Optimum f number for jupiter

[Andy Mitchell]

I've read that when taking photos of jupiter, you should use a camera with small pixels and use a barlow to get about f20 to f30.

After capturing some video on my hacked apart Xbox camera +2x Barlow + Skymax 90mm maksutov I couldn't help thinking isn't this way past diffraction limit...

So, would I be better off capturing without the Barlow and possibly dropping the resolution to get more and shorter exposures? That should increase the chance of getting lucky sharp images?

Also, how stable do other people find this camera with sharpcap. It was being very tempremental for me.

[Herzy]

Hacked apart xbox camera? Whats that?

[happy-kat]

I find sharpcap and my xbox quite stable using it on win 8.1 tablet. Nothing to lose having a go without the barlow to compare to the videos you have already taken.

[Physicist13]

rule of thumb is to try and match the optical resolution to the camera (pixel) resolution. Thts usually where the f20-30 comes from. faster u can cap the better..but S/N drops so trade off, plus pc throughput... 

[Tinker1947]

12 hours ago, Herzy said:

Hacked apart xbox camera? Whats that?

This is a hacked Xbox Camera, it quite simple to do, split the case apart remove the lens and plug it in so the led's light up, break the led's then glue on a 1.25" adapter put the case back together....job done.

[Marky1973]

12 hours ago, Herzy said:

Hacked apart xbox camera? Whats that?

Tinker beat me to it - but this link might help as well

 

 

 

[Andy Mitchell]

I've tracked down some of the stability problems to a dodgy USB connector/cable end and have chopped a few inches off the cable and fitted a new connector. Seems a lot better now.

For green light, the Rayleigh criterion gives a value of about 12.7um at f20, and 19um at f30. That's a lot of webcam pixels. Think about 4um for the xbox camera? Next time clear skies and life align I will try different settings. Would be nice to understand exactly how the recommended f numbers are derived.

 

 

[JamesF]

The intent is indeed to match the resolution of the camera to the resolution of the telescope, though it's not quite as simple as that.

A general rule of thumb is to configure the optical train to give a focal ratio of around five to six times the pixel size of the camera in microns (um).  So for example with the ASI120MC with a pixel size of 3.75um you might try to get a focal ratio of somewhere between f/18 and f/23.  The SPC900 has a pixel size of 5.6um, so you'd be looking at f/28 to f/34 there.  I'm not certain of the pixel size for the Xbox camera, but it may well be the same as the SPC900.

I'm not convinced there's any benefit in trying a higher focal ratio figure, but if you struggle to get to the lower figure it's hardly the end of the world and starting out it's often far easier to attempt a lower ratio to get the hang of things first.  You'll get a smaller image with less detail, but there's plenty of other stuff to get the hang of as it is and working at high focal ratios does add its own challenges that may be best avoided at first.

James