Celestron 8SE, Can't get a crisp focus

[Jrkoop4270]

Hey I just bought a new Celestron Nexstar 8SE and I'm having trouble achieving a good focus. The planets approach focus but then start getting bigger again when I've gone too far. The best focus I can get is a fuzzy blob even with the tiniest movements in the focusing knob. Collumation looks good but I'm new to SCT's so maybe I'm wrong. I'll attach an example of the best focus I can pull with Mars. Also this applies to both eyepiece observing and camera.IMG_1530.tif

[Cosmic Geoff]

Looking at planets visually is often a disappointment, as bad seeing often messes up the image, or the image looks 'too bright'.  Some observing experience is needed to get a good view.   A single shot camera image is almost guaranteed to be blurred, which is why serious planetary imagers shoot a video and then process it to extract a sharp image.

If you want to test your new 'scope, let it cool down outside for an hour or so, aim it at a not-too-bright star near the zenith, and using a high power eyepiece, look for concentric diffraction rings around a central dot. 

[johninderby]

The first couple of posts on thread on CN might be of use.

https://www.cloudynights.com/topic/526815-star-test-results-in-a-c8/

Edited September 26, 2020 by johninderby

[jonathan]

Inability to focus means either poor seeing conditions or collimation problems, I had this recently after I'd taken out the corrector plate of my 8SE to clean the inside surface.  It might be worth looking at the corrector plate to see if there's any visible dust or misting on the inside of the front glass, basically you should hardly be able to see the glass as you look down to the primary mirror.

The 8SE usually tops out for me at some point before my 10mm eyepiece due to seeing conditions (I am under Bortle 4 skies), on a very rare excellent seeing night I can use the 10mm to gain a sharp image on planets.  Recently I've had the chance to observe Mars in a 12" SCT and the best views of Mars in that were through a 12mm plossl eyepiece, I think that speaks volumes about SCTs in general and where you should be aiming your expectations and eyepiece budget (for planetary).  Admittedly I was a little disappointed that I couldn't push it to a 9mm, though the primary mirror and corrector plate were pretty filthy so that may have contributed somewhat.

To get around this there's really only one way to go - make sure you're out there every night that looks clear and good, and see what it's actually like through the scope.  The SCT is very sensitive to seeing conditions, but less bothered by the quality of the eyepieces used, plossl's are probably perfectly fine for most situations. 

One thing I tend to do is look to the horizon just as the sun is setting, if the atmosphere looks a bit hazy or thick then you can expect dew and mediocre seeing conditions, if it's perfectly clear all the way to the horizon then it should be a good night.

[dweller25]

The best way to maximize SCT performance is :-

1. Either point the scope down with both end caps off for at least an hour to allow it to cool or wrap it with refectix to improve thermal stability - this way you get good performance straight away.

2. Once thermally stable check the collimation at best focus at high power - ideally you will see a star with a perfect circle surrounding it.

3. Pray to the weather Gods for good seeing 😁👍

[Peter Drew]

You will be lucky to achieve a textbook star image with a large 8" SCT aperture, the passage of light three times through the main tube there seems always to be something that upsets the theoretical image wheras smaller refractors with one light passage do better.  I have access to 5",8",12" and16" SCT's and none give perfect star images in the seeing conditions in which they are used.  They do, however, give excellent lunar and planetary images on a good night.          🙂