Jump to content


Photo

Strengths of a long focal length Achromat

  • Please log in to reply
53 replies to this topic

#1 RGM

RGM

    Viking 1

  • *****
  • Posts: 935
  • Joined: 15 Sep 2003
  • Loc: Burks Falls, Ontario, Canada

Posted 13 November 2012 - 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?

#2 csrlice12

csrlice12

    Voyager 1

  • *****
  • Posts: 10582
  • Joined: 22 May 2012
  • Loc: Denver, CO

Posted 13 November 2012 - 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....

#3 Bonco

Bonco

    Gemini

  • *****
  • Posts: 3431
  • Joined: 17 Apr 2006
  • Loc: Florida

Posted 13 November 2012 - 03:55 PM

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

#4 hottr6

hottr6

    Vanguard

  • *****
  • Posts: 2341
  • Joined: 28 Jun 2009
  • Loc: 7,500', Magdalena Mtns, NM

Posted 13 November 2012 - 04:00 PM

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

#5 hottr6

hottr6

    Vanguard

  • *****
  • Posts: 2341
  • Joined: 28 Jun 2009
  • Loc: 7,500', Magdalena Mtns, NM

Posted 13 November 2012 - 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

#6 Andy Taylor

Andy Taylor

    Twisted, but in a Good Way

  • *****
  • Posts: 1858
  • Joined: 24 Sep 2008
  • Loc: Epsom - UK

Posted 13 November 2012 - 04:54 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?


Crikey!! What other reasons do you need?

How about:

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

#7 Andy Taylor

Andy Taylor

    Twisted, but in a Good Way

  • *****
  • Posts: 1858
  • Joined: 24 Sep 2008
  • Loc: Epsom - UK

Posted 13 November 2012 - 04:59 PM

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


OK,

13) Not recommended for lazy people... :grin:

#8 SteveG

SteveG

    Skylab

  • *****
  • Posts: 4266
  • Joined: 27 Sep 2006
  • Loc: Seattle, WA

Posted 13 November 2012 - 07:22 PM

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

#9 astrogeezer41

astrogeezer41

    Vostok 1

  • *****
  • Posts: 102
  • Joined: 27 Aug 2010

Posted 13 November 2012 - 08:30 PM

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

#10 mikey cee

mikey cee

    Cosmos

  • *****
  • Posts: 7997
  • Joined: 18 Jan 2007
  • Loc: bellevue ne.

Posted 13 November 2012 - 09:10 PM

#18. Never again needing to upgrade! :p :lol: Mike

Attached Files



#11 dlapoint

dlapoint

    Viking 1

  • -----
  • Posts: 999
  • Joined: 17 Aug 2003
  • Loc: Moncton NB Canada

Posted 13 November 2012 - 10:25 PM

The short answer is the view.

#12 stevew

stevew

    Now I've done it

  • -----
  • Posts: 4295
  • Joined: 03 Mar 2006
  • Loc: British Columbia Canada

Posted 13 November 2012 - 10:30 PM

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. :lol:

#13 m9x18

m9x18

    Apollo

  • -----
  • Posts: 1141
  • Joined: 12 Oct 2007
  • Loc: Abilene, Texas USA

Posted 13 November 2012 - 11:06 PM

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

#14 jsiska

jsiska

    Viking 1

  • *****
  • Posts: 670
  • Joined: 12 Jul 2006
  • Loc: NW Ohio

Posted 13 November 2012 - 11:49 PM

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

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

#15 RGM

RGM

    Viking 1

  • *****
  • Posts: 935
  • Joined: 15 Sep 2003
  • Loc: Burks Falls, Ontario, Canada

Posted 14 November 2012 - 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.

#16 chboss

chboss

    Viking 1

  • -----
  • Posts: 712
  • Joined: 24 Mar 2008
  • Loc: Zurich Switzerland

Posted 14 November 2012 - 07:09 AM

:gotpopcorn:

Keep them coming. ;)

regards
Chris

#17 RGM

RGM

    Viking 1

  • *****
  • Posts: 935
  • Joined: 15 Sep 2003
  • Loc: Burks Falls, Ontario, Canada

Posted 14 November 2012 - 08:04 AM

20. Works well with binoviewers

#18 vahe

vahe

    Viking 1

  • -----
  • Posts: 828
  • Joined: 27 Aug 2005
  • Loc: Houston, Texas

Posted 14 November 2012 - 08:07 AM

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



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

Vahe

#19 covey

covey

    Lift Off

  • -----
  • Posts: 24
  • Joined: 02 Jul 2011

Posted 14 November 2012 - 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.
:slap:

#20 Mr Onions

Mr Onions

    Two Time International Photographical Competition Winner

  • *****
  • Posts: 6624
  • Joined: 14 Apr 2007
  • Loc: Newcastle upon Tyne.

Posted 14 November 2012 - 11:24 AM

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

#21 DarkStar1984

DarkStar1984

    Explorer 1

  • -----
  • Posts: 88
  • Joined: 10 Jul 2012

Posted 14 November 2012 - 11:31 AM

Do you live in New England :roflmao:

#22 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43570
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 14 November 2012 - 12:45 PM

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


Slow refractors are great in the smaller apertures, 60mm is about right, I really like my Asahi-Pentax 60mm F/13.3. But for planetary detail, contrast is a function of aperture so while it does about all that can be done with a 60mm scope, it can only show so much.

Jon

#23 BarrySimon615

BarrySimon615

    Pa Bear

  • *****
  • Posts: 2788
  • Joined: 01 Mar 2004
  • Loc: New Orleans, LA

Posted 14 November 2012 - 01:10 PM

Strengths - classic look, long focal length or rather slow f/ratio (don't confuse the two - a large reflector can be very fast, but have a long focal length if it has a very large aperture) can make focusing easier and make it easier to correct out aberrations. Appearance makes for an impressive look at outreach events.

Weaknesses - more difficult to mount appropriately, harder to display or store, harder to transport, more prone to tube flexure particularly with extremely long tubes and with thin wall tubes. More difficult to set up. Much more difficult to get a large exit pupil. Advantages pretty much minimized by the introduction of faster apo refractors.

Don't get me wrong, I like fast achromats and I have several that are f/15 ratio or slower in 60mm, 76mm and 105mm. But, go much bigger than that and they beg for an observatory. Length and weight do not allow for easy transport. Go from a 4" f/15 Unitron to a 5" f/15 Unitron and you increase weight from 100 lbs. to 275 lbs. Something to consider when your age is going up and your physical abilities are going down.

Check out this photo of me with a 5" Unitron. The tripod legs had been shortened (not by me) to get the mount into a garage.

Barry Simon

Attached Files



#24 covey

covey

    Lift Off

  • -----
  • Posts: 24
  • Joined: 02 Jul 2011

Posted 14 November 2012 - 01:19 PM

No doubt they are great scopes.....
But I gave up masochism for a very good C8. :tonofbricks:

#25 nightfisher

nightfisher

    Mariner 2

  • -----
  • Posts: 201
  • Joined: 06 Nov 2011
  • Loc: Huddersfield UK

Posted 14 November 2012 - 02:53 PM

I guess my Antares 105 f15.......stopped down to 80mm f19 sort of counts :cool:






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics