Telescope Reviews: O.F.L.I. Eyepiece World Cup: Target Selection?

For the powers you will be using, I think you are going to be needing targets not much less than 1 arcsec, maybe more like 1 to 1.2", and for uneven doubles, perhaps targets of 1.3 to 1.5 arcsec. I doubt you would see a 0.8" pair at 200x in a 140mm scope. Maybe at 300x-400x, and maybe if it is a blue pair.

Here's an example:
Zeta Cancri, 16 Cnc, Stf 1196, 5.3-6.2, when it was at 0.98" PA59 F8 G0
Call it 1 arcsecond. Using a 5" SCT, st 240x, elongation was very obvious. It jumped right out at me. I saw this as very clearly elongated with ever-so-slight a notch, not a very deep notch, but definitely a slight shallow notch. So at 1 arcsecond, best I could see was just a slight notch with a 5" scope. And it was seen at 240x. Keep in mind, that is a 1" pair.

Here's another example: scope CR150 6" refractor

16Vul as 5.8-6.2 / 0”.8 PA115, The spectral class F2, yellow/white, supports the normal Airy disk formula.

Both 6mm and 5mm orthos produced an elongated image and I recorded the 4mm UO ortho as much elongated. The 4mm is 300x.

I've spent a considerable amount of time on 16Vul testing the magnification needed to detect the discrete parts of the diffraction pattern. One night I put a 2.5x Powermate in the diagonal and ran right down thru my box of eyepieces.

It was easy enough to identify the fainter component following even at relatively low magnifications of 250x to 275x. At 250x there appeared the very slightest dip between the two members. At 275x I could see partial diffraction ring.

At 300x, the components were easily notched, two members are noticed and diffraction ring is noticed, but not whole.

At 333x, the components are more notched, the secondary following is definitely seen as the smaller of the two. 375x confirmed the view. PA was recorded as 120 at several of these lower magnifications.

At 400x, the components are still touching, but now diffraction ring appears solid all the way around.

At 430x the notch appears fairly deep and I noted components appear to be overlapped by maybe only 20% to 25%.

At 500x, I still could not see separation. Diffraction and notching were about the same as 430x.

At 600x, although components were still touching, not only could I see the deep notching in the disks but also I could see a depression in the first diffraction ring. This was the only magnification that I was clearly able to identify the diffraction rings belong to two separate components.

The Airy disk for my 6" scope has a radius of 138/150 = 0.92. Even assuming light most readily stimulating scotopic vision at 510nm rather than 555nm photopic vision, the Airy disk for my scope still only has a best radius of 0.92 x 510/555 = 0.84 arcseconds. Depending on the magnitude of the pair, the visible disks are 50% to 60% of the diameter of the Airy disk, brighter magnitude pairs having a larger central disk.

The maximum separation I could see on a 0.8" even pair, using a 6" refractor, was about 20% overlap. It took over 400x to see it. At 250x I could barely see a dip between the diffraction rings.

SOOOO, this is why I think you are going to need pairs wider than the diffraction limit. You aren't using magnification high enough to support seeing the limit.

edz

I posted this list in the thread in the eyepiece forum,
Doubles for Testing Limits of 80mm to 150mm
Probably some of these need to be checked in the 6th Orbit Catalogue, since they have been changing, but perhaps by only a few hundreths arcsec. For example I have zeta Cnclisted at 0.98", but it is at 1.04" in 2009 and will be at 1.06" in 2010.

edz

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