Conlin Hill Observatory -- 42 7' N, 71 54' W

Techniques - Adventures with the ETX90


  2/24/04: I've been trying to use the 90 piggyback on the LX200 for guiding. The results were poor, with most images showing elongation and significant guiding problems. I thought that perhaps there was some movement or flexure when using the eyepiece hole on the scope and that attaching to the back might help improve things in that area.

I found a visual back for the ETX90 at Scopetronix that was just the ticket. They also had an F7 focal reducer built into a visual back. This is an interesting possibility: an ST8i on an ETX90 at F7 has a wide 50x75 arcmin FOV and a forgiving image scale near 3 arcsec/pixel. This is a nice size for many objects that are just too big for the ST8 on the LX200. I've been wanting a wide field scope for a while and am still thinking about purchasing one of the new Orion 80mm F7.5 apos.

The characteristics of the F7 ETX90 are even better than the apo, except for the central obstruction. The ETX90 has a nice Mak design with good focus and contrast, 90mm aperture vs 80, and it's faster at F7 vs F7.5. And to guide at F13, all I have to do is replace the focal reducer with the visual back, no swapping guide scope and wide field scopes! One more thing, the ETX90 has a built in flip mirror, so a star can be located manually with an eyepiece without ever having to move the camera or refocus. That's a very nice feature for a guide scope.

Overall it looked too good to pass up so I purchased a visual back and a focal reducer. I tried the reducer when it arrived and got terrible results. It was as if the optics were pinched. When focused, here's what the images were like:

Not good! I decided to try just the visual back and that worked well. I was able to achieve critical focus and ripped off a few 3 minute unguided shots of M65.

So, the visual back enabled the ST8i and color filter wheel to reach focus and take a series of good unguided shots, but the focal reducer didn't work at all. It's possible that the camera is flexing, I'm going to shim it with cloth or styrofoam or something to take any strain out of the system to see if that makes a difference. I'm also going to try some daytime shots to see what they look like. If it doesn't work, I'll ask Scopetronix if they've ever had someone successfully use this setup, perhaps this combo just doesn't work.

In addition, I still need to try guiding at F13 to see if the new visual back helps with the guiding results. Since the LX200 is producing good images unguided, there has to be some movement somewhere causing the poor guiding results. I suspect that the guide camera was moving around in the eyepiece port, I'm hoping the visual back helps me lock it down.


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