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# Help with Orion doubles please...

33 replies to this topic

### #26 WRAK

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Posted 25 December 2012 - 07:22 AM

... right on the first ring @ 1.2" arc...

Calculated with 550nm for yellow light the radius of the Airy disk is 0.92" and the first maximum is then 0.92*1.63 = ~1.5" - an error of about 35% compared to the advertised separation of 1.1". But to my experience the bright spot of a companion near the first diffraction ring tends to stick on it so this observation may be valid even if I would not buy it with a 150mm reflector with 28% CO.
Wilfried

### #27 Asbytec

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Posted 25 December 2012 - 07:54 AM

Wilfred, not sure where you keep getting that figure. Multiplying the Airy disc by 1.63 gives a result 1.22 times too large.

Indeed, if the first minimum is 1.22 lambda/D = .92", then the second maximum is 1.63 lambda/D ~ 1.2".

Rayleigh limit: 1.22 * 550 * 0.206/Dmm = resolution " arc, of course for the Airy disc.

Second maximum a = 1.63 Lambda/D
1.63 * 550 * 0.206 = 184.68/150mm = 1.23" arc

### #28 WRAK

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Posted 25 December 2012 - 05:31 PM

You are right, forgot to divide with the factor of the first minimum: 0.92/1.22*1.63 = ~ 1.23.
Best wishes.
Wilfried

### #29 Asbytec

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Posted 25 December 2012 - 09:11 PM

Thank you, Wilfred.

I wish you a Merry Christmas.

### #30 WRAK

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Posted 12 January 2013 - 11:40 AM

After four weeks of clouds and fog yesterday a few hours of clear sky around midnight. Although it was rather cold near freezing point and Orion was already rather low (about 25° altitude for Rigel) I gave it a try without great expectations as light pollution was as usual rather bad with NEML +2.8mag in the field of view (may be somewhat better at zenit but this does not help very much for observing at lower altitude).
The seeing was rather stable with solid spurious disks and the diffractions rings staying in place even if flickering a bit, but with magnifications above x140 the spurious disks got increasingly fuzzy blobs due to humidity and haze in the air.
Now to 42 Ori: I got clearly no split in terms of dark space between spurious disks and also no 8 but a very pronounced elongation pointing to 6:30 o'clock. As I use an alt/az mount this gives as I checked afterwards for my location and observation time including horizontal flip a position of about 210° corresponding very well with the advertised position. This observation corresponds very well with the image posted by Rutilus with an additional vertical flip (did he use no diagonal?) and a bit lower magnification.
Wilfried

### #31 Asbytec

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Posted 12 January 2013 - 11:49 AM

I don't know if he used a diagonal, I would think so. I did. His orientation is similar to mine, but I get confused with the flipped and reversed views in various scopes. So, I cannot say, nor whether he was alt az mounted and viewing at a different location in the sky. My scope is EQ mounted.

What scope were you using for elongation? A small refractor?

### #32 WRAK

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Posted 12 January 2013 - 12:23 PM

Sry, forgot: 140mm refractor.
Wilfried

### #33 fred1871

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Posted 13 January 2013 - 10:52 PM

Wilfried has posted a note in the "Some doubles in Tau and a bonus" thread, that gives access to a list he's made of Orion doubles, for testing telescope resolution, pairs bright and faint, even and uneven.

I thought I'd post here my observations of some of these as a way to give easier access (Orion in an Orion thread).

STT 133 in Orion: 0608.0, +2118: mags 7.35, 11.17 (delta-m of ~3.8) separation 3.3" - observed with 140mm refractor, no moon, fairly good seeing, it was surprisingly easy - I could see a hint of the companion at 80x, and it was a clear tiny speck just offset from the primary at 114x. [listed for 150mm aperture]

Also on the list is an old friend, much discussed - part of STF 748, which refers to various of the stars of the Trapezium in the Orion Nebula. The particular pair listed by Wilfried is the AE pairing, mags 6.55 and 11.1, at 4.6" - I've seen this one readily with the 140mm refractor on good nights, not too difficult. [listed for 138mm]

Harder, as often remarked, is the CF pairing, mags 5.06 and 11.5 at virtually the same separation these days, 4.5" (it's been widening) - I've also seen this one, though harder, with the 140mm when seeing cooperates. I don't think CF is in this list.

The 6-inch f/8 Newtonian that I had years ago also showed both C and F.

Next clear night (this week, I'm hoping) I'll try some more from the list along with more of the Taurus doubles.

### #34 WRAK

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Posted 14 January 2013 - 07:23 AM

... part of STF 748 ... the CF pairing, mags 5.06 and 11.5 ... I don't think CF is in this list...

Fred, this is correct. I restricted the list to doubles with delta-m of less than 6 to avoid it getting too big.
Wilfried

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