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I've been jealously reading post from owners of big dobs and something that often comes up is image scale. What does it mean? If my thoughts are correct, the same magnification across different scopes will be the same apparent size, and the field of view depends on the eyepiece and not the telescope, therefore faint objects should appear brighter with aperture not bigger (erm you will gather more light with a bigger aperture). So is image scale the ability to magnify more with a larger aperture without the view degrading? Is there something I'm missing? (purely visual of course- let's leave imaging for another day). I've often wondered but have always been reluctant to jump on the big boys' thread with a question. Thanks.

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31 minutes ago, domstar said:

I've been jealously reading post from owners of big dobs and something that often comes up is image scale. What does it mean? If my thoughts are correct, the same magnification across different scopes will be the same apparent size, and the field of view depends on the eyepiece and not the telescope, therefore faint objects should appear brighter with aperture not bigger (erm you will gather more light with a bigger aperture). So is image scale the ability to magnify more with a larger aperture without the view degrading? Is there something I'm missing? (purely visual of course- let's leave imaging for another day). I've often wondered but have always been reluctant to jump on the big boys' thread with a question. Thanks.

I think it relates to the fact that you can pump up magnification and still have bright image. With extended objects - the more magnification you use, darker it will be. This is important as both target and sky become darker and you are looking for a spot with greatest contrast. Having extra aperture will make higher magnification usable - which would otherwise (on a smaller scope) be too dim / too low contrast compared to sky.

That would be one explanation for image scale.

Other is related purely to imaging. Larger dobs have longer focal lengths, and enable with same camera to have "larger magnification" of targets - larger image scale.

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Dom - True FOV for a given EP depends on mag, so clearly the 'scope (its focal length) comes into this.

For extended objects (not pinpoints), the brightness/surface brightness is related to the exit pupil, but that alone is not enough.  A larger aperture allows the same exit pupil at higher  mag, so the overall or integrated brightness is greater.  This issue seems to split opinion considerably.  Some say the larger image stands out more and is easier to perceive.  

Anyway, my apologies for only addressing part of your query.  I'll let others comment on image scale (arcseconds per mm on film or ccd chip).

Doug.

 

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So the same focal length with a bigger aperture would give a bigger exit pupil because of the smaller focal ratio, therefore higher magnification would maintain a good-sized exit pupil allowing better high-mag views and thus a larger galaxy in the eyepiece. (What a sentence). That makes sense. 

I understand the idea of exit pupil but I'm not experienced enough to consider it in practice (I only use one scope anyway). That's interesting. Thanks you two. Floaters be gone (that's a different story, maybe)

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Yes - if the focal ratio drops, the exit pupil increases (it can go too high!).  Largish exit pupil is desirable for faint objects, but as I said, that alone is not enough, or it would be fine to view fuzzies with a very small aperture!  (And sky brightness/contrast all come in here too - an interesting topic.)

Intermediate values of exit pupil are a good compromise (maybe 2, 3, 4mm), and a larger aperture gives these values at higher mag.

Doug.

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I think.....what I'm actually hearing from you......if I can cut through all the jargon..........is what you're really saying.......is I should start saving for a 10inch dob (and a bigger car to put it in). I see your point and I tend to agree.:happy11:.

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6 minutes ago, domstar said:

I think.....what I'm actually hearing from you......if I can cut through all the jargon..........is what you're really saying.......is I should start saving for a 10inch dob (and a bigger car to put it in). I see your point and I tend to agree.:happy11:.

No question about it - and it will present you with many more small, faint objects too.  And better resolution.  You're not alone - I'm getting (eventually) a 12" Dob.  

As for image scale, I've spent the last hour or so looking into it.  Thanks for prompting me into a little research.  Interesting.  The AP members will no doubt have helpful remarks on that score!

Doug.

 

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Posted (edited)
2 hours ago, domstar said:

So the same focal length with a bigger aperture would give a bigger exit pupil because of the smaller focal ratio, therefore higher magnification would maintain a good-sized exit pupil allowing better high-mag views and thus a larger galaxy in the eyepiece.

Wow, my head hurts! :unsure: (anyone got a quiet little corner I can go take a lie down) :icon_biggrin:

2 hours ago, domstar said:

What a sentence

indeed! :help:

Edited by Redscouse
typo
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I can't say I've ever used 'image scale' when talking about visual observing. It's always been an astrophotographic term for me, though perhaps wrongly?

In the days of film you'd talk about 'plate scale,' so how many arcseconds of sky occupied how many mm on the photographic plate. In the digital age we talk about how many arcseconds of sky land on each pixel. (Arcseconds per pixel.) This is the definitive way to talk about the resolution at which you are imaging.

Olly

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