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91
Eyepieces Questions & Recommendations / Re: ES62 5.5mm vs. DeLite 5mm?
« Last post by David Pee on February 09, 2018, 11:15:40 AM »
yes that is it
92
Eyepieces Questions & Recommendations / Re: Why did you sell your Ethos?
« Last post by Chris Smale on February 09, 2018, 11:14:42 AM »
I bought the 21, 13 & 8mm Ethos eyepieces, I found them fine but I never actually viewed the full field as I didn't like moving my eye around. I will probably replace them with the Delos 12 & 8 and another N22T4
93
Reflectors Telescopes Forum / Re: Coma free zone calculation
« Last post by ryepittimy on February 09, 2018, 11:13:57 AM »
Read here:http://www.telescope.../Mak-Newton.htm , specifically, "...coma is reduced to ~30% of that in a comparable paraboloid."
94
Reflectors Telescopes Forum / Re: First Light with Zhumell Z10 10” DOB
« Last post by ecidjapa on February 09, 2018, 11:13:11 AM »
On using the RAF and Telrad; I picked up a "2-spot shoe" from Orion, along with the 4" extension for the Telrad, and have them mounted side-by-side, and love how it works.
95
Eyepieces Questions & Recommendations / Re: Planetary Eyepieces
« Last post by nuitropheneg on February 09, 2018, 11:12:19 AM »
televue plossl 7.4mm, why so many complexities lol
96
Quote
So would you say this sketch:http://www.asod.info...12-inverted.jpg is way off compared what you actually observe (this seems way to bright)?
or these:http://skytour.homes.../files/M51X.jpg,http://www.asod.info...-m-32-m-110.jpg

in a 14"/16"?
I have little doubt that with careful observation, perhaps at a variety of magnifications, and good skies, you can see all the detail in the first sketch with a 12.5" instrument. That doesn't mean it accurately and closely recreates the experience at the eyepiece. It certainly is showing too much contrast to be truly realistic.
Sketches that truly represent the eyepiece view are very difficult to produce (although they're more representative than most photographs). A sketch may represent the detail you pull out of observing over, let's say, 30 minutes of careful observation. A sketch that truly represents the eyepiece view would *also* take 30 minutes of careful observation to pull out the same detail! Even if you have the means and skill to produce such a representation, is that really the goal of the sketch? Probably not.
The 2nd M51 sketch is a little closer to the immediate experience at the eyepiece of a large-ish scope The last sketch of M31 is representative of the detail if you have very dark skies - seeing that second dark lane isn't easy.
97
ATM, Optics and DIY Forum / Re: Plans for a Portable a 6" F/8?
« Last post by ricoperte on February 09, 2018, 11:09:04 AM »
Quote
Maybe...for a reflector forum...And it is an old site.Why are you keep this OT going.
You might want to review the OP's original post.....he is looking for ideas on a new lighter setup for his 6" f/8. Would what we are talking about not fit as a consideration using T-SLOT to construct an OTA? It would be collapsible for transport, only require a couple or one short piece of 8" tube etc....ie a metal version of "scope on a stick".So I am not sure why you would be calling it OT.....you just feeling contrary today?
98
Reflectors Telescopes Forum / Re: Plan:Boundary fan suspended over 16" mirror.
« Last post by maogrinjorli on February 09, 2018, 11:08:21 AM »
I dunno if this is relevant or not, but it's related I guess...This guy has several demonstration videos of this tube current stuff..

https://www.youtube....h?v=UbH4AS-0ViE
99
Eyepieces Questions & Recommendations / Re: Eyepiece question
« Last post by juskemenbo on February 09, 2018, 11:07:31 AM »
OK, let me explain about eyepieces as afocal devices.

As you move your eye back from the eyepiece, the field gets smaller, but stays in focus.
That's because, once the focal plane of the eyepiece is coincident with the focal plane of the scope, the eyepiece is essentially afocal.
No matter what distance you are from the eyepiece, the field is in focus.

Nonetheless, as you back away from the eyepiece, the pupil of your eye becomes smaller than the size of the image
formed by the eyepiece (which becomes wider). You see less of the field because the edge of the eyepiece barrel is literally occulting the image.
You can move your eye back and forth laterally and see different parts of the image, and it stays in focus.
But the body of the eyepiece itself occults the edge of the field.
This sounds suspiciously similar to your description of drifting back and forth and having the edge of the field get occulted.

This only occurs when you are too far from the eyepiece, however, to see the actual field stop completely all the way around
(which occurs at the exit pupil).

Now, if you are at the exit pupil, you see the entire field to the field stop and, if the exit pupil is smaller than your eye's pupil, all the light from the eyepiece goes into the eye.

If you are too close, however, the light from the eyepiece becomes larger than the pupil of the eye and your iris starts occulting the image.
If you move back and forth, first one side, then the other, blacks out as the rays are intercepted by the iris and you get what are termed "black-outs".
This happens a lot with long eye relief eyepieces used by non-glasses wearers, or people who stand to observe or even just when you drift a bit too close to
an eyepiece. I can induce the blackouts on nearly every eyepiece that's made unless the eyepiece has such a short eye relief I automatically hold back
to preserve my eye.

So what it looks like at the eyepiece is dependent on where your eye is relative to the exit pupil.

If the eyepiece has spherical aberration of the exit pupil, however, then the exit pupil itself is not all at the same distance from the eyepiece, and moving in toward the eyepiece may make either the
center or edges of the field black out in an alternating way that resembles large kidney-bean-shaped shadows in the outer parts of the field. This is termed "kidney bean blackouts" or simply
"kidney-beaning". It is found in (primarily) widefield eyepieces without a flat exit pupil. This doesn't have to be severe (like the original 13mm Nagler)--it can be slight, but still result in an eyepiece
being sensitive to eye placement. I have run into many of these over the years.
http://www.handprint...O/ae4.html#SAEP

So, traditional "blackouts" are caused by the observer's eye position relative to the exit pupil.
"Kidney bean blackouts" are caused by SAEP in the eyepiece.
Other than cataracts or macular degeneration, those are the two sources of blackouts in eyepieces as I understand it.

The "walking past a keyhole" look simply indicates the observer is farther away from the eyepiece than the exit pupil.
100
Reflectors Telescopes Forum / Re: Focal ratio and Coma.
« Last post by consmagestma on February 09, 2018, 11:06:45 AM »
Quote
Jeff's descriptive term "convenient medium aperture " perfectly describes what I was thinking. I am looking for the "CMP" scope that has "refractor like" stars. Thank you for the input. I am leaning towards the dobs, I have had a 8SE and an 8CPC I don't think of them as convenient.

Bill:

This one is really calling out to me:

http://www.astromart...ified_id=937983

It's set up for photo and visual and could be easily converted to Dob use. But the second-to-last-thing-in-the-world I need right now is another scope!

Do you use bino-viewers?

Jeff
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