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90mm refractor vs. 8"SC

Started by jjacobso, 08/19/2002 12:32PM
Posted 08/19/2002 12:32PM Opening Post
Hi All -

Spent several nights up -way- too late observing from a dark sky location with my Meade DS-90 (90mm achromat) and my newer LX200 (8"). On Friday, I actually was up late enough to catch Saturn. Seeing seemed great as I was able to view Saturn very clearly with the DS-90 at 250X (cassini VERY clear, shadows on rings, cloud belts clear, etc.) - it was razor-sharp. Since I have never before used the 8" to view planets, I trained the LX200 at Saturn. At the same magnification, Saturn did not appear nearly as sharp. I could barely make out the Cassini division, and the image lacked the sharpness provided in the refractor (saturn has always been this sharp in the 90mm.) I checked collimation, and it appeared to be fine at this magnification.

Shouldn't Saturn appear clearer in the 8"? Is the 8" more succeptable to seeing? The one variable is that I was using different EP's on the two scopes: on the DS-90 I was using a 4.5mm Vixen Lanthanum (great planetary EP!), and on the LX200 I was using a Meade (series 3000) 16mm EP and a 2x barlow. The 16mm and 2x combination normally provides a sharp view on the DS-90, but it also is half the magnification with that scope (1000mm FL.)

Is this just the difference in EP's, or is the refractor just a sharper image by design, even though it has less aperture?

-Jake
Posted 08/19/2002 03:53PM #1
Jake, I had an experience similar to yours a couple weeks ago. Myself and another person stayed out practically until dawn at a star party and one of the last things we checked out was Saturn. He had a nice 10" LX-200 which had been providing very sharp views all night long. I had my Vixen FL80S (an 80mm fluorite refractor). The conditions were admittedly pretty crumby seeing wise, saturn was even inconsistent in the refractor. The views were quite similar in each scope aside from the much brighter and more colorful image in the LX-200. The image was more consistent in the refractor of course, due to the variable seeing. I was using a Siebert 7mm and a 3 element 2x Barlow (very nice combo for about 180x) and he was using Meade plossls. At first the image in the LX-200 was really a mess since the power was almost 300x? but then he dropped the power down to about the same that I was using.
I think the following factors had an influence on the comparison:
1. Seeing, which was admittedly bad
2. Collimation of the LX-200 which hadn't been thoroughly checked
3. There was a rapid temperature drop which may have exceeded the LX-200's ability to cool down.

My question for Jake would be how are you checking the collimation on your LX-200 ? From what I have seen it is extremely critical on objects like Jupiter and Saturn.

Joplin