Which Barlow should I use??

[Jammy]

I've taken a few planetary images using a x2 Barlow and C9.25 (2350mm focal length) and SPC900 webcam with Sharpcap.

I'd like to try getting larger images if possible. Which Barlow should I use with this scope for Saturn and Jupiter? x3 x4 x5???

Martin Mobberley's book on webcam imaging says to aim for 5000mm focal length which I nearly have with a x2 Barlow.

Will a longer focal length just degrade the images too much?

[Tinker1947]

You will need to play about with Barlows, get to much power and the image will degrade due to seeing ect.

[Cornelius Varley]

A quick calculation shows that you need the 2x barlow to achieve to optimum arcsec/pixel ratio for your telescope / camera, a 3x barlow would give the highest ratio without oversampling the image.

Peter

[JamesF]

I'd certainly try 3x, and perhaps 2x or 2.5x with an extension depending on the seeing. You do need to balance magnification against image quality, as you say, and what works for a given object some times may not work at others. If you had exceptional seeing then perhaps you could go to 4x focal length, but I'm really unconvinced you'd achieve much by doing so most of the time.

If you look at some of the images in Martin Mobberley's book I think the details give the focal ratio and camera used (the desired focal length will vary with camera pixel size). I'm sure there are some examples from Damian Peach taken using an SPC900 and C9.25.

If you're interested, I worked through some of the maths so it made sense to me here:

http://www.tanstaafl.co.uk/2012/03/focal-ratio-capture-length-planetary-imaging/

James

[Jammy]

Thanks James. The maths makes sense. I'll try a 3x barlow and see how I get on. I think Damian Peach uses f/40 for quite a number of images in Martin Mobberley's book, but I'll see how I get on at f/30.

Now all I need is someone to recommend a 3x barlow.

[JamesF]

I use a Tele Vue 3x barlow that I picked up second hand. Tal do one, as far as I know. If GSO/Revelation do one then that might be worth a look. I have no complaints about their 2x and 2.5x barlows.

James