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General Astronomy >> Beginners Forum

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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Hypothetical 4" budget APO choices.. WWJD? new [Re: lamplight]
      #5653888 - 01/31/13 09:23 AM Attachment (14 downloads)

Quote:

Jon the 4.7" range doesn't seem all that much bigger (??). The ones I'm looking at look really well built and everywhere I see good reviews of very low to no visible CA. I worry that if I went a triplet and stayed in the 100mm range (about same cost as these exceptionally good doublets) I would not get a noticeable upgrade in light gathering.




It all depends on what you are looking for in a telescope. In terms of light gathering, the difference between a 4 inch and a 120mm is not much if there is a 10 inch Newtonian sitting nearby.

The fact that it takes as much effort to setup the 120mm refractor as it does the 10 inch Newtonian is clue that maybe the 120mm is on the big side of easy to use and portable. Myself, I look to refractors to do the things smaller scopes do best, easily portable, compact, wide fields of view, with the ability to go to higher magnifications, remembering that there is a limit to what scopes this size can do.

When small is good, bigger is not necessarily better.

For the things I use a refractor for, an 80mm is a better choice than a 120mm and a fast 4 inch is about ideal. In terms of fast 4 inch refractors for visual use, the NP-101 is about as good as it gets.

Jon


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lamplight
Carpal Tunnel
*****

Reged: 09/18/12

Loc: western MA, U.S.
Re: Hypothetical 4" budget APO choices.. WWJD? new [Re: Jon Isaacs]
      #5654074 - 01/31/13 10:53 AM

( is that a vixen head on your TV? nice !)

thanks.. well if it will be smaller than what i have but only a bit heavier with bit more light and less CA it seems a no brainer for my needs. except cost which is contrary to practical. "but..."

this has been SO educational thanks! this hobby is a continuation of not knowing what i didnt know.. how many times is this going to keep happening? sheesh.


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CJK
professor emeritus


Reged: 12/05/12

Loc: Northeast TN
Re: Hypothetical 4" budget APO choices.. WWJD? new [Re: Jon Isaacs]
      #5654148 - 01/31/13 11:29 AM

Re: your photo -- Who knew Home Depot sold telescope mounts?

-- Chris


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kenrenard
Carpal Tunnel


Reged: 12/13/12

Loc: Dunmore, PA
Re: Hypothetical 4" budget APO choices.. WWJD? new [Re: Jon Isaacs]
      #5654260 - 01/31/13 12:43 PM

Quote:

Quote:

Quote:

I have owned a couple of 120mm refractors. I prefer a 4 inch, preferably faster, shorter.




And I will repeat my previous question. As you move up in the aperture leagues, what's wrong with a 6-inch reflector? More portable than a 120-mm refractor, much cheaper, and closely comparable light gathering and image quality.




Tony:

Indeed... over the years, the traditional 6 inch F/8 Newtonian gained the reputation as an "apo-killer." The classic Criterion RV-6 (last built about 1978) has a light weight phenolic tube, a small secondary and a simple clean aluminum mirror cell. The optical tube assembly weighs 8 lbs and these are very capable performers as planetary and double star telescopes.

Jon





Tony or Jon,
I have a question. Would a 6 inch reflector f/8 be better for planets than say a 8 inch f/6 or does the aperture cancel the longer focal length out? I have see quite a few ads for planet killer refractors f/11 or more. I have read how some people love these but the FOV is very limited. So would the a longer focal length help with planets or double stars on reflectors as well? I understand this would make them less portable. I am trying to understand the trade offs over wide field or better planet viewing. From what I am understanding it sound like only a select few refractors like the NP-101 does both things well.

Ken


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Mike4242
Pooh-Bah


Reged: 11/02/11

Loc: Memphis, TN
Re: Hypothetical 4" budget APO choices.. WWJD? new [Re: kenrenard]
      #5654405 - 01/31/13 02:12 PM

The main benefit of a longer focal length is that you get a higher magnification with a given eyepiece; however, this also restricts your maximum field of view.

