be uncomfortable.
If the beam is then expanded out so less than half of it is inside the circle, there is very noticeably less light striking that circle.
There is no more or less light leaving the flashlight, but a lot less of that light is striking the circle on the wall (the exit pupil).

If you have something that shows I'm wrong I'll be glad to read it and will change my stand if it's convincing. However for now common sense, simple math, plus multiple experiments show me that an exit pupil larger than the eye means dimmer views.

Thanks,
Mark.[/quote]
Not going to do all the research for you - but if you read up on exit pupils, you will see that the brightest possible view of an extended surface area object (ie, non-point source objects) is obtained with the naked eye. A telescope cannot increase the surface brightness that the object puts out - it merely magnifies it. So long as the telescope is fully illuminating the exit pupil, and the exit pupil is equal or greater to your pupil, the brightness you will see is the maximum possible (unless you found a way to increase your pupil size).

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12 is easier to transport, 10 is quicker to setup.

Gee, I agree--short, sweet, and to the point.

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The 48 would hugely benefit from a factory rubber eye cup.

Maybe the new owners will see fit to do that?

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Beginners Forum / Re: New Guy, New Scope
« on: January 31, 2018, 09:51:50 AM »
Ok. Thanks. It sounds like I should stick with a refractor if I am looking for something smaller and more portable. $200 is a bit more than I want to spend since I am planning to by a $300-$400 reflector soon but if I can talk him down it may be work $150. Only problem is the seller is on the very very west side of town and I am in Gilbert. That is about an hour drive. Maybe I can shoot that way after work one day but the thought of the traffic kills me. Or perhaps if they work in Phoenix they can meet me with it. I will see what is available after my next paycheck.

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Beginners Forum / Re: "Refractors are best for planetary viewing"
« on: January 31, 2018, 09:38:12 AM »
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my best view of Jupiter was a 6" refractor
my best view of Saturn was a 10" Mac
my best view (and only) of Pluto was a 22" dob
my best view of comets was with binoculars
my best view of meteors and Northern Lights was naked eye

as stated above-it is complicated

edj

I like this answer. Different tools for different jobs. 

There isn't one perfect scope that does it all, which is why people wind up with two or three (or 10) telescopes.

I'm a refractor guy. Love my 6" f/8 achro. But it is a big scope on a big mount, and sometimes I'm not up for all that. Which is why I grabbed the MUCH smaller 6" SCT.

Between a C8 and a 6" achro, that's a tough call. In most fights aperture wins. The C8 sure would be more compact and easier to mount, and it has no CA. CA is a personal thing though... some are bothered by it, some aren't.
I think that it comes down to a combination of cost, quality, portability and observing goals

edj
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ATM, Optics and DIY Forum / Re: Looking for very compact travel dob ideas
« on: January 31, 2018, 08:56:13 AM »
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Hi,
you might want to have a look at this one - collapses into a carry on sized box (albeit a bit heavy at 17kg) or two boxes of 10 and 8 kg
Joachim

Joachim,

You forgot the link !

Colin

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It would seem that offering split-ring mounts would be the next step in "garden Dob" evolution. I realize that a split-ring is an order of magnitude more complex than a basic dob but it hardly seems insurmountable...

A couple of seemingly small, but actually quite complex problem become evident almost immediately: A split ring equatorial is difficult to make adjustable to a wide range of latitudes. It also needs to have a rotating OTA or upper tube section, so you can rotate the focuser into a comfortable position, when the mount tracks across the meridian. Both of these make it massively more complex, and thus expensive, compared to a dobsonian. It will also be much heavier. If that is what people wanted, the models that have been available, would have sold well. In reality, the dobsonian outsells any other large aperture scope by several orders of magnitude, because it is cheap, simple and reliable, which is what people want.Clear skies!
Thomas, Denmark

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A very good eyepiece in many ways. I use it, along with 28 20 and 16mm + 2x focal extender, in a 16" Skywatcher Dob with ESHR coma corrector.
Before prices go up do yourself a favor and buy the ES 34 68º and get 3.5º TFOV....

Matt

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ATM, Optics and DIY Forum / Re: $533 to recoat a 10 inch mirror?
« on: January 30, 2018, 05:53:37 AM »
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Thanks! Optic Wave Labs also seems to have a good price. Nova is almost within driving distance though.

Nova no longer is in the coating business..

[/quote]
That's what I read, too.

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Only man seeks to bend nature to his will, rather than trying to live within nature like every other living thing.....

That's an exaggeration. Beaver dams are one example to the contrary. One of the hard science fiction authors made the observation that when beavers build dams for their purposes, thereby flooding the land immediately upstream, it's considered natural but when humans do the same thing, it's considered to be interference with nature.

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ATM, Optics and DIY Forum / Re: help with diy dew heater system
« on: January 26, 2018, 06:33:26 AM »
the moment it senses the temp is above ambient, the circuit shuts off automatically. this is better than most commercial solutions.

I found the circuit

http://web.archive.o.../pcb/index.html

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Light Pollution Topics / Re: Darkest Places on Earth
« on: January 26, 2018, 12:23:11 AM »
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<p class="citation">Quote"Only about 10 percent of the U.S. population has seen a truly dark sky"

..... I wonder what their definition of "truly dark" is......

I think there’s a lot more than 10% of North America that can provide “truly dark skies” under the right conditions. The real issue here is that I’d have to agree that only one in ten people are interested enough to make the effort to go to one of these locations and look. The rest either don’t know, or don’t care.

<strong class="bbc">For the most part today “dark sky” = “very few people”…. and pretty rough living conditions…. which is the reason there are few people. [/b]Hopefully, our anti-LP actions will close the gap between dark sky and population centers. [/quote]Where I live remotness tends to equal wealth. The town I stay in during my 15 days "on" is about a 20 minute drive to grey/black skies. Lots of people up here work under super dark skies, yet no one usually talks about them. Everyone is too busy trying to look cool in their $70 000 pickups.

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ATM, Optics and DIY Forum / Re: Reliable way to test sagitta?
« on: January 25, 2018, 07:01:32 PM »
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One implicit assumption when using a spherometer is that the surface is spherical. This is often not the case during course grind.

The same implicit assumption is made when measuring sag with a ruler and pennies.
No, you measure either RoC of some sphere and by assuming it co-incides with the surface deduce a sag. With a ruler and pennies you measure the actual depth, still no sphere though...

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Beginners Forum / Re: Crack in new LightBridge 16" mirror
« on: January 25, 2018, 06:19:46 PM »
Even if the scope is fully functional, cosmetic damage to a brand new product you just received is upsetting. It could have been a dent in the optical tube. Whatever. The Meade warranty claimstheir products will be free of defects in material and workmanship. Some issues are worth pursuing, and some aren't. It's a personal thing.Call Meade directly.

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