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I think I screwed up !

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#126 osbourne one-nil

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Posted 10 February 2008 - 04:20 PM

Post deleted by RandyR

#127 Cenk

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Posted 10 February 2008 - 05:05 PM

hahaha. I got a better idea, will post in due course. According to beginners forum this is normal for a SCT.

#128 Starlighter

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Posted 10 February 2008 - 05:30 PM

Here's a enlarged example of coma affecting a typical SCT:

http://starizona.com...optics/coma.jpg

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:

http://starizona.com...astigmatism.jpg

And finally, what you see through most cheap scopes:

http://www.trailerpa...Images/Star.jpg


:lol:

#129 Cenk

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Posted 10 February 2008 - 05:46 PM

hmmm. What I see is something like the attached.

Attached Thumbnails

  • 2181789-coma.jpg


#130 Starlighter

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Posted 10 February 2008 - 05:59 PM

Sure it doesn't look like this?: http://www.brazinlad...sun-squigle.JPG

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.

#131 b1gred

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Posted 10 February 2008 - 06:41 PM

I'm not SURE how they did it, but my 9.25 SCT has virtually no coma. I think it's done with the corrector plate.

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.

#132 Starlighter

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Posted 10 February 2008 - 06:53 PM

That doesn't explain why my 6SE has coma and that it goes almost away when using a field flattener.

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."

#133 b1gred

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Posted 10 February 2008 - 07:04 PM

I won't disagree with

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...

#134 Tel

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Posted 10 February 2008 - 07:06 PM

Does anyone by now get a strong feeling of "Deja-vu" ? :question:

Regards,
Tel

#135 b1gred

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Posted 10 February 2008 - 07:09 PM

Ya..

Ironic, eh?

#136 Starlighter

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Posted 10 February 2008 - 07:24 PM

My wife claims I wake up every morning from a coma.

#137 rick rian

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Posted 10 February 2008 - 07:42 PM

Does anyone by now get a strong feeling of "Deja-vu" ? :question:

Regards,
Tel


And that brings us back to the original post ... :smirk:

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.

'nuff said. :ubetcha:

#138 Starlighter

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Posted 10 February 2008 - 08:02 PM

Maybe the 8's and larger have less coma. What I do know is I've been told that it's to be expected. I was told this by three scope salesman at three different stores, all Celestron dealers. When I called Celestron and spoke with a service advisor explaining it, he told me there is some in my scope.

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.

#139 rick rian

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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 ... ;)

#140 b1gred

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Posted 10 February 2008 - 11:15 PM

A MAKCAS will have less coma, precisely for the reason Starlighter mentioned. There are other drawbacks, but coma isn't one of them.

If you want a totally coma free scope, you have to spend more $$. That's all there is to it...

#141 Starlighter

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Posted 10 February 2008 - 11:26 PM

Which is why I'm seriously thinking of buying that TeleVue NP 101 that has such a nice write up here on CN. Now, if I can just hide it from the ol' wife....

#142 b1gred

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Posted 10 February 2008 - 11:27 PM

Ok, let's get back on topic....

#143 Cenk

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Posted 11 February 2008 - 02:59 AM

:) Love the image Starlighter. Yes just like that one. Depending on how much I had to drink. I made some progress. When I turned off all lights around me and made sure there was no dew on ep's or scope and re-collimated the coma reduced to a tiny size around the edge of FOV.

#144 Starlighter

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Posted 11 February 2008 - 11:28 AM

My coma (not the physical kind :lol:) is small. When I look through the eyepiece I see the periphery of stars showing little tails.

Somewhere on CN there was a good, informative discussion on how to determine if what one is seeing is coma, spherical aberration or astigmatism.

#145 Cenk

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Posted 11 February 2008 - 01:54 PM

Where where where ? BTW, where in the world are you Starlighter ?

#146 Starlighter

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Posted 11 February 2008 - 02:24 PM

Southern California.

#147 Cenk

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Posted 11 February 2008 - 02:40 PM

what sort of scope do you have ?

#148 Starlighter

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Posted 11 February 2008 - 03:16 PM

Like you, I have a 6SE as well as several refractors.

#149 Cenk

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Posted 11 February 2008 - 03:31 PM

Do you find that the focuser is a little difficult to get right at high magnifications ? Do you get sharp views of Saturn/Jupiter/Mars at high magnification ? With an 8mm Televue Plossl my views of Mars/Saturn are fairly dull and looked unfocused. Even worse when I add a Barlow. What does anyone else get ?

#150 Starlighter

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Posted 11 February 2008 - 04:28 PM

I've never had a problem with focusing except for some shaking at very high magnification. There's no image shift and it's very smooth to operate. I've used a 5mm with a 2x barlow and although it takes a little doing what with the shake, I always get it right on. Perhaps pads would help, but I think it's the tripod legs which are not as robust as the two inch monsters on my refractor. With that scope, tapping on it even at high magnification only shows me about a second of shaking. It dissipates almost as quickly a I remove my hand. I got rid of shaking when focusing on that scope by adding an Orion Accufocus motor. With the SCT there are a couple of motorized focusers out there. JMI sells one as do Celestron dealers. Celestron's website sells one direct: http://www.celestron...D=44&ProdID=277

At $209.95+shipping and any applicable tax it's rather pricey.


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