Skywatcher Startravel 102 Refractor Focus

[Mozza]

I currently own a Skywatcher Startravel 102 on a GOTO mount.

I have a couple of problems:-

1) When focussing on stars, I cannot get a good resolution, they appear as crosses,I cannot seem to get them in good focus. Jupiter is bit better, the moon is okay. Any ideas?

2) I appreciate that these types of scopes suffer from Chromatic Abberation, but is there anyway of minimising this?

I am using the eyepieces, barlow and prism supplied with the scope.

Thanks in advance

[GazOC]

Are the "crosses" the same both sides of focus Mozza?

[Mozza]

Yes, I seem to get close, then goes away.

[johninderby]

When I had one it did focus quite well with stars being fairly sharp with no "crosses". Soulds like the objective could be out of collimation. Also the diagonal that comes with this scope isn't the best. Is there someone who could lend you a decent diagonal to see if it helps?

Here's a simple moon image taken with a 102. Is this similar to what you see visually?

John

[Mozza]

Using the supplied optics, I get as pretty good view of the moon with purple fringing.

My main problem is with stars. streaky1 has given me a link to abberations, and from a quick glance, I would say my problem is Astigmatism.

Will need to investigate further

[MushroomBill]

I've recently purchased a ST102 and haven't had much chance to play yet. I believe either a semi-apo or fringle killer filter may help with the CA. Its something I'd like to look into soon.

Also as John mentioned -it would be well worth trying another diagonal.

[John]

I'm assuming that these stars with crosses appear right across the field of view - is that correct ?.

It could be that the objective lens retaining ring is too tight.

You can check this by pointing the scope upwards and locking it in position, remove the dew shield (it slides off the objective lens cell), loosen the objective lens retaining ring a turn or so, tap the sides of the lens gently cell to settle the lens elements, then gently re-tighten the retaining ring so that it it just tight enough to stop the objective lens from rattling but no more.

If you can borrow a better quality diagonal that is worth a try as well.

CA is something that all achromatic refractors have in varying degrees, more so with faster ones and F/5 is fast. Filters will lessen it a little but it's will still be there on brighter stars and planets and the lunar limb. It's not an issue for low to medium power views of deep sky objects, which is what these scopes were designed for.

[Mozza]

Thanks for you all for your assistance, I have some things to try now. Weather permitting.

[Mozza]

Forgot to answer.

In answer to Johns (jahmanson) question. Yes, it is across the field of view.

[Peter Drew]

I think Jahmanson is spot on with his diagnosis, it sounds like a classic case of a pinched objective. Follow his advice and leave the objective just loose enough to rattle. Good luck.

[ronin]

The prism is for getting an erect image and really is unnecessary in an astro scope.

If the system can be made to work without it, I think it can, then discard it and set it up without the prism. There should be an extender tube to compensate for the path length added by the prism bit.

[Mozza]

I have loosened the objective and will give the scope a try. I will also take the prism out of the path and try a tube extender.

Many thanks

[Macavity]

A fun, if slightly maligned scope? I did loosen (nay "tapped") the objective cell, to relieve any potential pressure on the objective. I had a go at (vaguely) "collimating" the thing too - With DYMO tape shims. The focusser can benefit from a general... "aligning and tightening"? Added a 2" diagonal. Whatever! My first scope - Always to remain in my affection?

[Mozza]

An update

I have been in contact with the shop that I brought my scope of, and with a bit of email tennis, discussing what the problem may be, the scope has been taken to OVL (Skywatcher Agent) for inspection to see what the problem is.

So hopefully, in a couple of weeks I can be viewing again.