• Cameras

  • Which Barlow should I buy for planetary imaging?

I have a Celestron Nexstar 8SE 8" SCT. I am getting a ZWO ASI120MC to use for planetary imaging. I'd like to image Jupiter, Saturn, and Mars.


Which Barlow should I get for this? I've read that it's a trade-off between pixel density and seeing conditions. More zoom allows for more detail, but it's more susceptible to atmospheric smearing. What do you guys recommend? Needless to say a TeleVue is out of my price range.

Every pixel size has optimal f-ratio at which it catches image at telescope max resolving power - and that resolving power doesn't change. For smaller than usual pixels in 120MC/MM you can try using f/15 or bit more. So a Barlow lens on camera nosepiece could do that for f/10 SCT (Baader Q-Barlow, GSO 2x Barlow - has filter threaded lens cell)

riklaunim wrote:
Every pixel size has optimal f-ratio at which it catches image at telescope max resolving power - and that resolving power doesn't change. For smaller than usual pixels in 120MC/MM you can try using f/15 or bit more. So a Barlow lens on camera nosepiece could do that for f/10 SCT (Baader Q-Barlow, GSO 2x Barlow - has filter threaded lens cell)

So how can I determine what the resolving power is given the scope, cam, and barlow? I'm trying to decide between an ASI120MC and an ASI034MC. Given what you said, theoretically wouldn't the 034MC and 3x barlow be roughly equivalent image quality to the 120MC and 2x barlow?

pixel size / 0.275 will give you theoretical optimal f/ratio (where pixel size is either its size or diagonal of a pixel in micrometers).

riklaunim wrote:
pixel size / 0.275 will give you theoretical optimal f/ratio (where pixel size is either its size or diagonal of a pixel in micrometers).

So, the ASI034MC has a pixel size of 5.6um. So 5.6/0.275 = 20. So a 2x barlow would make my f/10 scope f/20, right?


And the ASI120MC has 3.75µm pixels, so that would be f/14, which is like a 1.5x barlow. Does that make sense?


And given the right barlow and the right focal resolution, would both cameras essentially give equal quality output?

ASI034MC is not as sensetive as ASI120MC

you will get nearly the same brightness with the same F ratio


the good seeing here usually allow me to capture at F20 for ASI120MC and F30 for ASI034MC

mostlyemptyspace wrote:
riklaunim wrote:pixel size / 0.275 will give you theoretical optimal f/ratio (where pixel size is either its size or diagonal of a pixel in micrometers).

So, the ASI034MC has a pixel size of 5.6um. So 5.6/0.275 = 20. So a 2x barlow would make my f/10 scope f/20, right?


And the ASI120MC has 3.75µm pixels, so that would be f/14, which is like a 1.5x barlow. Does that make sense?

Yes, thats what the equation says. The result value is "theoretical". At very good conditions you can go beyond it, at average it's good to stick to it.

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