F/ratio for ASI120MC.

[Starlord19]

I currently use this camera with my Mak127 and a 2x Barlow getting around f/24. I was wondering if I changed scope to a C6 sct, f20 with the same Barlow, would it make a noticeable difference to my imaging to justify the expense? 

[knobby]

The extra light gathering should help bump up your frame rates hence more to stack and be picky with the good frames .

[riklaunim]

Aperture changes only resolution. Image brightness comes only from f/ratio. If both would be f24 the SNR or "brightness" would be equal, but the bigger telescope would give bigger and more detailed planet image.

[JamesF]

I'd probably not want to go as high as f/24 with the 120MC, but it's hardly extreme.

However, do you know that you're actually getting f/24?  Maks and SCTs change focal length depending on the position of the focal plane (unlike a newt or frac where the position of the focal plane is fixed and you move the eyepiece/camera to meet it, with a Mak or SCT you keep the eyepiece or camera fixed and move the focal plane to it).  If you have the barlow in the visual back of the OTA and the camera in that then it may well be the case that the focal length is shorter than expected and your focal ratio is therefore somewhat lower than you think.  FireCapture estimates the focal length for you if it knows the target and saves it in the text file with the image data, but if you can look up the apparent diameter of your target and measure the size of the image in pixels (and then convert to mm of image sensor) you can work it out for yourself.  It's relatively simple maths.

With the ASI120 I'd personally probably be looking to achieve a focal ratio in the range of somewhere around f/20.  A little either way is probably neither here nor there.  From memory I think Piotr (riklaunim, above) prefers a little less and some prefer a little more, but if you're in the right ballpark that's good enough to start with.  To achieve that with the C6 you'd probably need an extension (I'd guess somewhere in the region of 50mm, but the only way to be sure is to experiment) between the barlow and the OTA, but you may find that the results with the camera directly in the back of the OTA are quite acceptable.

I have to admit that if I were thinking of changing from the 127 Mak I'd be saying to myself "The C6 is only a little big bigger.  If I'm going to the expense of changing, can I justify the C8?"   Of course the down side is that you may also need a bigger mount.  Aperture fever is a terrible thing

James

[Starlord19]

Thank you very much for the comprehensive explanation. I have been using the camera in the diagonal, in place of the ep, I'd never even considered using it directly into the ota. 

You are absolutely right, aperture fever is a terrible thing that really exists! I'd love a C8 but then I'd need a beefier mount aswell. People would then be saying "why not get a 9.25?". I am not exactly loaded and have quite a small house and storage is an issue, so I've got to draw a line somewhere. Maybe if I worked harder I could get a bigger house and scope but then I wouldn't have as much time to use it, it's a catch 22!