30 replies to this topic

[Uwe Pilz]

I always started with glass only and switched then to NVD. Between different objects, I waited half an hour or so.

[sixela]

Good choice.

[jesse 3]

An IR cut filter (shortpass) does. An IR pass is the opposite and is a longpass filter, and is useful in light pollution where the IR airglow is less bad than the manmade light pollution in visible light.

You mentioned Astronomik L1 is a good filter due to 720nm cutoff. Can you please describe the type of objects that can benefit from this filter under dark sky? I use 642nm pass filter for galaxies. It would be great that IR cut offers more contrast.

[Mauro Da Lio]

Mauro,

You didn't answer my question. What do you see when you observe NGC 6749 with glass?

I have no logs. So probably I have never observed it. If it is behind dust it may well be that you see it in the IR nd not in the visible band. However, the fundamental problem of using NV without filters is the loss of contrast because the sky shines at 20 mpsas.

 

In the other thread there is a comparison on NGC 6217 between 25 cm + NV (no filter) and 60 cm. I see the same stars, but the glow of the unresolved stars is visible with 60 cm (sky 21.5) and invisible with 25 cm (sky 20). You can lower the sky brightness to 21.5 in NV by using an IR cut filter, but you also cut the IR light and see less stars.

[Mauro Da Lio]

And while we're on the subject of globulars, I can sure resolve more stars in NGC 5053 with the NVD, but do I like that view over the glass eyepiece one (in a good sky, and with lots of patience)? Not really, because it's missing the faint underglow almost completely.

Exactly. A globular is made of stars bright enough to be seen and dimmer stars that form a background unresolved glow. Being able to see more stars while the glow disappears looks unreal to me.

Also, for sake of completeness, stars looks different in the two: in the glass they are tiny points, colored, and the typical speckles due to turbulence make them scintillate, move and pop in and out of view. In NV they are not as pinpoint as in the glass, the are colorless, the look more static (more similar to a stellar map).

Edited by Mauro Da Lio, Yesterday, 02:18 PM.

[sixela]

You mentioned Astronomik L1 is a good filter due to 720nm cutoff. Can you please describe the type of objects that can benefit from this filter under dark sky?

Galaxies, mainly (at least that's what I've used it for).

I use 642nm pass filter for galaxies. It would be great that IR cut offers more contrast.

At a truly dark site a 642nm pass filter is usually worse than no filter. Without man made light pollution there isn't anything particularly bad about the visual range (save for some really narrow Oxygen skyglow bands), so why filter it?

But all of this depends on the airglow and even wihch airglow dominates. If Oxygen skyglow dominates, then it's quite possible for an IR-pass to be better; if OH skyglow dominates, an IR-cut will be better. At most dark sites I am at OH skyglow is more prevalent except in a narrow band at the horizon (but there is man mad light pollution there too in some directions, so I try an IDAS LPS-D3 and lower magnification as well close to the horizon).

It could also well be that experiences change over time too. Colleagues of mine are doing visual observing at La Palma at 2000m and at this solar maximum are seeing a lot more airglow than they're used to see, and it's also plainly obvious in all-sky photos they're taking. Perhaps "no filter" will again become competitive when the Sun quietens down a bit...haven't had the NVD and the L1 filter for long enough to know.

The ideal filter would be one like the Astronomik L1 and two extra really narrow notches blocking the two main O airglow lines. Sadly the LPS-D3 isn't that, it also tries to get rid of sodium light pollution and the worst of LED light pollution, so it is quite aggressive and tosses away too much just before its H-alpha passband (which also goes only to 700nm). The Orion Skyglow Imaging Filter has an extra passband where you'd want the LPS-D3 to have one (if there is no light pollution), but then its band around H-alpha is a lot more narrow, so you don't get much net gain.

Edited by sixela, Yesterday, 02:45 PM.