Jump to content


Photo

Strengths of a long focal length Achromat

  • Please log in to reply
53 replies to this topic

#26 astrogeezer41

astrogeezer41

    Vostok 1

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

Posted 14 November 2012 - 05:21 PM

Yes, that image steadiness can be wonderful. I find the following about f-ratios with my scopes:
I can work with an f/7 when the seeing is fair-good (SV80F7 Aplanat)

I can work with f/9.4 if the seeing is fair or better (SV80/9D)

I can work with f/11 when the seeing is fair or better (SV102F11)

If the seeing is poor-fair only my old 75mm, f/16, Unitron will do.
In fact the depth of focus is so deep with the f/16 that I could make a U-turn with a semi in it (or so it seems).

I have owned several f/6 scopes over the years but they were so tweeky in focus that I never fell in love with them.

I would also add that my 6"f/8 Planetary Newtonian has served me pretty well but I wish that I had been able to make it an f/9.

Steady views,
Robert

#27 jrbarnett

jrbarnett

    Eyepiece Hooligan

  • *****
  • Posts: 20320
  • Joined: 28 Feb 2006
  • Loc: Petaluma, CA

Posted 14 November 2012 - 05:32 PM

21. Doubles as a tool for removing cobwebs from the rafters of the observatory.

22. It compensates...er...for something.

23. So long as the two elements of the objective are roughly in the same county, collimation is adequate.

24. Using one allows you to (credibly) pull off the cardigan sweater and briarwood pipe look in the field.

25. Visitors will address you as "sir".

26. Really kind visitors will instead address you as "Sir".

27. You can pull off "mine's bigger" and "some are longer than others" jokes at star parties.

28. They're great if you like to talk to your guests at a star party. You'll be describing endlessly what it is those little dots and dim patch represent. Dobs are for introverts; they let the image do the talking.

29. People ask you about your grand children. Even if you don't have any grand children. Or children. Or haven't even completed secondary school yet.

30. They created a market for MV filters.

31. They created a market for apochromats.

32. Most of all, they are long on nostalgia as much as focal length. While you can never go home, you can briefly relive the observations of yesteryear.

#28 rockethead26

rockethead26

    Gemini

  • *****
  • Posts: 3338
  • Joined: 21 Oct 2009
  • Loc: Arizona, USA

Posted 14 November 2012 - 05:48 PM

:funny:

#29 stevew

stevew

    Now I've done it

  • -----
  • Posts: 4337
  • Joined: 03 Mar 2006
  • Loc: British Columbia Canada

Posted 14 November 2012 - 09:58 PM

33. Most long F-ratio achromatic lenses are better figured than their shorter counterparts.
Reducing seidel aberrations to very low levels.


Steve

#30 Sky Muse

Sky Muse

    Mariner 2

  • -----
  • Posts: 239
  • Joined: 26 Oct 2012
  • Loc: De Soto County, MS

Posted 15 November 2012 - 01:31 AM

:rofl2: :roflmao: :rofl5:

#31 Ziggy943

Ziggy943

    Gemini

  • -----
  • Posts: 3291
  • Joined: 11 Aug 2006
  • Loc: Utah

Posted 15 November 2012 - 02:18 PM

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


That's just mean!

#32 ken hubal

ken hubal

    Apollo

  • *****
  • Posts: 1079
  • Joined: 01 May 2007

Posted 15 November 2012 - 03:45 PM

Stevew hit the nail square on the head!! :grin:

#33 Ziggy943

Ziggy943

    Gemini

  • -----
  • Posts: 3291
  • Joined: 11 Aug 2006
  • Loc: Utah

Posted 15 November 2012 - 04:25 PM

34. They double a clothes rack

Post # 3,000

Attached Files



#34 Andy Taylor

Andy Taylor

    Twisted, but in a Good Way

  • *****
  • Posts: 1873
  • Joined: 24 Sep 2008
  • Loc: Epsom - UK

Posted 15 November 2012 - 04:49 PM

34. They double a clothes rack


Blimey!!! You could pole dance with that one... :roflmao:

Now there's an idea. :question:

#35 dan_h

dan_h

    Surveyor 1

  • -----
  • Posts: 1972
  • Joined: 10 Dec 2007

Posted 15 November 2012 - 07:45 PM

Post # 3,000


Congrats!

#36 t.r.

t.r.

    Skylab

  • *****
  • Posts: 4452
  • Joined: 14 Feb 2008
  • Loc: 1123,6536,5321

Posted 16 November 2012 - 08:14 AM

35. They get you closer to the object your viewing. Always a bonus! :grin:

#37 csrlice12

csrlice12

    Voyager 1

  • *****
  • Posts: 10822
  • Joined: 22 May 2012
  • Loc: Denver, CO

Posted 16 November 2012 - 03:55 PM

35. They get you closer to the object your viewing. Always a bonus! :grin:


Nah; haven't you read: Objects APPEAR closer then they actually are. Of course, that was a reflective surface and not an objective lens...

#38 rwiederrich

rwiederrich

    Goldfinger

  • *****
  • Posts: 12622
  • Joined: 17 Nov 2005
  • Loc: Gorst Washington(center of the Universe)

Posted 16 November 2012 - 05:13 PM

Strengths of long focus refractors.......Hmmmm...let me see.

Attached Files



#39 chboss

chboss

    Viking 1

  • -----
  • Posts: 720
  • Joined: 24 Mar 2008
  • Loc: Zurich Switzerland

Posted 16 November 2012 - 07:45 PM

You have to own a weekend house? ;)

cheers
Chris

P.S. I love the look of this scope, I am sure the views are fantastic?

