Your minimum mag for splitting Double-Double

I am able to split the Double-Double in Lyra on nights with good seeing at 75X with my f/14.2 LOMO Astele 150 and f/6 80mm Stellarvue NH II. The seeing was no better than 4 out of 5. Associated equipment includes dielectric diagonals, Siebert binoviewer, Ultima and Star Splitter EP's. Similar mags have made the split in my Meade SN8 with JMI dual-speed Crayford focuser. I live at about 200 meters above sea level and approximately 35 degrees north latitude. While I do star test and star collimate my scopes (I own H.R. Suiter's book on star testing), when it comes to actual performance on objects I actually view, the Double-Double split is my standard test for actual system visual acuity. I have to see black space between both star pairs during moments of steady seeing, and not merely definite elongation or two connected blobs to consider it a split. I did not have my LOMO scope last summer when Lyra was high in the sky, and I am sure I can improve on that 75X mag when I get a night of excellent 5/5 seeing as sometimes happens here during the dog days of summer. I have done 81X splits with the SN8 and Ultima 10mm even before the JMI focuser was installed (I used a Borg helical focuser then) and present critical collimation was achieved.

What are your lowest mags for the split, and what gear was it done with and the seeing conditions?

Phil

Hi Phil,
Lyra's Double-Double is indeed a fun star to see how "low you can go." My personal best is 56.4x under excellent seeing and transparancy, although I do live at sea level in a suburban neighborhood with light pollution. My equipment was a Stellarvue SV102ABV (790mm fl), Stellarvue 2" dielectric diagonal, Stellarvue binoviewer, and 14mm Orion Epic ED-2 eyepieces. I saw black between both splits.

David E

I think that splitting bright 2-3 arc second doubles at low magnifications is more dependent on your eye than on the optics of the scope. I also think that there is an optimal aperture for splitting the double double at low magnfications.

Last summer I routinely split the double-double at 67x with my Burion 100F6 (Burgess Mechanical/Orion Achro objective.) At times I was able to split it at 60x. The optics in this scope are decent but nothing special, the diagonal is the stock 2 inch Burgess and the eyepiece a the 9mm Synta Widefield.

With larger, more capable scopes, I think the image is just too bright, the exit pupil too large to split the double double at low magnifications. A 10 inch scope at 60x produces a 4mm exit pupil and that pushes the eyes capabilities, big bright stars appear larger...

At 60x a 4 inch scope is producing a 1.6mm exit pupil which is still bright but in the region that is often considered optimal for the human eye.

It also depends on what is considered "splitting" a double. In my mind I consider it split if I perceive that there is a gap between the two stars. At 60x on a stable night, I think I am seeing that gap...

Unequal doubles are harder and benefit more from larger apertures...

The double-double is one of my favorites, every night when Lyra is visable, it's one of the first targets. Smallest scope I have managed a split with is my 60mm Asahi-Pentax from the 60's.. Don't think there will be anything smaller.

Oh well,

jon

44x with my poor old ST80 (the seeing was good though).

Dmitri

Phillip,

I split it easily at 133x with my 90mm achro refractor. I will try it this weekend at 80x. My eyesight is not the best, though . . .

Rick

Just went out and tried to split the double-double. Seeing is a bit worse than normal. I know my scope is collimated. I could not split at 79x, 119x, 133x or 158x. I finally got a clean split at 237x (14.5", f4.5, and Pentax XL 7mm. - I am absolutely sure of the collimation, the seeing is that bad here. Some of the time tonight, I could make out the bands of Jupiter at 184x, at times it would get mushy enough that I would almost loose sight of the bands.)

The stars they are a twinkling tonight!

(I live along the eastern slope in Colorado)

Michael