Budget wide angle EP suggestions?

I bought a scope for a friend's kids - Orion's 6" dob; 150mm and fl 1200mm = f8.

The focuser holds only 1.25" eyepieces. It came with the 10 & 25 mm sirius plossls.

Can anyone suggest a budget eyepiece that would offer a FOV that would take in open cluster targets like the Pleiades?

Also, I'm looking for enough eye relief so that they have an easier time viewing.

Thanks!
Dave

Dave Badrick said:

I bought a scope for a friend's kids - Orion's 6" dob; 150mm and fl 1200mm = f8.

The focuser holds only 1.25" eyepieces. It came with the 10 & 25 mm sirius plossls.

Can anyone suggest a budget eyepiece that would offer a FOV that would take in open cluster targets like the Pleiades?

Also, I'm looking for enough eye relief so that they have an easier time viewing.

Thanks!
Dave

Dave:

The bad news: That maximum True Field of View Possible with the XT-6 is about 1.3 degrees, enough to show most of the Pleiades but not all of them. In my experience a proper view of the Pleiades requires a 2 degree+ field of view.

The good news: The goal of an eyepiece which provides the maximum TFoV is a good one, both to view larger clusters and as a finder eyepiece.

More good news: For a budget eyepiece which maximizes the TFoV, just about any decent quality 32mm PLossl will do a wonderful job, sharp from center to edge, the F/8 focal ratio is very forgiving. I enjoy the views through a 32mm Plossl of the larger clusters like the Beehive in a 6 inch F/8 Newtonian. The exit pupil is 4mm, quite reasonable. Eye relief will be generous in any 32mm Plossl.

As far as which budget 32mm Plossl, the important thing is to choose one with a full sized field stop, the corresponds to a ~50 degree AFoV. There are some 32mm Plossls that have only a 44 degree AFoV, you don't want one of those.

Here's a good deal on a used good quality used 32mm Plossl:

http://www.astromart.com/classifieds/details.asp?classified_id=594870

Note: There are 40mm Plossls but because the size of the field stop is limited by the size of the eyepiece barrel, the TfoV possible will be no greater than that possible with the 32mm Plossl.
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The maximum True Field of View is determined by the focal length of the telescope and the diameter of the field stop of the eyepiece. (The field stop is the ring you see at the edge of the field of view. If you take the eyepiece out and look though it backwards, you'll see the ring)

The 1.25 inch eyepiece format limits the size of the field stop to 28mm, no more. To compute the maximum field of view possible, the equation is

Max TFoV = 57.3 deg/radian x (Field stop / Focal Length scope)

For the Orion XT-6:

TFoVmax = 57.3 deg/rad x (28mm/1200mm) = 1.33 degrees.

JOn

With a 1.25" focuser, the absolute maximum field size will be 27-28mm diameter.

You can field this image with a 32mm plossl and there are a good number of those to choose from today. The Orion or GSO plossls would do quite nicely.

However, a 27mm image is not going to show the whole of the Pleiades. Maximum field will be somewhere around 1.25 degrees. The Pleiades are rated at almost 2 degrees. M31 is the only Messier object that is larger. Still a 32mm plossl is a good fit in a scope this size and it provides good eye comfort. I think the added field over the 25mm Plossl is worth the price.

have fun,
dan

Edmund RKE 28MM. Cheap, good, readily available, decsnt field of view and will work well at F8.

Also, try the Garrett SWA 20 & 15 MM as well as the new Sterling Plossls.

Jeff