Why do I see colour fringing?

[JulianB]

Hi all,

Just wondering if anyone can help me re the colour fringing I described in this thread:

http://stargazerslounge.com/observing-planetary/96778-first-look-mars.html#post1372304

Cheers

Julian

[geoff_k]

Julian

Several things immediately spring to mind:

The conditions.

Allowing sufficient time for the optics to settle.

Collimation.

Geoff

[JulianB]

Thanks Geoff,

Ok so I collimated it twice during the session using a Hotech collimator so I'm fairly sure that was ok, and I left the scope for about an hour cooling down before use, is that enough would you say? it came from a warm house into the freezing cold garden, maybe it needed more.

So what about the conditions would cause this exactly?

Cheers

Julian

[geoff_k]

I was sure you had done adequate collimation but had to ask.

What you describe is more common with achromatic refractors but can occur when conditions are not optimal or the object in question is low in the sky. Also, some people are more susceptible to the effect than others.

Its not easy to say not knowing your location, etc, but I would think that one hours cool down time is not enough for such a large mirror. Have you had the chance to check the Naglers in another scope? I am sure they are excellent eyepieces but you never know.

I was trying to image Mars about a week or so ago (I have a 10 inch Newtonian) and the results to use a technical term were pants. I couldn't get a proper focus even when Mars was fairly high in the sky. On that occasion there was a lot of water on the optics and that sort of thing has made me give up more than once. I live in the West Country and dew is a real problem between November and February and can adversely affect an imaging or viewing session but luckily not all the time.

On the occasion in point I had set the scope up at about 5 in the evening and didn't image Mars until at least six hours later. The trouble is that it's not always possible to do something like that. I knew that Mars wouldn't be in the right position for observing until much later, so I took a chance on the conditions being OK. Also my scope is locked away in a shed with no heating so cool-down is not such an issue for me. Even so, I still think I will need to invest in some sort of dew heater when funds allow.

I must say that operating a large scope like that without motors requires some skill on your part so I take my hat off to you!

Geoff

[LukeSkywatcher]

Collimation doesnt seem to be an issue. I'd say the cooling down period was not long enough and also the seeing conditions (mostly the seeing conditions) are the main problems.

"CA" is normally a problem with refractors, but will be an issue with reflectors that are not cooled down enough. Even if the scope is well cooled, forget observing Mars until it at its highest point in the sky.

[David Pavlich]

Hmmmmm....reflectors are 100% apochromatic. From the description, it has to be the moisture on the eyepiece. Televue uses the best in optical glass. I use several and have never seen fringing on my SC.

One bit of advice, have a hairdryer by your side. Wiping your eyepiece is not a good practice. The coatings are fairly robust, but continued wiping will cause damage in time.

David

[johninderby]

You shouldn't be seeing any colour fringing around stars etc. I agree it's probably moisture on the eyepiece or it could be grease from a fingerprint.

John