Can of worms - Fracs and Dobs

[cloudsweeper]

The smaller the focal length, the more you see.  

Doug.

[cloudsweeper]

.....and the bigger the aperture, the brighter the image.

[SilverAstro]

- - cant go bigger than 40" with fracs, at least they couldnt when they built Yerkes

anyone for a diy segmented frac ?

[Moonshane]

15 minutes ago, cloudsweeper said:

.....and the bigger the aperture, the brighter the image.

At the same magnification

[Alien 13]

There is another benefit with an aperture mask on a dob and that is the change of F ratio can make eyepieces perform better especially the cheaper ones.

Alan

[glowjet]

2 hours ago, JOC said:

OK, so that makes sense, so then why do frac users deploy those long thin telescopes when they could be using a Dob with a huge mirror and lots of possibility to resolve detail.  Where does the Frac design score over the Dob. design? (Or does that open too many can's of worms?)

IMHO, at the end of the day it is not "where does the Frac design score over the Dob", or where does the Dob score over the Frac. The two optical instruments function to gather photons from distant stellar objects and bring them to focus. Two different designs with the same purpose. There are devotees of both designs and it then becomes a question of horses for courses. The amateur who wants to observe just some of the many variable subjects in our night sky, will choose the instrument governed by resolution and clarity for his or hers chosen subject.

For example, the light gathering power of the large mirrors, that can scoop up those fainter photons, will be the scope of choice for the dedicated DSO observing amateur. On the other hand, the smaller Fracs, triplet or otherwise, including the smaller Newtonians, having a reduced objective size / weight, much the choice of our imaging and observing community. Have the ability to provide excellent resolution and clarity, but can also be installed on an  EQ driven mount

[JOC]

2 hours ago, Alien 13 said:

There is another benefit with an aperture mask on a dob and that is the change of F ratio can make eyepieces perform better especially the cheaper ones.

Alan

So is it worth me experimenting with the aperture mask?  What will I notice?  Until tonight I didn't even realise what it was.  I just assumed it was a dust cover with a perplexing hole in it.  

[Alien 13]

Just now, JOC said:

So is it worth me experimenting with the aperture mask?  What will I notice.  Until tonight I didn't even realise what it was.  I just assumed it was a dust cover with a perplexing hole in it.  

Its worth experimenting and costs nothing, the moon is a good target to try.

Alan

[Moonshane]

it makes a big differences in doubles

[JOC]

11 minutes ago, Alien 13 said:

the moon is a good target to try.

You mean there's actually a moon up there!!?  LOL

[JOC]

1 minute ago, Moonshane said:

it makes a big differences in doubles

Ooo.....I have a favourite double that I can do - I'll try it on Castor!

[Moonshane]

If you make a card mask, ensure the opening is circular and try about 75 or 80mm in a position between the vanes and avoiding the secondary. That gives an 80mm f15 dobfractor.

[SilverAstro]

6 minutes ago, Moonshane said:

 ensure the opening is circular

Well actually,,, it could be square, but then we are back to diffraction spikes   well  some people like them I am told

,,,, coat >>>

[iPeace]

Dobs are beautiful. And I am sure one would show me many more fuzzy smudges. But I would have to cool it. And collimate it. And live in it.

I am happy for those who can cope and are rewarded. Really, you are the real deal. The hard core. I am not worthy.

But me and Callahan, we have to know our limitations. So a smallish frac it is. But mostly binoculars to peek through the clouds. And take on holiday. Everywhere. Anytime.

Carry on.

[cloudsweeper]

7 minutes ago, iPeace said:

Dobs are beautiful. And I am sure one would show me many more fuzzy smudges. But I would have to cool it. And collimate it. And live in it.

I am happy for those who can cope and are rewarded. Really, you are the real deal. The hard core. I am not worthy.

But me and Callahan, we have to know our limitations. So a smallish frac it is. But mostly binoculars to peek through the clouds. And take on holiday. Everywhere. Anytime.

Carry on.

.........and store household waste in it.

Doug.

[iPeace]

2 minutes ago, cloudsweeper said:

.........and store household waste in it.

Doug.

Full marks, but keep it clean.

[SilverAstro]

If you had two holes in the mask with two secondaries and two eyepieces would you have a binoDob ? I think there must be a fallacy in there somewhere -

- I'll think of it in a minute or three, maybe, 

[SilverAstro]

17 minutes ago, iPeace said:

 And live in it.

But mostly binoculars to peek through the clouds.

     

Ohhh ! can they do that ? I thort 'twas only FLO's Zarkhov Gun that could do it.

 

[iPeace]

1 minute ago, SilverAstro said:

     

Ohhh ! can they do that ? I thort 'twas only FLO's Zarkhov Gun that could do it.

 

Indeed. I should have written "between the clouds". Glad someone is paying attention. Good man, I owe you a dram.

[SilverAstro]

1 minute ago, iPeace said:

 I owe you a dram.

  a virtual one no doubt ?

 

[iPeace]

Just now, SilverAstro said:

  a virtual one no doubt ?

 

No, I will treat you to the beverage of your choice, and join you, next time we meet.

[estwing]

3 hours ago, cloudsweeper said:

.....and the bigger the aperture, the brighter the image.

nope...same brightness...bigger image scale.

[cloudsweeper]

5 minutes ago, estwing said:

nope...same brightness...bigger image scale.

It's a complicated matter, I believe.  For a larger aperture and the same focal ratio, you get the same exit pupil at higher magnification.  That means the same surface brightness at higher mag.  Surface brightness is brightness per unit area, so a more magnified image will have a greater integrated or overall brightness.  

Doug.

[estwing]

its a candle 10ft away.....bring the candle closer....its still the same brightness but now the candle is 2ft away

[cloudsweeper]

The light flux from a source is inversely proportional to the square of the distance from the source, so if you're closer, you pick up more light flux.  Does that not constitute greater brightness?

Doug.