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Atl
professor emeritus
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Reged: 04/13/12

Re: Aperture.. who cares new [Re: Cames]
      #5642240 - 01/25/13 10:00 AM

Quote:

On crisp, clear winter nights when those with the large apertures cringe at the effect of the nightís turbulent atmosphere and its scintillation on their images, your brave little soldier will experience hardly a hiccup.




This is a myth as far as I can tell. During times of mediocre seeing large scopes still have a better view. I own and enjoy having a C90, but it can't match my 12.5 dob under any conditions.


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BigC
Carpal Tunnel
*****

Reged: 09/29/10

Loc: SE Indiana
Re: Aperture.. who cares new [Re: Atl]
      #5642363 - 01/25/13 11:17 AM

Well the small scope does gather less light so the fainter details won't be visible at all and therefore won't waver in and out of focus!

A long tube 50mm could be made with no visible CA except most scope manufacturs shorten the 50mm tube and sell f12 models.

A 50mm ground for use in a common 900mm long tube would be f18 .Wonder if there have been any?

I know there were f20 80mm sold in the pre-internet age.


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csrlice12
Postmaster
*****

Reged: 05/22/12

Loc: Denver, CO
Re: Aperture.. who cares new [Re: BigC]
      #5642398 - 01/25/13 11:39 AM

I saw an old used Russian made 70mm, think it was 1000mm f/l, at S&S Optica a while back. Nice looking scope, great optics. Can't remember the name off the top of my head, started with an "I" (I think). It was Russian made.

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Cames
sage


Reged: 08/04/08

Re: Aperture.. who cares new [Re: Atl]
      #5642429 - 01/25/13 11:57 AM

ATL,

Magnification may have something to do with the effect.

When I look up on those nights, I see a sharp twinkling star.

When I look at the same with my 10x50 binoculars, the star image still looks good.

Same look with my dob, star looks like glowing ball of frizz that will not come into sharp focus. I'm not exactly enjoying what I'm seeing because I'm spoiled by the the view I get when seeing is great. That's all I'm saying is what looks good to me.
-------
C


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Aperture.. who cares new [Re: BigC]
      #5642498 - 01/25/13 12:35 PM

Quote:


A long tube 50mm could be made with no visible CA except most scope manufacturs shorten the 50mm tube and sell f12 models.




Chromatic aberration increases with aperture, decreases with focal ratio. A 50mm F/12 achromat will be essentially color free, comparable to a 100mm F24 in terms of CA. A rule of thumb is that if the focal ratio is at least 5 times the aperture as measured in inches, the chromatic aberration will minimal and not distract from the image. This ratio is sometimes called the Chromatic Ratio. A 50mm F/12 has a Chromatic Ratio of 6.

As far as the seeing goes. A 50mm is quite small so it is unlikely that a 50mm would ever outperform a 12.5 inch. There are situations when the seeing is so bad that a 4 inch might outperform a 12.5 inch but in such situations, neither one is providing reasonable views. Of course this assumes the larger scope is thermally stable... Still.

I do care about aperture... I also care about portability.

Jon


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Eric63
scholastic sledgehammer


Reged: 06/16/12

Loc: Ottawa, Ontario
Re: Aperture.. who cares new [Re: Jon Isaacs]
      #5643248 - 01/25/13 07:51 PM

My 102mm F5 achromat is horrible on Jupiter at high power. I cut the aperture down to 50mm and turned it into an F10. This pretty well eliminated all of the violet and turned a washed out image into a pleasing image. A dim..but very pleasing image.

Eric


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jgraham
Postmaster
*****

Reged: 12/02/04

Loc: Miami Valley Astronomical Soci...
Re: Aperture.. who cares new [Re: Eric63]
      #5643389 - 01/25/13 09:24 PM

One thing that I've noticed about my 16" is that the view always shows excellent resolution. When the seeing is bad it is just bouncing all over the place.

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Gert K A
sage


Reged: 07/16/12

Loc: Copenhagen, Denmark
Re: Aperture.. who cares new [Re: kenrenard]
      #5643684 - 01/26/13 01:22 AM

Quote:

You can't go wrong with the Dwarf Star. A great mount. Larry is a great guy to work with too. Be sure to get the eyepiece tray it is a nice add-on




@ kenrenard
The Dwarf Star is a manual alt/az mount, only clutches no gears.. right?
Itís a real sweetie it fits well for a tiny setup and to my surprise very reasonably priced too.


@ Jon
You just know your stuff, I know you do and I do read all you advice with the outmost care
it is appreciated thx

Just to get back to this light cheapo mount I got, It is a huge compromise to use and is really only useful at the very lowest powers 25x at best,
I tested it out yesterday and I knew it was gonna require care,
it got a settle down time way in the +5sec, it shows me stars like this Light show and nice as that is
I will surely get to use the heavier mount the most.


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kenrenard
Carpal Tunnel


Reged: 12/13/12

Loc: Dunmore, PA
Re: Aperture.. who cares new [Re: Gert K A]
      #5644058 - 01/26/13 09:55 AM

Gert,
Yes the dwarf star is alt az weights about a pound and works very well. Larry makes an eyepiece tray and a handle for it. You can also use it as a simple binocular mount with the UA l bracket.


Ken


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brianc99
member
*****

Reged: 11/27/12

Loc: Southern NH
Re: Aperture.. who cares new [Re: kenrenard]
      #5644363 - 01/26/13 12:54 PM

I bought a used one from another CN member, and the spousal unit says I can have it for our anniversary. After trying to make do with an old video head and the scope mounted "side-saddle," I relented and decided to get something purpose-built. To say that it's "built" is an understatement. It may only weigh one pound, but I've got a feeling that someone will be able to make use of that mount long after my observing days are over.

Brian


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kenrenard
Carpal Tunnel


Reged: 12/13/12

Loc: Dunmore, PA
Re: Aperture.. who cares new [Re: brianc99]
      #5644436 - 01/26/13 01:47 PM

I bought mine directly from Larry at his shop. He really hand makes every part. Huge piles of steel dust everywhere. I don't think you could find a better piece of equipment.

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