15 replies to this topic

[mmagrunmo]

Hi gang
A friend of mine decided to tweak his collimation last night on his nexstar 8 gps he bought used a while back. It was the first time he had touched the collimation. stars had shown a bit of a flare to one side in focus.

He tweaked at low power and then went to to the mid 400s i think. It definatley got better and he was able to make the rings on one side of focus look pretty good. The moons of jupiter became more point like than they had ever been in this scope.

However. he couldnt really get rings on one side of focus at high power, it was more like a fuzzy blob looking thing. the other side looked ok and it seemed the scope was in better shape than before he did anything.

Anyone have anny suggestions about being able to get rings on one side of focus and only a fuzzy looking roundish thing on the other side of focus??? I know this isnt much information but i thought it might ring some bells with someone. thanks

[Starman1]

This is a symptom of spherical aberration, either overcorrection, or undercorrection. A Ronchi grating in the focuser would show evidence of non-parallel lines.
But, inadequate cooling of the optics can have this same effect.
Without active cooling (such as the Lymax SCT Cooler), some modern SCTs, if put in conditions where the temperature is steadily falling, may not show completely equilibrated optics over an entire night (especially the latest versions with carbon tubes).
You did not say how soon after setting the scope up you performed the collimation.
Here's a site with pictures of what I mean. Look down the page to see examples of the aberration you describe: Aberrations in the star test
Of course, no man is an island, and neither is any scope aberration. You likely saw a hodgepodge of seeing problems, spherical aberration, thermal issues, and possible a slightly turned edge. Unless the seeing is exceptional, it may not be possible to identify which, exactly.
As a doctor's oath says, "First, do no harm". Collimating the scope (it sounds like a major improvement was made) did no harm. Only after addressing the thermal issues can he begin to pin down the source of his aberrations (which may be quite minor if the moons of Jupiter were pinpoints).
Let's see what happens the next perfectly still night. Please report back.

[nreid]

I have never seen a perfect night with my sct. They all look like that first one.

[garyc11]

ya... im still waiting to see an airy disc

[c131frdave]

Saw the airy disk last night for the first time!

I had a similiar problem and scoured the world for an answer. Ended up that my corrector had a finger sized smudge on it.

Good luck!

[pstarr]

Read this article. So many things affect the star test. The scope must be totally cooled down before the optics will show the true result of the amount of correction. thats if the seeing is steady enough to let you interpret the test correctly. It takes patients and many tries to finally pass judgement. SCT's by definition don't star test well. I have gone through this faze with my scope and after much testing and some mechanical ajusting, it performs extreamly well. Bottom line is you can't judge it by one look or one nights testing. I kind of got off your collimation question but the ring differnce you are seeing could be caused by many factors, including seeing, cool down problems and optical errors that might not effect the final image.
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Paul
C-9.25 CF XLT on EQ-6 star test results

[Derwin Skotch]

Temperature is critical to SCT performance. I usually leave my scope (a Celestron 9.25) outside at least an hour before observing if it's cold outside. I have also found that once the scope has been outside for a long time that the right amount of heat from my regulated dew heater improves the star test. Before I had it, my scope almost always looked to be undercorrected. With good conditions and a little heat I can't tell the difference between inside and outside of focus. Anybody else noticed this?

[loo27]

For those of you who have never seen the airy disk, your scopes are either not cooled or not collimated. If these don't prove the be the problem,and sky conditions are good, you got a lemon.

[sixela]

For those of you who have never seen the airy disk, your scopes are either not cooled or not collimated.

A bold statement to make, if you also consider larger scopes. I'd *love* to live in conditions where average seeing conditions always supported 800x .

I can see Airy discs, mind you - but I have to stop down my scope.

[loo27]

For those of you who have never seen the airy disk, your scopes are either not cooled or not collimated.

A bold statement to make, if you also consider larger scopes. I'd *love* to live in conditions where average seeing conditions always supported 800x .

I can see Airy discs, mind you - but I have to stop down my scope.

I've seen Airy disc at lower powers than 800x, try around 300x. It's more of a function of atmosphere conditions than magnification.

[jrcrilly]

I've seen Airy disc at lower powers than 800x, try around 300x. It's more of a function of atmosphere conditions than magnification.

The magnification required to see the Airy Disk varies directly with aperture. 300X won't do it in larger telescopes.

[sixela]

I've seen Airy disc at lower powers than 800x, try around 300x. It's more of a function of atmosphere conditions than magnification.

Well, you have an 8", and I have a 16"...as I said, if I stop down my scope, I do see Airy discs much more easily.

[garyc11]

will this work for any size scope?

[jmcdonald]

Isn't the rule of thumb 40x-50x per inch of aperture?

[Samir Kharusi]

I always check my collimation simply by sticking in a Nagler that yields 300x magnification. A 7mm when I had a C8 and now a 13mm in a C14. Slowly send a brightish star around that wide field. It becomes quite obvious where the star looks centrally symmetric and where it goes distended (also which direction). All you need is that centrally symmetric region to be in the middle of your eyepiece. Collimation done. I think even if it was for this purpose alone an ultra-wide high power eyepiece is a good investment. The fact that it makes chasing a planet a breeze is gravy. Of course, on particularly bad nights the star may look just awful everywhere. Heck, some nights you cannot even split the Double-Double. All IMHO of course.

[Psa19one]

I had a real time trying to get my 12" LX200 collimated after it's journey to my home in Wisconsin from the fella I bought it from in AZ. The "donut" from my star test was more like an oval, and the "donut hole" from the secondary shadow was also not round. Turned out it was due to nights of not-so-good seeing, not some permanent physical aberration on the primary. On a night when the seeing was 4 out of 5 (see the Clear Sky Clock for your area at http://cleardarksky.com/csk/ ), I was able to crank up the power to about 600x on Polaris and, after careful adjustment, get a nice diffraction pattern...and no more ovals! Now the optics are performing VERY nicely. A good seeing night is really important for good collimation.