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Equipment Discussions >> Refractors

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RGM
scholastic sledgehammer
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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
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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
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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
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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
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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
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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
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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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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
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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
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Re: Strengths of a long focal length Achromat new [Re: astrogeezer41]
      #5518706 - 11/13/12 09:10 PM Attachment (51 downloads)

#18. Never again needing to upgrade! Mike

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dlapoint
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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
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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
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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
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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
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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
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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
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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
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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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DarkStar1984
member


Reged: 07/10/12

Re: Strengths of a long focal length Achromat new [Re: covey]
      #5519400 - 11/14/12 11:31 AM

Do you live in New England

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Jon Isaacs
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Re: Strengths of a long focal length Achromat new [Re: Mr Onions]
      #5519491 - 11/14/12 12:45 PM

Quote:

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


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BarrySimon615
Pa Bear
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Loc: New Orleans, LA
Re: Strengths of a long focal length Achromat new [Re: Jon Isaacs]
      #5519519 - 11/14/12 01:10 PM Attachment (84 downloads)

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

Edited by BarrySimon615 (11/14/12 01:14 PM)


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covey
member


Reged: 07/02/11

Re: Strengths of a long focal length Achromat new [Re: BarrySimon615]
      #5519531 - 11/14/12 01:19 PM

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


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nightfisher
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Reged: 11/06/11

Loc: Huddersfield UK
Re: Strengths of a long focal length Achromat new [Re: Mr Onions]
      #5519641 - 11/14/12 02:53 PM

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

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astrogeezer41
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Re: Strengths of a long focal length Achromat new [Re: stevew]
      #5519833 - 11/14/12 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


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Strengths of a long focal length Achromat new [Re: astrogeezer41]
      #5519846 - 11/14/12 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.


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rockethead26
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Re: Strengths of a long focal length Achromat new [Re: jrbarnett]
      #5519862 - 11/14/12 05:48 PM



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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: rockethead26]
      #5520226 - 11/14/12 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


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Strengths of a long focal length Achromat new [Re: jrbarnett]
      #5520464 - 11/15/12 01:31 AM



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Ziggy943
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Re: Strengths of a long focal length Achromat new [Re: mikey cee]
      #5521316 - 11/15/12 02:18 PM

Quote:

#18. Never again needing to upgrade! Mike




That's just mean!


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ken hubal
Pooh-Bah
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Re: Strengths of a long focal length Achromat new [Re: Ziggy943]
      #5521458 - 11/15/12 03:45 PM

Stevew hit the nail square on the head!!

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Ziggy943
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Re: Strengths of a long focal length Achromat new [Re: ken hubal]
      #5521515 - 11/15/12 04:25 PM Attachment (39 downloads)

34. They double a clothes rack

Post # 3,000


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Andy Taylor
Twisted, but in a Good Way
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Loc: Epsom - UK
Re: Strengths of a long focal length Achromat new [Re: Ziggy943]
      #5521564 - 11/15/12 04:49 PM

Quote:

34. They double a clothes rack




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

Now there's an idea.


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Strengths of a long focal length Achromat new [Re: Ziggy943]
      #5521806 - 11/15/12 07:45 PM

Quote:


Post # 3,000




Congrats!


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t.r.
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Re: Strengths of a long focal length Achromat new [Re: dan_h]
      #5522407 - 11/16/12 08:14 AM

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

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csrlice12
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Re: Strengths of a long focal length Achromat new [Re: t.r.]
      #5523104 - 11/16/12 03:55 PM

Quote:

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




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


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rwiederrich
Goldfinger
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Reged: 11/17/05

Loc: Always Dark skies of Belfair W...
Re: Strengths of a long focal length Achromat new [Re: csrlice12]
      #5523217 - 11/16/12 05:13 PM Attachment (34 downloads)

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

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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Strengths of a long focal length Achromat new [Re: rwiederrich]
      #5523370 - 11/16/12 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?


