Megrez 80mm f/6 Refractor

Hi,
I have a Celestron Ultima 8 PEC. The mount is even more solid than the newer Ultima 2000. I am taking good piggy back astrophotos. Prime focus is tough, except for the brightest objects because of guiding with a radial guider.
Do you think that the Megrez 80mm f/6 Refractor could work mounted piggy back to my SCT scope? I haven't heard of anyone trying this. I think the shorter focal length would give me a much more forgiving guiding tolerance and better field of view. I love my mount and it's capabilities, but since it is a fork, I am limited to the SCT unless I can come up with a good piggy back scope. The equivelent nikon lenses are just to expensive.
Thanks,
Chris

Although the Megarez is probably a little heavier than my ST80 F5 it should work. You will definitely need to get a counter-weight system on the bottom side because this becomes a must have.



Robert W.

Attached Image:

I am a little unclear about whether you want to guide with the Megrez and photograph thru the 8" or guide with the 8" and photograph thru the Megrez.

Assuming that you will be using the Megrez in lieu of the radial guider, your photographic "power" is approx. 40 at prime focus, so an appropriate guiding power would have to be at least 6 times that or 240x. To get 240x out of a Megrez (focal length 480mm) you need a 2 mm guiding eyepiece. I don't think such an animal exists. Takahashi has a very nice 5mm guiding eyepiece, but even this high power guiding eyepiece will only give you 96x in the Megrez, so your guiding ratio (guiding power to photographic power) would only be little more than 2 to 1 with this setup instead of the minimum recommended 6 to 1. A standard 12mm guiding eyepiece would only give you a 1 to 1 ratio, totally unacceptable and a 9mm guider would be little better at 1.33 to 1.

Now if your plan is to photograph using the Megrez as the camera lens and the 8" Schmidt-Cass as the guide scope, a standard 12mm guiding eyepiece would give you a nice 17 to 1 ratio and a 9mm guiding eyepiece would give you an even better 22.5 to 1 ratio.

Other problems you may face is the potential image shift with the Schmidt-Cass and the achromatic color error you will see with the Megrez (not too bad according to some).

Due to the image shift potential, an off-axis or radial guider was always considered the best choice, because it takes the image shift issue out of the equation. Additionally your guiding ratio becomes 5.5 to 1 with a 9mm guiding eyepiece which is just getting to the acceptable range. You can guide off-axis with a 12mm but your allignment has to be "spot on" and you have to be very dilligent as your guiding ratio is reduced to about 4 to 1.

Barry Simon