#40 jrbarnett

jrbarnett

    Eyepiece Hooligan

  • *****
  • Posts: 20320
  • Joined: 28 Feb 2006
  • Loc: Petaluma, CA

Posted 16 November 2012 - 08:33 PM

36. If you should ever be so fortunate as to have a beach house (I don't) you'll have the right kind of telescope to set up in the family room next to the ocean-view window.

37. They provide a reason to hang on to all of those old design eyepieces like Plossl, Orthoscopics, Erfles, Kellners, Ramsdens, Huygenians and such.

38. There is no better weapon for instilling "shock and awe" on lines of ants.

39. They give you instant credibility. Folks at star parties figure that there must be some reason your scope is so different. They're afraid that you know something they don't, so they reflexively treat you as an "authority".

40. They add weight to your case to the spouse that you need a new, larger vehicle.

41. They are highly "floater" resistant. The TFOV is typically so small that the probability of it and a floater occupying the same space at the same time is almost nil.

42. They resist dew without need for heated dew strips and batteries. All you need do is slew near zenith and allow the objective to poke through the upper atmosphere.

42. They are lovely scopes in action. For outreach I put the 105/1500 Antares on a CG5-GT head, atop a 60" tall pier. This places the tube saddle at almost 6.5-feet off the ground and the objective when aimed at Polaris just over eight feet up. It's absolutely majestic to watch the massive slow arcs the objective carves as the scope slews from horizon to zenith. It's like mechanical ballet. I found that by going from a 48" to a 60" pier, my outreach lines almost doubled. Talk about "advertising". :grin:

- Jim

#41 BigC

BigC

    Gemini

  • *****
  • Posts: 3213
  • Joined: 29 Sep 2010
  • Loc: SE Indiana

Posted 17 November 2012 - 01:09 PM

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.


One of the first things I noticed was how much "touchier" focusing a fast scope is than any of my cheap ,long tubes 60mm f15.

Got much nicer planetary views with a 80mm f11 than an 80mm f5.

#42 astrogeezer41

astrogeezer41

    Vostok 1

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

Posted 17 November 2012 - 02:20 PM

RE: #42

I guess style does count for something!

Enjoyed it,
Robert

#43 Bonco

Bonco

    Gemini

  • *****
  • Posts: 3438
  • Joined: 17 Apr 2006
  • Loc: Florida

Posted 17 November 2012 - 03:43 PM

Size DOES matter !!!
Bill

#44 rwiederrich

rwiederrich

    Goldfinger

  • *****
  • Posts: 12622
  • Joined: 17 Nov 2005
  • Loc: Gorst Washington(center of the Universe)

Posted 17 November 2012 - 06:12 PM

Size DOES matter !!!
Bill


Especially when you have a 5" refractor and it needs a scope to be a spotter on...... :smirk:

#45 astrosys16

astrosys16

    Lift Off

  • -----
  • Posts: 20
  • Joined: 27 Nov 2008

Posted 12 December 2012 - 11:04 PM

OK, new to the Refractor world. About to get my hands on an f15 apo. Curious about the shaking and wondering what is the inherent problem, tube stiffness, hysteresis in gears, stiffness of mount…what would you focus on to minimize this problem?

Lot to learn, reading the forum I saw an interesting post about a fringe filter. Seems something worth the money, right? Curious about what value a paracor would be. I have one on a f4 20 inch DOB that works wonders.

#46 astrogeezer41

astrogeezer41

    Vostok 1

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

Posted 12 December 2012 - 11:34 PM

Ed,

I doubt that you would need a fringe filter on an f/15 achro, certainly not if your achro is 4" or less.

Clear skies,
Robert

#47 Sky Muse

Sky Muse

    Mariner 2

  • -----
  • Posts: 239
  • Joined: 26 Oct 2012
  • Loc: De Soto County, MS

Posted 13 December 2012 - 12:17 AM

A 3" to 4" f/15 requires a beefier mount than an f/10, and certainly more than an f/5 or f/6, and to improve stability. Narrower spacing of tube rings would contribute to instability issues, too, as a dovetail and rings act as a brace for the OTA. Where the telescope joins the mount is another factor.

A 6" f/8 achromat could benefit from use of the filter, as would a 4" f/6, with false color less noticeable the longer the focal length, per objective diameter.

Regards,

Alan

#48 hottr6

hottr6

    Vanguard

  • *****
  • Posts: 2399
  • Joined: 28 Jun 2009
  • Loc: 7,500', Magdalena Mtns, NM

Posted 13 December 2012 - 10:46 AM

Double post..... :foreheadslap:

#49 Ziggy943

Ziggy943

    Gemini

  • -----
  • Posts: 3291
  • Joined: 11 Aug 2006
  • Loc: Utah

Posted 13 December 2012 - 02:31 PM

OK, new to the Refractor world. About to get my hands on an f15 apo. Curious about the shaking and wondering what is the inherent problem, tube stiffness, hysteresis in gears, stiffness of mount…what would you focus on to minimize this problem?

Lot to learn, reading the forum I saw an interesting post about a fringe filter. Seems something worth the money, right? Curious about what value a paracor would be. I have one on a f4 20 inch DOB that works wonders.


Is the F/15 apo from Switzerland? Or do you mean F/15 Achro?

#50 bluestar

bluestar

    Surveyor 1

  • -----
  • Posts: 1715
  • Joined: 06 Nov 2005
  • Loc: Maryland Eastern Shore

Posted 13 December 2012 - 08:08 PM

Every girls crazy 'bout a long refractor man (i.e. chicks dig 'em ;) )

And yes...seldom a need to upgrade and you're set for life.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics