New Supernova in M51

[mdstuart]

Yep the F number is just the shape of the mirror. Its the size that matter! When it comes to light collection for visual use.

Mark

[cathalferris]

I've bagged it with an 8" but it wasn't easy, especially as my scope battery cable had failed so I was hand-guiding the lxd-75, and it does not have slo-mo controls without power.

Something to be aware of as well, would be that some scopes have not got as good control over the handling of scattered light as they should, and this scattered light can often rob the contrast that would otherwise allow something faint to be seen through the eyepiece.

I know that if I point my scope in the general vicinity of a streetlight, I can see this in the eyepiece as a brightening of hte field of view - and in my list of things to get over the summer is a sheet or two of the Protostar flocking paper or plastic sheets, to put these into my scopes and focuser tubes to minimise this kind of stuff.

[jamespels]

Regarding the focal ratio debate, is it possibly related to the size of the final image at the ep? I thought wider image gives increased brightness (seems to with bins, though I am not an expert and am probably missing several things!).

Final image diameter (whose correct name I forget!) is governed by the ratio of scope and ep focal length so, using the same ep, it is very possible the perceived image brightness varies with two scopes of similar aperture and different f ratio if the previous paragraph is correct.

[Moonshane]

hi James

I don't this so but may be wrong as I don't know if afocal image scale (is that what you meant?) has any implications visually. A galaxy gives off a fixed level of light (ignoring supernovae of course!) so 6 inches of aperture will gather 28" square of photons from that source. This will not change no matter what the focal ratio. I was assuming that the two scopes were using different eyepieces to get the same magnification so e.g. my f11 would need a 32mm eyepiece to give the same image size as the f5 using a 15mm both at 50x magnification.

[jamespels]

Thanks Shane.

OK, found the term I was grasping so hard for there - exit pupil. After a little hunting I found this explanation of what I was trying to get at: Nikon | Sport Optics | Exit pupil

In short, it isn't the actual amount of light collected so much as our ability to interpret it correctly. Where the exit pupil is smaller then the eye pupil, in low light conditions the image will appear less bright than where the exit pupil (with the same amount of light) is equal to or larger than the eye pupil. Based on this, I THINK it must be possible for two scopes of similar aperture to have give images that appear brighter or darker than each other simpy by varying the f ratio (assuming the same ep, etc.).

Waiting to be shot down here as I am venturing into new territory now - still, how else do you learn?

J.

[John]

... Where the exit pupil is smaller then the eye pupil, in low light conditions the image will appear less bright than where the exit pupil (with the same amount of light) is equal to or larger than the eye pupil...

I think it might be the other way around. If the exit pupil produced by the scope / eyepiece combination is larger than the fully dilated pupil then not all the light will get into the eye and the image will appear less bright.

This is why you see quite a lot of discussion on over-sized exit pupils when long focal length eyepieces are proposed for use with fast scopes.

[jamespels]

That would seem logical to me, too, but it isn't what Nikon seem to think!

Will have to ask my optician father in law.

[John]

It might be worth starting a fresh thread on light grasp / exit pupils in the Equipment discussion section. Buried down here at the end of a thread on a supernova it probably won't get the attention it deserves

[Moonshane]

ahh, I just look with my eyes and see what I see rather than worrying about what I should be able to see

[John]

ahh, I just look with my eyes and see what I see rather than worrying about what I should be able to see

Yep - me too Shane

[Moonshane]

or I should say in this particular case, not see what I cannot see!

[Moonshane]

quite excited as I now have a dark(ish) site on the edge of the Peak District - my wife's cousin's husband's uncle (Cleatus - not really - i'm joking) has a farm which has an excellent southern horizon and is raised above the surrounding land! hope this SN stays around until my large scope is sorted!

[swamp thing]

I hope it fades a bit for ya to make it more of a challenge.

Regards Steve

[nitram100]

I've seen this one a few times with my 10" F/4.8 newtonian now.

Good to know as I have the same scope as you, so I should be in luck! Might have to escape the city lights though..