Focus - Nexstar 6SE

[Ryohei56]

Hi. I have a Celestron Nexstar 6SE, not brand new but pretty close. It's never been wonderful at focusing, but I lack the skill (and confidence) to put that right.

Essentially, the 'sweet spot' is almost non-existent - you go from slightly blurry back to slightly blurry without much sign of sharpness in between.

I'm guessing this might be a collimation issue, but I could be wrong. I'd be grateful for any suggestions you guys can come up with.

[johninderby]

First thing is have you colimated the scope? Does need collimating when you first get it as shipping will knock it out of colimation but being a small SCT holds collimation pretty well.

Edited April 14, 2020 by johninderby

[Ryohei56]

Hi. As far as I know it hasn't been collimated (certainly not by me). That could well be the issue. I'll need to do some research in that case, as I'm unsure how to proceed.

Thanks for the help.

[John]

What were you trying to focus on and what eyepiece were you using ?

Or has this been the case with a number of targets and eyepieces ?

[Chris]

Next time you're out under the stars, de focus a star in the centre of an eyepiece at high ish mag. If you don't see a concentric doughnut then the scope needs collimating. 

Here is a good guide to collimating an SCT. 

 

Edited April 14, 2020 by Lockie

[Ryohei56]

I was out last night observing Venus. If I defocused, I could see a concentric donut, but maybe that's not a good guide seeing as Venus is a planet (not a point source like a star). Will try again tonight, with Polaris.

[Ryohei56]

On this occasion, it was a 25mm eyepiece, and only Venus was visible at the time. To be honest, I tend to use an imaging camera much more often than I do eyepieces - on a recent outing I did get Venus, but getting a decent focus wasn't really happening.

[Ags]

Aside from collimation, you can try this hack I did with my C6 to get shake-free fine focus. It works really really well.

By the way, I don't think Venus is a good test "star" as it is not a point - it is going to mask any asymmetry in the donut. Pick a bright star like Regulus, Arcturus or Polaris instead. I find it easier to examine the donut on my laptop screen using a camera and Sharpcap. Also Sharpcap gives you crosshairs so you can be sure the donut is in the center of the field.

Edited April 16, 2020 by Ags

[Ryohei56]

Hi Ags. I did wonder if Venus was suitable, it's just that there was nothing else visible at the time (haze, not fully dark etc). I'll try again next clear night, and your suggestion of using a laptop with Sharpcap is definitely worth taking up. As my telescope work is mostly around imaging - with imaging cam and laptop - getting a sharp image on the screen is key.

I also have to try your gloriously hi-tech solution for fine focusing. Pure genius......

[RT65CB-SWL]

Or you could use one of these micro-focussers...

<--- @johninderby image of the helical focusser.

 

You will also need a 1.25" or 2" to T-ring nosepiece as well to attach to the telescope visual back...

 <--- 1.25" T-ring nosepiece.

 

[johninderby]

Or get a T2 to SCT adapter.

http://ensoptical.co.uk/--baader-sct-to-t2-adapter?search=t2 to sct

[newbie alert]

I'd first make sure it's at ambient temp before you adjust anything, it's no good taking it out of the house and using it..also was Venus over the top of your rooftop? As I had issues with focus on the moon a week or so ago when it was above the roof, once it had moved away and clear of any sort of heat release it was fine..

Check your collimation,  de-focus on a star at the zenith, with the star in the center of the fov check the doughnut..if it's pushed to one side then it needs adjusting until it's in the center, with concentric circles

[johninderby]

One thing to keep in mind when star testing you need good seeing for it to give meaningfull  results.