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Messages - coreanoguf

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Good posts, Don, as always!

It seems that dielectric diagonals, 2" or 1.25" are more popular than prisms. Going by the selection available. Roland's comments seem to indicate lesser quality with the dielectrics, especially for detail views, if I read it correctly.

This has always been an area of confusion for me.

Only at the edge.
I note that the AP MaxBright star diagonal coating has a dielectric coating, and RC could use anything he wanted to, including silver or enhanced aluminum.
But his comments about secondary mirrors in newtonians are apropos, given that the entire secondary mirror is used in many applications.
[However, many newtonian secondary holders cover the edges of the mirror. When I take into account the 1/10" loss all the way around on my own secondary mirror
and look at the interferogram of the mirror, 90% of the mirror's deviation from perfect is covered by the holder. If that is the case for dielectric-coated secondary mirrors,
they might not be as much a negative as RC implies.]

Just purchased Astro-Tech dual ED 8mm and 5mm for $57 each and all I can say is WOW! Superb eyepieces for the money. Excellent sharpness across the FOV. I found these slightly better than my X-Cel eyepiece. 4" F/10 is very forgiving. Why pour hundreds of dollars on expensive eyepieces. These Astro-Tech EPs or the X-Cel line or Agena's similar series all should do very well if you are content with 60 degree FOV.

Reflectors Telescopes Forum / Re: Celestron Omni XLT 150
« on: February 08, 2018, 04:10:28 AM »
Would love to put a scrubber fan on the side, but I am not sure how to work with the metal.

bimetal holesaw. slowly. then a plywood transitional piece, belt sand the concave shape of the tube install a 40mm fan, 60mm may be too big, or buy them both and return the one you don't use, or use the 60 in the back and the 40 on the side.

I installed an 80mm on the back, but I have a cool cell for it.

Eyepieces Questions & Recommendations / Re: Vexing AFOV calculations
« on: February 08, 2018, 03:46:10 AM »
what is 1.04?

Reflectors Telescopes Forum / Re: Problem: Can't quite reach focus
« on: February 08, 2018, 12:44:32 AM »
Never heard of similiar issues with the 350P. Mine has a perfect focus snap and planetary performance is nothing less then breathtaking. Sorry to hear that.

A known issue is a certain amount of mirror shift in the bolted central connection due to large tolerances and small tightening forces. Enough to ruin your collimation.
But I personally avoided tightening the bolt further and looked for a alternative solution.

Was the astigmatism/undefined focus there - before you tightened the bolt?

Beginners Forum / Re: The plan?
« on: February 03, 2018, 12:50:21 PM »
Oh, to answer your question I need to occupy 4 brains and she wants two scopes, one for me and her to share and one for the kids to share while having some binoculars to cycle to while the others get scope time. I wanted one nicer scope, but I can't argue with her logic about how quickly a disagreement can happen especially with youth involved, and that can ruin a night in a hurry.
One scope isn't an option and sacrificing a pair of binoculars for an extra $30 on a scope isn't going to get me much if I choose wisely. It's not like I'm getting a one and done scope.

With my 4" mak: 150x on Jupiter; 220 on Saturn and Mars.

The foam needs to be somewhat flexible. Styrofoam would not work, or it would not work for long.

I would cut it so that it wraps around the tube a bit and then attach it with tape for the test. Cut it so that it is somewhat of a pressure fit between the tube and the side mount but not so tight that it can't slide. You might consider some clear packing tape to cover the foam where it contacts the side mount. This will allow pressure but reduce friction so you can track with the scope.

The goal is a light preload so that any vibration is absorbed by the foam to dampen any bounce.

I work with foam with my model airplanes for various things.  I save foam out of electroncis packing, appliance packing and the like. The open cell type that can be compressed and bounces back would be the best for this.  Or, as I said earlier, urethane foam used in couches.

I don't think ti can hurt anything and it might just fix the problem.

Beginners Forum / Re: Image size based on aperture
« on: February 03, 2018, 12:59:23 AM »
"I have seen several posts where the poster states that image size is somehow related to aperture and I don't understand this. Let me share my understanding and you tell me where I have gone wrong."

Your first question had to do with aperature and image size. Then you gave two examples of scopes.
Aperature just like in cameras DOES affect image size AND wide angle.
The term aperature can combine the ratio lens/mirror diameter to focal length (f) ratio OR it can just refer to lens/mirror diameter ie: 80mm lens, 10inch mirror.
Just like in a camera, a "fast" lens will be bigger in diameter and will have a shorter focal length and be wider angle. Same applies to telescope optics.
However, in telescopes, refractors seldom are "fast" ie: (f) ratio less than (f 8) whereas reflectors are often "fast" ie: (f) ratios often down to (f 4).

