Unitron Images.

[kevy02]

Hi all, just had my first view through my 4" Unitron since replacing original focuser with 2" Crayford type.The scope was out in the night air for around 2 hours,I have a dew heater at the main objective end.The images were far from great.The image of Saturn wasn't stable, seemed to be double imaged at times.Is this all related to cooling down time or is it to do with the optics,collimation,etc.The session was eventually interrupted by the neighbours floodlight,so packed up for the night.Cheers.Kevin.

[alocky]

The optical quality of the later 4" scopes is variable. I have one that is superb, and delivers images nearly as good as my Tak. The other had almost no anti-reflective coatings between the crown and the flint, causing intolerable ghosting on anything brighter than 2nd mag stars - sounds a bit like what you're describing. I spent a fortune getting it recoated and got rid of the ghosting, but it still has a bit more spherical abberation than it should. Unitrons have adjustable collimation, and as such, it also goes out of collimation relatively easily, so you need to ensure your focuser is square to the objective with a laser or sight tube, and check the collimation using a cheshire eyepiece. Still, at f15 they're fairly tolerant of collimation errors.
Cheers,
Andrew.

[astronut]

Kev,
I would check the collimation....this scope gave me the most brilliant views of Saturn, and many other objects.
It may also be the fault of a unsteady atmosphere.

[kevy02]

Thanks guys, I'll have to check the collimation, not much I can do about seeing conditions at the moment, I remember also that there appeared to be quite a bit of condensation on the 2" eyepiece to start with and also the main lens didn't look clear when shining a light on to it,not sure if there was moisture on the inside or outside of the lens.All to do with cooling down time I would think.Just wondering also,if it is advisable to have the dew heater working flat out to warm the dew cap near the main lens, or whether its better just to let the whole thing cool down to ambient temperature.I read that the metal surfaces of the telescope cool much more than the air temp,which would transfer to the glass of the main objective.If thats the case moisture in the warmer air would always condense on the cooler lens.Am I rambling too much here?Cheers.

[kevy02]

Just did a collimation check with laser, new focuser was way out ,did some adjustments,seems ok now,will have to wait until weather clears again