Blured round the edges

Just a quick note: what David originally described was field curvature, which is unreleated to coma or vignetting.

Field curvature is a difference in focal length from the outside of the image to the center. By refocusing, you can produce a sharp image at the outside (although at the expense of sharpness on axis). Field curvature is not uniformly variant with focal length. An f/10 instrument can easily have more field curvature than an f/5 instrument -- it just depends on the design.

Shorter focal length instruments often exhibit higher degrees of coma, spherical aberration and chromatic aberration (and perhaps astigmatism, although I don't know if it's directly related to focal length or not). These are what give simpler (read: "less dear") eyepieces fits with shorter f/l instruments.

Lastly, vignetting has to do with whether or not all the light rays can reach the image plane (not whether or not they're displaced from where they ought to be -- so it's an illumination question, not an aberration). It generally shows up as a darkening of the outside of the field (although I think the secondary shadow of an SCT or Newt is also technically vignetting). Faster instruments produce a steeper light cone, and are as such much more susceptable to vignetting (particularly as the tubes the light passes through get either narrower or longer).

OK, so it didn't turn out to be such a "quick" note after all. But, perhaps someone will find it useful nonetheless....

-- Jeff.

Hmmm think I understand you jef (not quite sure) :lol:

So do you reckon its still eyepiece or telescope design of SCTs

Hmmm think I understand you jef (not quite sure) :lol:

So do you reckon its still eyepiece or telescope design of SCTs

David --

In your specific case, I think it's mostly a combination of the scope and your eyes. SCTs do produce field curvature, and quite a bit of it compared to other telescope designs. The majority of people aren't bothered by this because their eyes automatically accomodate (ie: adjust their focus) for the field curvature. But, that still leaves the minority.

To correct it, you've got a couple of options:
1) experiment with enough eyepieces to find some that have the opposite field curvature of your scope
2) use an f/6.3 focal reducer/flattener
3) buy a scope with a flatter field

If you don't have any 2" eyepieces, option (2) looks pretty good. (With 2" wide-field eyepieces, the f/r is going to vignette.) Note that it will lower the power of all your eyepieces, but you can compensate by getting some shorter focal-length ones.

Note: I just re-read the entire thread. You only report this effect on M37, which suggests the possibility that the effect isn't too pronounced. I think I remember reading that accomodation only works in one direction (ie: your eye can shorten the focus, but not lengthen it, or the other way around). So before doing anything drastic, try approaching the best focus (in the center) from both directions and see if that helps.

-- Jeff.

Hi

A quick question for all the experts I was out last night observing (brrrr...) and I noticed that in some of the views the image seemed to be blurred towards the edge of the field of view and focused towards the centre.

When I focused in the edges, the centre then became blurred. Is this a normal thing in SCTs

Thanks

Hi Dave,

One possibility not covered yet is tube currents. It has been particularly cold lately and your scope may be relatively warm. This would manifest itself as a blur in the top portion of the field of view and would be most noticable with higher magnifications. You can eliminate eyepiece aberations by simply rotating the eyepiece (and/or diagonal) in the focuser and seeing if the blur also rotates. If not, then the blur is being caused somewhere within the telescope itself. How long did you let the scope cool down before observing?

Hope this helps,

Phil.

Hi Phillip

Hope you are well. In my haste to beat the clouds and wow the girlfriends family with their first look at saturn I didnt leave it to cool down for long at all I dont yet have a dew shield so this could also be the culprit.

It was only really noticable for me with M37 so it could also be just my imperfect eyesite!