Can of worms - Fracs and Dobs

[iPeace]

[Ricochet]

24 minutes ago, cloudsweeper said:

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.  

That is the only bit you need. Brightness is proportional to the exit pupil area. Focal ratio doesn't matter either. 

Aperture / exit pupil = magnification 

If you increase aperture then either exit pupil (brightness) or magnification must increase if you keep the other the same. 

[cloudsweeper]

7 minutes ago, Ricochet said:

That is the only bit you need. Brightness is proportional to the exit pupil area. Focal ratio doesn't matter either. 

Aperture / exit pupil = magnification 

If you increase aperture then either exit pupil (brightness) or magnification must increase if you keep the other the same. 

Yes, as I said earlier.  

But I must repeat, I believe this to be a complicated issue in which several factors come into play.

And in relation to a previous post, there is the matter of luminosity, or total energy output, which is constant as it depends only the source itself.

Doug.

[Stu]

4 hours ago, JOC said:

So I'm already understanding with Astronomy that a lot (most) of the apparent magnification is down to the EP's - do the frac users see their objects as apparently large (or significantly, smaller or larger) than the Dobs do - If someone had an F6 Frac (that may not even be possible b.t.w. I don't know enough) would they get the same view through the same EP as I would through my F6 Dob?  I imagine their overall view of the sky must be potentially smaller even with the least magnification - their OTA tubes are not as wide - I still struggle to find a given star that I know with my Dob - the Frac users must have a really tough time as surely they don't see as much of the sky in one go - or do they?

I don't think anyone has specifically answered this question so I'll give it a go.

Firstly magnification is magnification, an object at say x100 will appear the same size in either type of scope. The only thing that I can think might give the impression of different sizes is a using eyepieces with different apparent field of view, which is of course totally unrelated to the scope type!

The focal ratio actually says nothing about the size (i.e. Magnification) of an object in the eyepiece. That's because you can reach this ratio in a wide variety of scope sizes with varying focal lengths (and focal length is, after all, what determines the magnification, along with eyepiece focal length). For instance, an f6 scope could be a 70mm aperture, 420mm focal length refractor, or it could be a 300mm aperture, 1800mm reflector.

With a 10mm eyepiece, the frac would give x42, the newt would give x180. You can see a lot more sky in a short focal length refractor than in most newts which is one of the attractions of faster fracs, providing some compensation for what you lose in brightness given the loss of aperture.

So, what's the significance of the f6 focal ratio? Actually what it says, is that the same 10mm eyepiece will give you the same exit pupil in each scope and so extended objects will have the same apparent brightness. That might seem strange, but remember that the object appears far larger in the 300mm newt than it does in the 70mm frac. That is one of the main reasons for a larger scope, you can magnify objects far more whilst maintaining brightness. Image scale helps your eye detect contrast better so hunting small faint galaxies for instance is the domain of big dobs and dark skies.

Hope that is of some help.

[cloudsweeper]

4 minutes ago, Stu said:

 

So, what's the significance of the f6 focal ratio? Actually what it says, is that the same 10mm eyepiece will give you the same exit pupil in each scope and so extended objects will have the same apparent brightness. That might seem strange, but remember that the object appears far larger in the 300mm newt than it does in the 70mm frac. That is one of the main reasons for a larger scope, you can magnify objects far more whilst maintaining brightness. Image scale helps your eye detect contrast better so hunting small faint galaxies for instance is the domain of big dobs and dark skies.

Hope that is of some help.

I think we're together on this Stu - you get more mag at the same surface brightness (related to exit pupil), and this in turn means the integrated or overall brightness is greater.  Would you agree?  A one-LED torch and a ten-LED torch have the same surface brightness, but the latter is definitely brighter.

Doug.

[laudropb]

Help. I have run out of popcorn.

[John]

5 minutes ago, laudropb said:

Help. I have run out of popcorn.

Me too. On to the Maltesers now !

[JOC]

Hi Stu (& all), but def. Stu - wow!  I've read it three times and it seems to make sense although I feel as though I might turn all that into a diagram tomorrow.  So its almost counter intuitive: The F6 frac in your example with the 10mm EP shows more of the sky because the magnification is less - thats OK,  I've got  my head round the concept that magnification is really the amount of sky you see tgrough the EP.  So a frac is OK if you want to see lots of sky, but not for seeing really small or distant objects?  Yes/no?

[Ricochet]

1 hour ago, cloudsweeper said:

I think we're together on this Stu - you get more mag at the same surface brightness (related to exit pupil), and this in turn means the integrated or overall brightness is greater.  Would you agree? 

Surface brightness is brightness. It is by definition per unit area so magnifying the image ( by using a larger scope) doesn't make it brighter, it just makes it bigger at the same brightness. However, your brain is better at recognising larger objects which makes it easier to see. 

[John]

33 minutes ago, JOC said:

...  So a frac is OK if you want to see lots of sky, but not for seeing really small or distant objects?  Yes/no?

I don't think that is the case or that Stu was saying that. He gave some examples of scopes with certain specifications and then explained the impact that those specs would have on image scale, field of view size and brightness. I think those examples would apply regardless of the design of scopes, ie: with a refractor, newtonian or catadioptric etc. I may be incorrect but I don't think Stu was implying anything specific about the refractor design as a whole ?

Back to my Maltesers I think - I'm probably confusing things rather than making them clearer !

 

 

 

[niallk]

I've, erm, never looked through a frac!

Well, I've never looked through any scope other than my two dobs (10" & 15").  So I'm blissfully ignorant of what I'm missing out on

However, I'm intrigued by the resounding recommendations for say a 4" frac to compliment a dob.  I like the idea of a wide field, ultra portable grab and go, no cool down, wider tfov views, with good white light solar potential; even possibilities of Ha with a quark.  All good, sound. Contrast and pinpoint stars - great.

