Nexstar 5se focusing issues. Any advice much appreciated.

[Matt I]

Hello all. Bought myself a second hand nexstar 5se to add to my SW Dob 200p as fancied a goto mount. The mount is great and easy align and does the job spot on. The issue I'm having is I can't get a crisp image when focusing. I can't get to the point of almost focusing as like using my Dob but it just doesn't get there. I'm using a star diagonal which appears to be in fine working order.Can only think that the diagnol may be positioned wrongly or may be an issue with one of the mirrors. Anyone come across this same issue? And how did you resolve it? Thanks in advance.

[Oddsocks]

How does the image look through the eyepiece when getting close to focus?

If the secondary collimation is correct you should see the image on the left.

 

If the secondary collimation is off then you will see the image on the right.

If the defocussed image looks like the the one on the right then you need to adjust the three collimation screws on the front of the corrector until the large black centre donut sits exactly central to the bright outer ring.

[Matt I]

Thank you. Will check it out.

[Peter Drew]

Be sure to centre the star accurately otherwise a well collimated out of focus star as shown on the left can look like the one on the right. 

[Stu]

Matt, are you saying you are running out of focus range before you get to focus, or you go through focus without obtaining a sharp image?

[barkis]

If your secondary screws require a hex.key, or screwdriver for adjustment, get a set of Bobs Knobs,

much easier to fine adjust the secondary then.

Ron.

[Joves]

There's also a possibility you're comparing the crispness of stars in your dob, which is a Newtonian reflector, with that of your new Schmidt Cassegrain? I love my LX90, but there is no denying star images are a lot "softer" and slightly bloated when compared to my newt or refractors. It often gives the sense of not quite reaching the same level of focus that the other scopes do. Absolutely blows them away at high mag views of planets, star clusters and galaxies though (due to greater aperture and resolving power, not quality of optics).

[Ags]

Fine-focusing SCTs and Maks can be tricky. Try adding a fine-focus wheel to the SCT - just a larger diameter disc blue-tacked to the small diameter focus knob. This should give you more fine control.

[SkyGibbon]

On poor seeing nights my 6" and 8" sct both aren't sharp at all on planets. I will pull out a different scope or I will view something else.

On above average seeing or when the temp isn't dropping 30 degrees over a 2 hr span they are great. Sometimes I feel my 8" sct never comes to thermal equilibrium in the winter here in the desert. My c6, not so much and has as pinpoint stars as much as a sct will ever have.

Being used though, definitely check collimation first.

[barkis]

Planetary detail will suffer if  the SCT's  collimation is off.

It has to be done by star test, or in daylight with simple eye judgement.

Not always easy.

If the scope is in an outdoors environment, ie Obs, then cooling is not a problem,

but indoors to out for a session, will require a substantial cooling period.

[barkis]

Hello all. Bought myself a second hand nexstar 5se to add to my SW Dob 200p as fancied a goto mount. The mount is great and easy align and does the job spot on. The issue I'm having is I can't get a crisp image when focusing. I can't get to the point of almost focusing as like using my Dob but it just doesn't get there. I'm using a star diagonal which appears to be in fine working order.Can only think that the diagnol may be positioned wrongly or may be an issue with one of the mirrors. Anyone come across this same issue? And how did you resolve it? Thanks in advance.

Why not check without the diagonal on an object, and compare to when the diagonal is in place.

You might need an extension tube with the diagonal out though.

Choose an object on  or about the celestial equator, then you won't strain your back and/or neck too much.

[Matt I]

Thanks for the advice folks! It was indeed the secondary mirror. All sorted now!