Your question about image size refers to which:
1) actual size of an observable image magnification OR
2) size of viewable amount of sky (width of view) measured in degrees.

1) image size is affected by magnification ie: focal length of lens/mirror and eyepiece used 4mm to 40mm
2) image width (wide angle) is affected by (f) ratio F4 to F15 of lens/mirror and viewing angle of eyepiece 42 degrees to 100 degrees.

Todays large 16" diameter Dobsonian scopes referred to as light buckets means that the scope has a huge mirror which gathers huge amounts of light with a short focal length of about F4 and can "see" a wide angle of sky.

Todays 80mm Dia. by 400mm FL refractors may have a F8 to F12 lens which can't gather near as much light as the huge reflector but may have near the same useful magnification 200X but won't have the wide angle view of the sky.

Theoretically those 16" light buckets have a wider field of view compared to refractors.
The actual image size has everything to do with, focal length of lens or mirror and the eyepiece, with or without a barlow lens.

As has been described in the earlier part of the discussion, many of the concepts of lenses, speed and the use of focal ratiosapply differently in how we use them in astronomy forobservation through telescopes from how they are used in photography. Fast and slow are carry over terms that really have no practical use in visual astronomy. There is nothing fast or slow about a telescope. Those terms are legacy terms that carry over from film and exposure times.

My question about image size was not intended to be related to field of view but of the size of a given object. Mars at 50X would be the same size in an 80 mm scope as a 120 mm scope. The field of view would depend on the AFOV of the eyepiece used but the size of Mars would be the same.

Eyepieces Questions & Recommendations / Re: Filter Slide / Filter Wheel
« on: February 02, 2018, 09:51:52 PM »
If you have a dobsonian with spider arms not directly above the focuse; you can get a 2" astrocrumb filter side with 4 to 8 slots (one open non-filter, and the rest filters). Works with both 1.25" & 2" eyepieces, as well as binoviewers (oprional Denk infocus adapter which I keep on my scope at all times). Can buy additional filter sides and filter case.

So you would need to reach into the top of the OTA to switch filters with the filter slide? This would not work well for me, because I attach a long dew shield extension to the front of the OTA.


Reflectors Telescopes Forum / Re: Mega-mod thread for Zhumell dobs
« on: January 31, 2018, 01:38:30 PM »
Finderscope dewshield in place

Attached Thumbnails

General Astronomy & Observing / Re: Dark Adaption
« on: January 31, 2018, 12:38:47 PM »
In front of my house there is so much light I am confident I have never been fully dark adapted there.
Re your first sentence - same here.  My suburban observing is never truly dark.  A small consolation prize (?) is that I feel no guilt in using my phone to star hop!  No night vision to ruin.
Don't be so absolutist! It's true that you can never get fully dark-adapted at a suburban site, but that doesn't mean that you don't get dark-adapted at all. It's still important to avoid looking at bright lights, even in the middle of a major city. And if you use that phone, make sure it's on red mode and as dim as you can manage.

I do notice some dark adaptation while viewing from my back yard, it is true. While there I do set up away from direct light (as much as I can) and use phone at dimmest and red.

Beginners Forum / Re: Red or green for preserving night vision?
« on: January 31, 2018, 12:09:56 PM »
I like red. But I have white, red, green, and blinking red on my headlamp.
I also have a beany with white LED's in it. (Called a Power Cap)

I'm an imager and look at a laptop screen.
Also operate out of my back yard in light pollution suburbia.
So it makes nearly no difference really.

I saw a laptop at Costco by Lenovo. It's a gamers Laptop.
What I liked was the red lighted keyboard.
Throw away the head light. But it cost a thousand bucks.
Still, it could be cool.

Since I'm putting my Baby Dell in my storage container I use as a battery box cover during the day, I've been imaginering an LED red strip on a potentiometer to dim it, and mount so it shines down on my keyboard just to make the caricatures glow.
But I'll probably just stick with the headlamp.

General Astronomy & Observing / Re: Bear fears
« on: January 31, 2018, 11:53:15 AM »
OTOH, bear human interaction makes for great comedy.

My solid tube Z8 holds collimation pretty good. My observing site is close to my home buteven thoughI have to transport all stuff. It doesn't affect collimation.I'm also surprised that evenlong road trips (~1500+ miles) in cargo bin on the roof of my SUV don't affectmy Z8 collimation at all.That how it is. Of course, I used tocheck collimation every timewhen I'm out, but its usually spot-on. And I use only Cheshire (at home) and stock Zhumelllaser + 2x barlow(in the field). I'm not surprised reading that owners of larger Dobs have to make adjustments more frequently and using more expensive tools because even small collimation errors seem more critical for bigger mirrors.

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