Orion 120ST - Color (Violet) Fringing

Hi All,

I just bought a 120st from Orion. This is the first 5" short refractor F5 that I've bought. I have been using either an MCT or SCT. I mated this scope with a two inch diagonal with a 2" 32mm eyepiece.

I've been hearing that refractors generate colors. I had some concerns about the scope, since I started using it. I called Orion and they had advised me to test the scope during the day and focusing it on an object atleast 300ft away. I did that and focused it on a tree. Here are the results:

If I look straight through the eyepiece, there's no color
If I move my eyes (head) slightly off center towards the right I get the violet color

Also - just to note at night when using the 2" eyepiece, I get good focus in the center of the fov but doughnut stars around the edge.

Are these normal behaviour for these types of scopes? I still have a couple of weeks to return it. Should I?

Any advice would be appreciated.

Thanks
Ed

You know how this scope works and the question is, are you satisfied with it? Color will increase with magnification so you have pretty much purchased a widefield scope.

My guess is that the color is excessive and that you would be happier with a slightly smaller scope with a slightly slower focal ratio. Say 100mm F6. Of course you can turn your 120mm F5 into a 100mm F6 with an aperture mask, but why bother..

This is a decision only you can make. Do you have other scopes, what would you want to do with this one....

jon

I have an Orion Space Probe 130ST OTA which has been modified with a 2 inch focuser. This is a 130mm Newtonian with a 650mm focal length, quite similar to your scope in specifications but of course it is a reflector. Trade offs but it does have the advantage of being color free and when used at higher magnifications it does a good job with double stars. I have nailed the usual suspects, Rigel, Zeta Orionis, the double double, Izar etc with it. A nice combination of wide field and decent high power viewing.

With my 32mm Televue Widefield it gives nice big views of the night sky, 3.3 degrees at 20X. The nebulosity of the Milky Way is really quite amazing, at 20x with a 6.4 mm exit pupil, the gradients which are lost at higher powers are readily seen.

Some people Pooh-Pooh wide field viewing, claiming that there are few objects worth seeing. The reality is that the there is plenty to see on the big scale, those poor folks are just stuck zooming in to look at the leaves and have missed the forrest.

jon