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: Strengths of a long focal length Achromat new [Re: chboss]
      #5523437 - 11/16/12 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".

- Jim

Edited by jrbarnett (11/17/12 01:44 PM)


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BigC
Carpal Tunnel
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Reged: 09/29/10

Loc: SE Indiana
Re: Strengths of a long focal length Achromat new [Re: RGM]
      #5524344 - 11/17/12 01:09 PM

Quote:

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.


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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: jrbarnett]
      #5524455 - 11/17/12 02:20 PM

RE: #42

I guess style does count for something!

Enjoyed it,
Robert


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Bonco
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Reged: 04/17/06

Loc: Florida
Re: Strengths of a long focal length Achromat new [Re: astrogeezer41]
      #5524583 - 11/17/12 03:43 PM

Size DOES matter !!!
Bill


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rwiederrich
Goldfinger
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Reged: 11/17/05

Loc: Always Dark skies of Belfair W...
Re: Strengths of a long focal length Achromat new [Re: Bonco]
      #5524820 - 11/17/12 06:12 PM

Quote:

Size DOES matter !!!
Bill




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


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astrosys16
member


Reged: 11/27/08

Re: Strengths of a long focal length Achromat new [Re: covey]
      #5568431 - 12/12/12 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.


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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: astrosys16]
      #5568470 - 12/12/12 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


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Strengths of a long focal length Achromat new [Re: astrogeezer41]
      #5568513 - 12/13/12 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


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hottr6
Carpal Tunnel
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Reged: 06/28/09

Loc: 7,500', Magdalena Mtns, NM
Re: Strengths of a long focal length Achromat new [Re: jrbarnett]
      #5568988 - 12/13/12 10:46 AM

Double post.....

Edited by hottr6 (12/13/12 10:47 AM)


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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: Strengths of a long focal length Achromat new [Re: astrosys16]
      #5569382 - 12/13/12 02:31 PM

Quote:

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?


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bluestar
Carpal Tunnel


Reged: 11/06/05

Loc: Maryland Eastern Shore
Re: Strengths of a long focal length Achromat new [Re: Ziggy943]
      #5569858 - 12/13/12 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.


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Mr Onions
Two Time International Photographical Competition Winner
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Reged: 04/14/07

Loc: Newcastle upon Tyne.
Re: Strengths of a long focal length Achromat new [Re: bluestar]
      #5570380 - 12/14/12 07:00 AM Attachment (31 downloads)

I like the way it beats the seeing conditions.
Just as sharp as my LOMO.
Just as contrasty as my TV Oracle 3 { which is more contrasty than LOMO }
I bought it with my Xmas tips last year so,it was free!


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Ziggy943
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Reged: 08/11/06

Loc: Utah
Re: Strengths of a long focal length Achromat new [Re: Mr Onions]
      #5571067 - 12/14/12 04:03 PM

A beauty for sure.

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terraclarke
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Reged: 05/29/12

Loc: Just South of the Mason-Dixon ...
Re: Strengths of a long focal length Achromat new [Re: Ziggy943]
      #5571099 - 12/14/12 04:23 PM

That is a gorgeous scope! It is profoundly gorgeous. In anyone's mind who would see it, they would say to themselves, "now there is a telescope."
There is nothing like the look of a classic refractor.


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terraclarke
Post Laureate
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Reged: 05/29/12

Loc: Just South of the Mason-Dixon ...
Re: Strengths of a long focal length Achromat new [Re: terraclarke]
      #5571120 - 12/14/12 04:39 PM

One other strength that I failed to find listed (and if its there and I overlooked it, I most humbly apologize), is that in my opinion, long focus refractors are the best telescopes for solar observation. One seldom needs apertures greater than 4 inches given daytime seeing constraints, the ability to achieve focal ratios of between f15 and f35 are more easily met without central obstruction (a must for post objective narrow band etalons), chromatic aberration is not an issue with narrow band (sub-A) filters, and their is maximum contrast and a black sky as a background.

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