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Equipment Discussions >> Refractors

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RGM
scholastic sledgehammer
*****

Reged: 09/15/03

Loc: Burks Falls, Ontario, Canada
Strengths of a long focal length Achromat
      #5518154 - 11/13/12 03:30 PM

I have really enjoyed reading the current threads regarding achromats. Every optical configuration has it's strengths. For those that have never owned, or looked through a long FL achro, and are wondering what all the fuss is about, let's tell them.

Here are a few to start the conversation off.

1. Flat field of view due to long FL
2. Do not need costly eyepieces
3. Pinpoint star images
4. Because of #3, great for double stars
5. Because of #3, great for open star clusters
6. High contrast - "diamonds on black velvet"
7. Moons of Jupiter are resolved as discs

Any other strengths?


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csrlice12
Postmaster
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Reged: 05/22/12

Loc: Denver, CO
Re: Strengths of a long focal length Achromat new [Re: RGM]
      #5518164 - 11/13/12 03:36 PM

8. They're a lot cheaper then their Apochromatic counterparts.
9. They make a great complimentary scope to a larger Dob.
10. Can be used as a club in case of emergency (Dobs just don't cut it).
11. In the daytime, you can take out the objective lens and burn paper or ants with it....


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Bonco
Post Laureate
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Reged: 04/17/06

Loc: Florida
Re: Strengths of a long focal length Achromat new [Re: csrlice12]
      #5518192 - 11/13/12 03:55 PM

12. They impress the masses who marvel at the beautiful long tube.

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hottr6
Carpal Tunnel
*****

Reged: 06/28/09

Loc: 7,500', Magdalena Mtns, NM
Re: Strengths of a long focal length Achromat new [Re: Bonco]
      #5518203 - 11/13/12 04:00 PM

13. (lucky 13) Good for muscle building as you haul around the heavyweight mount and tall tripod.

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hottr6
Carpal Tunnel
*****

Reged: 06/28/09

Loc: 7,500', Magdalena Mtns, NM
Re: Strengths of a long focal length Achromat new [Re: hottr6]
      #5518209 - 11/13/12 04:06 PM

14. Don't need dual-speed focussers
15. Never have to collimate
16. No finder-scope needed; just sight along the tube
17. Gotta love observing while sitting/lying in damp grass


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Andy Taylor
Twisted, but in a Good Way
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Reged: 09/24/08

Loc: Epsom - UK
Re: Strengths of a long focal length Achromat new [Re: RGM]
      #5518295 - 11/13/12 04:54 PM

Quote:

I have really enjoyed reading the current threads regarding achromats. Every optical configuration has it's strengths. For those that have never owned, or looked through a long FL achro, and are wondering what all the fuss is about, let's tell them.

Here are a few to start the conversation off.

1. Flat field of view due to long FL
2. Do not need costly eyepieces
3. Pinpoint star images
4. Because of #3, great for double stars
5. Because of #3, great for open star clusters
6. High contrast - "diamonds on black velvet"
7. Moons of Jupiter are resolved as discs

Any other strengths?




Crikey!! What other reasons do you need?

How about:

8) They look like a REAL telescope - you just WANT to look through it.


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Andy Taylor
Twisted, but in a Good Way
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Reged: 09/24/08

Loc: Epsom - UK
Re: Strengths of a long focal length Achromat new [Re: hottr6]
      #5518309 - 11/13/12 04:59 PM

Quote:

13. (lucky 13) Good for muscle building as you haul around the heavyweight mount and tall tripod.




OK,

13) Not recommended for lazy people...


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SteveG
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Reged: 09/27/06

Loc: Seattle, WA
Re: Strengths of a long focal length Achromat new [Re: Andy Taylor]
      #5518562 - 11/13/12 07:22 PM

#7 only applies to maybe 4" diameter or better - right?

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astrogeezer41
super member
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Reged: 08/27/10

Re: Strengths of a long focal length Achromat new [Re: RGM]
      #5518648 - 11/13/12 08:30 PM

8. Depth of focus
9. Depth of focus
10. Depth of focus
Thanks RGM,
Robert


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mikey cee
Postmaster
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Reged: 01/18/07

Loc: bellevue ne.
Re: Strengths of a long focal length Achromat new [Re: astrogeezer41]
      #5518706 - 11/13/12 09:10 PM Attachment (52 downloads)

#18. Never again needing to upgrade! Mike

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dlapoint
scholastic sledgehammer


Reged: 08/18/03

Loc: Moncton NB Canada
Re: Strengths of a long focal length Achromat new [Re: mikey cee]
      #5518819 - 11/13/12 10:25 PM

The short answer is the view.

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stevew
Now I've done it


Reged: 03/03/06

Loc: British Columbia Canada
Re: Strengths of a long focal length Achromat new [Re: astrogeezer41]
      #5518828 - 11/13/12 10:30 PM

Quote:


8. Depth of focus
9. Depth of focus
10. Depth of focus

Robert



This is something that has to be experienced to really appreciate. But it is quite remarkable how steady the images can be in a long focus refractor.
And just so we are all on the same page, F-8 is not a long focus refractor.


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m9x18
Pooh-Bah


Reged: 10/12/07

Loc: Abilene, Texas USA
Re: Strengths of a long focal length Achromat new [Re: RGM]
      #5518875 - 11/13/12 11:06 PM

19. Experiencing the classic views enjoyed by our refractor-using forefathers.

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jsiska
professor emeritus
*****

Reged: 07/12/06

Loc: NW Ohio
Re: Strengths of a long focal length Achromat new [Re: stevew]
      #5518931 - 11/13/12 11:49 PM

Quote:


And just so we are all on the same page, F-8 is not a long focus refractor.



So what is the minimum focal ratio to be considered a long focus refractor


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RGM
scholastic sledgehammer
*****

Reged: 09/15/03

Loc: Burks Falls, Ontario, Canada
Re: Strengths of a long focal length Achromat new [Re: jsiska]
      #5519105 - 11/14/12 06:31 AM

I forgot to mention depth of focus in my original post. It is something that has to be experienced.

My initial thought of what a long FL achro would be, is anything over f10. IMO, a lot of the positive qualities really start to show themselves at f12 and up.


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Strengths of a long focal length Achromat new [Re: RGM]
      #5519120 - 11/14/12 07:09 AM



Keep them coming.

regards
Chris


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RGM
scholastic sledgehammer
*****

Reged: 09/15/03

Loc: Burks Falls, Ontario, Canada
Re: Strengths of a long focal length Achromat new [Re: chboss]
      #5519149 - 11/14/12 08:04 AM

20. Works well with binoviewers

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vahe
scholastic sledgehammer


Reged: 08/27/05

Loc: Houston, Texas
Re: Strengths of a long focal length Achromat new [Re: jsiska]
      #5519151 - 11/14/12 08:07 AM

Quote:

So what is the minimum focal ratio to be considered a long focus refractor





For achromats F/15, for apos F/9, these F ratios are also the sweet spot when it comes to performance optimization.

Vahe


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covey
member


Reged: 07/02/11

Re: Strengths of a long focal length Achromat new [Re: vahe]
      #5519367 - 11/14/12 11:06 AM

On the other hand...unless you have a whopper of a mount, anchored practically all the way to the Earth's core, they quiver and shake every time you inhale, and exhale.

And if a mosquito should happen to land on one and then fly away, they take 3 minutes or settle down afterward.


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Mr Onions
Two Time International Photographical Competition Winner
*****

Reged: 04/14/07

Loc: Newcastle upon Tyne.
Re: Strengths of a long focal length Achromat new [Re: covey]
      #5519393 - 11/14/12 11:24 AM

I'm very happy with my 60mm Skylight using the Carton lens.
Razor sharp and super high contrast.


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