184 replies to this topic

[osbourne one-nil]

Post deleted by RandyR

[Cenk]

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

[Starlighter]

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

[Cenk]

hmmm. What I see is something like the attached.

Attached Thumbnails

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[Starlighter]

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.

[b1gred]

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.

[Starlighter]

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."

[b1gred]

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...

[Tel]

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

Regards,
Tel

[b1gred]

Ya..

Ironic, eh?

[Starlighter]

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

[rick rian]

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

Regards,
Tel

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.

'nuff said.

[Starlighter]

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.

[rick rian]

The Meade I peered into showed extremely sharp, pinpoint of light with virtually no astigmatism or coma to speak of.

Sounds like my 8i ...

[b1gred]

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...

[Starlighter]

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....

[b1gred]

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

[Cenk]

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.

[Starlighter]

My coma (not the physical kind ) 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.

[Cenk]

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

[Starlighter]

Southern California.

[Cenk]

what sort of scope do you have ?

[Starlighter]

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

[Cenk]

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 ?

[Starlighter]

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.