But: if I say I want to maintain an exit pupil >0.75mm for floaters, then max mag is ~135x.  Seems lowish - I find that I use 200x to 400x a lot.  For night time viewing, will I be satisfied with the views?? That's my conundrum... Are stars reported as more pinpoint because of lower magnification??  The CO in my 15" is ~17% by diameter.

I guess I need to get to a few star parties to see for myself!!

[mikeDnight]

9 hours ago, Alien 13 said:

You can always use a smallish newt without having the mirrors silvered for solar, might still need an additional filler though.

Alan

Unaluminized mirrors can be used for solar projection, but a full aperture filter would be needed if looking through the scope. Also, I think 4.5" is the usual max aperture as large mirrors gather far too much light. ?

[mikeDnight]

[jetstream]

The surface brightness for extended objects is fixed I believe (?) Making an object bigger- Clark says around 100 arc min- will make it easier to detect but if eye illumination (exit pupil) drops below a certain point the gain is lost.

http://www.clarkvision.com/articles/visastro/omva1/index.html

[jetstream]

53 minutes ago, mikeDnight said:

I just had some excellent lunar views @ 350x in my 15" dob and binoviewers I would have got my frac out but I didn't want to waste the good seeing

[cloudsweeper]

4 hours ago, Ricochet said:

Surface brightness is brightness. It is by definition per unit area so magnifying the image ( by using a larger scope) doesn't make it brighter, it just makes it bigger at the same brightness. However, your brain is better at recognising larger objects which makes it easier to see. 

Sounds good to me - so it's perceived  as being brighter.   My brain is now aching, so I think I'll join the popcorn gang!

Doug.

[estwing]

2 hours ago, cloudsweeper said:

Sounds good to me - so it's perceived  as being brighter.   My brain is now aching, so I think I'll join the popcorn gang!

Doug.

...and the penny has dropped ?

[Stu]

9 hours ago, cloudsweeper said:

I think we're together on this Stu - you get more mag at the same surface brightness (related to exit pupil), and this in turn means the integrated or overall brightness is greater.  Would you agree?  A one-LED torch and a ten-LED torch have the same surface brightness, but the latter is definitely brighter.

Doug.

I think we are basically saying the same thing Doug, yes. it is the surface brightness that is maintained at the higher mag.

[Stu]

1 hour ago, estwing said:

...and the penny has dropped ?

I think Doug's penny had dropped earlier  

[niallk]

5 hours ago, mikeDnight said:

 

Looks like the Newt must be giving absolutely mind blowing views of a resolved to the core globular cluster going by yer man's face. The other fella does seem to be quite happy viewing the moon though... 

 

[Stu]

8 hours ago, Ricochet said:

Surface brightness is brightness. It is by definition per unit area so magnifying the image ( by using a larger scope) doesn't make it brighter, it just makes it bigger at the same brightness. However, your brain is better at recognising larger objects which makes it easier to see. 

@Ricochet I agree with this, but if you have 1 unit area of brightness (surface brightness) in the small scope, and five units of area at the same surface brightness in the large scope, what is the measure of total 'output' if you like? I think that is what Doug is referring to.

There is presumably a difference if the object either fills or does not fill the field of view? If just fills the field in the smaller scope, but overfills it in the larger scope then I would expect the views to look identical (excluding any detail differences due to the mag)

[estwing]

21 minutes ago, niallk said:

Looks like the Newt must be giving absolutely mind blowing views of a resolved to the core globular cluster going by yer man's face. The other fella does seem to be quite happy viewing the moon though... 

now,now mate lets not get into a Frac bashing thread.....

[Stu]

7 hours ago, niallk said:

But: if I say I want to maintain an exit pupil >0.75mm for floaters, then max mag is ~135x.  Seems lowish - I find that I use 200x to 400x a lot.  For night time viewing, will I be satisfied with the views?? That's my conundrum... Are stars reported as more pinpoint because of lower magnification??  The CO in my 15" is ~17% by diameter.

I use exit pupils down to 0.5mm in my fracs regularly. Yes, floaters can be a problem but I have learnt how to look around them. Using a frac you will be using lower powers than the newt, but that's the thing, the frac would be complementary. It would give you wider fields of view, and no, the stars are not pinpoints because of low mag; have a look through a good frac, you'll see what I mean  

[estwing]

a 40w bulb is still a 40w bulb no matter how close you look at it folks...

8 hours ago, Ricochet said:

Surface brightness is brightness. It is by definition per unit area so magnifying the image ( by using a larger scope) doesn't make it brighter, it just makes it bigger at the same brightness. However, your brain is better at recognising larger objects which makes it easier to see. 

at last we have a winner...

[cloudsweeper]

7 minutes ago, Stu said:

@Ricochet I agree with this, but if you have 1 unit area of brightness (surface brightness) in the small scope, and five units of area at the same surface brightness in the large scope, what is the measure of total 'output' if you like? I think that is what Doug is referring to.

 

Yes Stu, this is exactly what I have been referring to.  The total/overall/integrated brightness might be expected to be greater, while the brightness per unit area is constant.  As I said before, the analogy is the LED torches - 1 LED is the same brightness/area as 5 LEDs, but the 5 LED torch has more output, and would lighten your shed up more in the dark.

Apart from this consideration, it is undoubtedly true that the larger image at the same SB would appear  brighter to the brain.

I find all this interesting and challenging, and I still have nagging doubts, but the friendly exchange of ideas and knowledge can help to clear the mists!

Doug.