Re: (SNES) Object Brightness

[Albireo54@aol.com]

Hi Ernie,

You wrote <<Would an extended object (e.g., nebula, galaxy) appear to have 
the same brightness to the eye when viewed through an f/8 scope with a 16mm
eyepiece as compared to an f/4 scope of the same aperture using an 8 mm
eyepiece?>>
 
As Bob N. pointed out, the objects will have the same apparent diameter in 
the two different scopes, so their appearance should be similar - all other 
things being equal. 

A factor that may change your visual impression, however, is the eyepieces' 
true field of view - which is often greater in longer focal length eyepieces. 
In other words, your magnification can be the same, but because one eyepiece 
may show you more sky area the visual impression can be quite different. More 
surrounding "stuff" will be in view - but this also includes more background 
sky. So if your location doesn't rank well on the Bortle Dark Sky scale on a 
particular night, you will see more of what you'd rather not see - sky 
brightness (which you alluded to in your question).  And this can have a 
negative effect on the appearance of a particular faint fuzzy, as contrast is 
reduced. (As you know, when it comes to photography, short vs. long scope 
focal lengths becomes a different and important issue.)

Of course, as anyone who has used a Nagler or similar eyepiece will tell you, 
when conditions are good (nice black background) the wider true field is a 
real bonus. M27, as an example, when viewed with lots of surrounding sky 
blackness, can look much more impressive than it does when it completely 
fills your field of view.

Regards,
Doug Stewart
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