I think I screwed up !
Posted 10 February 2008 - 05:05 PM
Posted 10 February 2008 - 05:30 PM
This is exactly what I see around the periphery when using wider angle eyepieces. I'd say the Lumicon reduces it in half.
And here's an example of off-axis astigmatism:
And finally, what you see through most cheap scopes:
Posted 10 February 2008 - 05:46 PM
Posted 10 February 2008 - 05:59 PM
Seriously, that looks like coma to me. I assume you're not using your hand-drawn example to scale or I'd say if it's that big you'd better do as suggested. Drop it, claim it on insurance and get another one.
Posted 10 February 2008 - 06:41 PM
Thus, Cenk, I think it's time for you to start figuring out what the proper orientation of your corrector is.
I wasn't buying the concept that they're all the same and that they're randomly installed, I believe firmly that there is some hand-orienting going on. Since the orientation of Cenk's corrector has been changed, that's where the coma is coming from. Additionally, the scope shop I go to is incredibly meticulous about maintaining the orientation of the corrector when they remove one for cleaning. This is because they know the configuration is critical.
Cenk, it's time to devise a method, be it aiming at polaris, or using an artificial source, and start rotating your corrector until you return to the orientation that results in the least coma. You'll find it, but it's going to take some time, and experimentation.
Posted 10 February 2008 - 06:53 PM
Perhaps the 9.25 does a better job reducing spherical aberrations. One scope salesman I deal with told me the 9.25 is the "sweetest SCT on the market."
Posted 10 February 2008 - 07:04 PM
the 9.25 is the "sweetest SCT on the market."
The 6SE is a different "animal" but I've known people who have them and see no coma in theirs, so I wonder if yours might not have been through the "final adjustment" or may have had a slip in shipping, etc...
A field flattener, might be correcting for something that shouldn't have to be corrected in the first place...
Posted 10 February 2008 - 07:06 PM
Posted 10 February 2008 - 07:42 PM
Does anyone by now get a strong feeling of "Deja-vu" ?
And that brings us back to the original post ...
I have never seen coma in my 8i. Everything I have read says the position of the corrector plate is critical. Sounds like a trip to Celestron should seriously be considered.
Posted 10 February 2008 - 08:02 PM
Now, I looked through a Meade 125 ETX Mak and saw hardly any. Could it be that the curved front corrector plate used on a Mak reduces spherical aberration better than what's on an SCT? I've heard this claim and also that they tend to be somewhat sharper. The Meade I peered into showed extremely sharp, pinpoint of light with virtually no astigmatism or coma to speak of.
Posted 10 February 2008 - 08:34 PM
The Meade I peered into showed extremely sharp, pinpoint of light with virtually no astigmatism or coma to speak of.
Sounds like my 8i ...
Posted 10 February 2008 - 11:15 PM
If you want a totally coma free scope, you have to spend more $$. That's all there is to it...
Posted 10 February 2008 - 11:26 PM
Posted 11 February 2008 - 02:59 AM
Posted 11 February 2008 - 11:28 AM
Somewhere on CN there was a good, informative discussion on how to determine if what one is seeing is coma, spherical aberration or astigmatism.
Posted 11 February 2008 - 01:54 PM
Posted 11 February 2008 - 03:16 PM
Posted 11 February 2008 - 03:31 PM
Posted 11 February 2008 - 04:28 PM
At $209.95+shipping and any applicable tax it's rather pricey.