Trouble with Fucus

phillymanjaro

I am having some issues trying to get a clear image of Jupiter with my ASI120MC. In the samples you can see that Prime Focus is just fine but as soon as I add a Barlow the image does not come through clear. I have tried with a 2" and a 3" Barlow. I am using a Celestron CGEM-800 with an added Crayford focuser. I am not sure what I am doing wrong but any help would be appreciated. Oh, one other thing I am using ShrapCap and Registax. Thanks, all the best, Phil

riklaunim

SCT 8" ? And what brand of Barlow are you using and how it looks like when you use it (can't focus, fuzzy image...)

phillymanjaro

Yep SCT8. The 2" Barlow is a Meade and the 3" Barlow is a Celestron X-Cel LX 3x. The image during focusing is generally fuzzy but it also goes in and out of focus with the atmosphere as you know.

Thanks, Phil

riklaunim

f/30 would a lot for small pixels of this camera. What you may see is the focus point blurred by seeing (and overscaled image). F/20 should good if the seeing is good. With SCT it must be able to focus :)

ndb

If you added a crayford, this lengthens the optics/sensor distance, and the focusing may not be achieved any more. Is it temporarily removable for another try with only a Barlow and the original SCT focuser?

And I agree about f/20 (C8+small pixels).

Your prime focus images are fine.

Nicolas.

Sam@ZWO

seeing is everything for such high magnititude imaging!

jamesf

NDB wrote:
If you added a crayford, this lengthens the optics/sensor distance, and the focusing may not be achieved any more.

This doesn't work quite the same with an SCT or Mak. As Piotr alludes to in an earlier post, SCTs focus by moving the focal plane to meet the eyepiece or camera whereas newts and refractors for example focus by moving the eyepiece or camera to the focal plane. This means that generally an SCT can reach focus whatever position your camera is in. My C9.25 for example will reach focus with the camera directly on the visual back, or with a secondary focuser, flip mirror, 2x barlow and filter wheel in front of the camera.

What does happen is that as you change the camera postition, the effective focal length of the system changes when the image is in focus. The only really reliable way for the average person to work out the actual focal length used for an image from a camera on an SCT or Mak is to count the number of pixels in the image of the target and from the current angular size of the target work out the number of pixels per arcsecond for the image, then work backwards to find the focal length that would give that image scale.

James

phillymanjaro

Sorry for my very late reply in getting back to everyone. Thanks so much for all the comments and incite. I have captured a few images since I posted the question so I will post them soon. Thanks again , Phil