Focal length also comes into play with achromatic refractors because longer focal lengths reduce the amount of chromatic aberration.

In the case of reflectors collimation is more forgiving with a longer focal length.

An 8" f/6 should give better planetary views than a 6" f/8 if both scopes are cooled, properly collimated, and the seeing conditions permit. The 8" will be more affected by atmospheric seeing than the 6".

I'm probably missing some points here, but I'm sure Jon and/or Tony will chime in.


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Mike4242
Pooh-Bah


Reged: 11/02/11

Loc: Memphis, TN
Re: Hypothetical 4" budget APO choices.. WWJD? new [Re: Mike4242]
      #5654409 - 01/31/13 02:14 PM

Here's a good article I just remembered about the pros and cons of short and long focal lengths:

http://www.chuckhawks.com/telescope_focal_length.htm


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kenrenard
Carpal Tunnel


Reged: 12/13/12

Loc: Dunmore, PA
Re: Hypothetical 4" budget APO choices.. WWJD? new [Re: Mike4242]
      #5654420 - 01/31/13 02:26 PM

Thanks Mike,
Good article. When I put the different focal lengths comparing say a XT8 f/5.9 and XT6 f/8 in an eyepiece calculator they have the same magnification. So in essence looks like a wash? At least thats the way I'm understanding things.

Ken


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Mike4242
Pooh-Bah


Reged: 11/02/11

Loc: Memphis, TN
Re: Hypothetical 4" budget APO choices.. WWJD? new [Re: kenrenard]
      #5654426 - 01/31/13 02:29 PM

You have to look at the actual focal length value instead of the ratio for magnification. Since the F ratio is the focal length divided by the aperture, a larger aperture will have a lower/faster F ratio with the same focal length.

But, yes, it is essentially a wash. The collimation tolerance of the 8" will be tighter, but the 8" should give better views if atmospheric seeing permits.


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kenrenard
Carpal Tunnel


Reged: 12/13/12

Loc: Dunmore, PA
Re: Hypothetical 4" budget APO choices.. WWJD? new [Re: Mike4242]
      #5654444 - 01/31/13 02:37 PM

OK I understand. Thanks for clearing it up. I found a few other old post explaining why we don't have long focal length reflectors. Size and weight seem to be the biggest answer. As well as better mirrors today.

Ken


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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Hypothetical 4" budget APO choices.. WWJD? new [Re: kenrenard]
      #5654468 - 01/31/13 02:51 PM

Quote:

Thanks Mike,
Good article. When I put the different focal lengths comparing say a XT8 f/5.9 and XT6 f/8 in an eyepiece calculator they have the same magnification. So in essence looks like a wash? At least thats the way I'm understanding things.

Ken




Ken:

The article was comparing telescopes of the same aperture and is not really applicable in this situation because the question here is involves two scopes of the same focal length but different apertures. It is also rather dated so many of the comments about eye relief and eyepieces are no longer valid.

This is a favorite saying:

"The best 6 inch F/8 is an 8 inch F/6."

In terms of viewing the planets, the benefit of the larger aperture is greater resolution, increased planetary contrast and a brighter, potentially more colorful view. The resolution and contrast are proportional to the aperture, so assumming the seeing (atmospheric stability) is not the limiting factor, the expectation is that an 8 inch F/6 Newtonian will provide noticeably better planetary views than a 6 inch F/8 Newtonian.

This is very consistent with my experience. I do have the classic 6 inch F/8 but if want the good planetary views, it's not the 4 inch apo nor the 6 inch F/8 Newtonian but rather the 10 inch F/5 or something even larger.

Jon


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kenrenard
Carpal Tunnel


Reged: 12/13/12

Loc: Dunmore, PA
Re: Hypothetical 4" budget APO choices.. WWJD? new [Re: Jon Isaacs]
      #5654557 - 01/31/13 03:36 PM

Thanks Jon,
As always you are a wealth of information.


Ken


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