Limiting magnitude of CCD cameras

[lukebl]

Hi there. I use a SXVF-H9 mono CCD cam with my 250mm f/4.7 Newtonian, and am wondering if a newer model would be noticeably more sensitive.

I'm particularly keen on capturing faint asteroids, but find that even with long exposures I can only capture point objects down to around magnitude 17.5. I am in a moderately light-polluted area, but can usually easily see the Milky Way and M31 with the naked eye, so it's not too bad, and I can capture 15 minute exposures with little sign of fogging. Presumably, there's a point at which the background sky illumination drowns out fainter objects, but I don't think I'm there yet.

Any ideas on a suitable replacement, or should I stick with the H9? As I said, I'm more interested in astrometric measurements of asteroids, rather that attempting publication-quality deep sky images, so don't need a massive sensor.

[Tiki]

Would hoping to boost SNR with something like an Ha narrowband filter be any good?

[lukebl]

Would hoping to boost SNR with something like an Ha narrowband filter be any good?

No, I dont think that would work. Being illuminated by the solar spectrum, that would just reduce their magnitude significantly, making them even more difficult to capture. ...I think!

[davew]

If you have a look at the attached graph you'll soon see which chips will give you a boost. The 285 you're using isn't too bad but the Sony 694 looks very good. The KAF 3200 is a beast at certain wavelengths but is NABG ( If that bothers you ) and very expensive.

Sony or bust ?

Dave.

[skybadger]

That's a bit surprising to me, I'd expect mag19. In those conditions. Is your tracking avoiding smearing?

[lukebl]

That's a bit surprising to me, I'd expect mag19. In those conditions. Is your tracking avoiding smearing?

Tracking/guiding is good. I wonder if there's something faulty with the cam? If so, how can I tell. It seems ok.

[pete_l]

Short of going for a back illuminated CCD, like something from Apogee, you're only going to see small improvements.

What I'd suggest is that you consider the scope + CCD as a "system" and looks for ways of improving the overall efficiency and light gathering capabilities.

[Tiki]

I hope that this little bit is of some interest:http://starizona.com/acb/ccd/projectsfaint.aspx.

You could check the SNR in a similar fashion in your own images (perhaps using ImageJ freeware) in an attempt to see where things are not adding up. You should certainly see deeper.

[narrowbandpaul]

You need high QE and low read noise. The KAF3200ME or the sony 674/694 fit the bill. Smaller pixels would allow for higher positional accuracy if that's what you are in to.

[Macavity]

I think part of the problem is the plethora of different units / definitions re. camera chips. lol.

(I know what an integrating video camera of sensitivity "0.000x Lux at f/1.4" can do, but!)

The Good news? There's (usually) "headroom" above Sky Brightness - Even in the UK? 

[lukebl]

Cheers, guys. So it looks like I'd get better sensitivity, and maybe better resolution, with something with the sony 674/694 sensor, i.e. an Atik 428EX or a Starlight Xpress Trius SX-674. The former seems to be £200 cheaper than the SX model, so that's probably the one to go for?

[Uranium235]

Small pixels will give you the opportunity to bin (especially at your focal length) for added sensitivity, trouble is you need lots of pixels to make the resulting image scale worthwhile. The 428 might be a bit small for you (its FOV isnt much bigger than the 285 chip), so the 460 would be better. A quick check reveals that your 250pds will yield a resolution of 0.78" p/p unbinned with the 460, slighly oversampled?

Just a heads up though, once you get a bigger chip you might end up fiddling with things to get the corners right.... swings and roundabouts ey!

[lukebl]

Interesting stuff. Perhaps I should save my money and just clean the mirror! It looks pretty grubby, but I gather that it has to be really bad before it causes a noticeable detrimental effect.

[Uranium235]

Thats what I thought too, it would need some pretty ugly marks on it before it needs a clean. The most I ever do is to take a rocket blower to the mirrors to try and dislodge any bits of fluff that gather.

[lukebl]

In retrospect, I think I might have been maligning my poor old SXVF-H9.

It was clear earlier this evening, so I tried a capture at the zenith (theoretically, at least, the darkest part of the sky) and astrometric measurements show that I have managed to capture stars down to at least 19th magnitude after all. I guess it's down to the sky transparency which, in the rare clear nights over recent weeks, hasn't been very good round here.

I also unexpectedly captured 18th magnitude Asteroid 65669, which Astrometrica showed me was coincidentally in the same field of view of a trio of another asteroids I was imaging, 15.7 magnitude Zvezdara, 15.7 mag. Danubia, and 14th mag Iva. I haven't captured so many asteroids in a single narrow field of view before. Here's a rough stack of 7 x 5 minute exposures, stacked on the asteroid's position using Astrometrica so the stars and other asteroids are trailed somewhat. It's come out pretty clear, so I guess I could go a fair bit deeper.

I don't think my scope would cope with a larger sensor, given the coma issues I am having even with a coma corrector. Think I'll stick with the SXVF for the time being!

[davew]

" Think I'll stick with the SXVF for the time being! "

With images like that, so would I ! Lovely work.

Dave.