Jump to content


Photo

Minimum aperture for splitting 90 Her

  • Please log in to reply
10 replies to this topic

#1 WRAK

WRAK

    Apollo

  • -----
  • Posts: 1172
  • Joined: 18 Feb 2012
  • Loc: Vienna, Austria, Europe

Posted 27 October 2012 - 04:06 AM

I was not successful in splitting 90 Her (WDS17533+4000, HIP 87563, RA 17:53:18, Dec 40:00:29) 1.6"sep +5.3/8.8mag with my 140mm refractor but this seemed of no surprise to me - the rule of thumb for this one would give 116/1.6*3.5= ~254mm for a reflector and about 170mm for a reflector.
On the other side there are always these irritating reports of splits of difficult doubles with incredible small apertures.
Did you ever observe 90Her and if yes what aperture did you need to split it or with which aperture did you fail?
Wilfried

#2 VanJan

VanJan

    Ranger 4

  • *****
  • Posts: 384
  • Joined: 09 Jul 2008

Posted 29 October 2012 - 09:27 AM

90mm refractor at 230X, not split. 200mm reflector at 250X, split. Best regards.

#3 PJ Anway

PJ Anway

    Double-Star Observer

  • *****
  • Moderators
  • Posts: 2662
  • Joined: 04 Jun 2003
  • Loc: North Coast

Posted 29 October 2012 - 07:47 PM

Wilfried,

Burnham added this pair to his catalog in 1879 (BU130) using his 6" refractor saying: "The components have a striking difference in color - golden and blue"; a beautiful pair, even with a small aperture." Of course his 6" scope was made by the Clark & Sons!

#4 WRAK

WRAK

    Apollo

  • -----
  • Posts: 1172
  • Joined: 18 Feb 2012
  • Loc: Vienna, Austria, Europe

Posted 30 October 2012 - 02:48 AM

PJ, thanks - seems then not so difficult with an 152mm refractor under dark skies. My naked eye magnitude loss due to light pollution of about 3 mag makes it maybe more difficult.
Thanks to VanJan for the observation report. Is light pollution also of relevance for you and what percentagr of diameter is the CO of your Newton?
Wilfried

#5 VanJan

VanJan

    Ranger 4

  • *****
  • Posts: 384
  • Joined: 09 Jul 2008

Posted 30 October 2012 - 10:46 AM

Wilfried,

My NELM is about 4.5 mag. I no longer own the Newt, but I believe the CO was 29 percent.

VanJan

#6 fred1871

fred1871

    Viking 1

  • -----
  • Posts: 893
  • Joined: 22 Mar 2009
  • Loc: Australia

Posted 30 October 2012 - 07:10 PM

Wilfried, I would not expect light pollution to have much effect on 90 Herculis. Your telescope loss will be less than the 3 magnitudes of naked-eye loss so I think the main problem will be air steadiness, though I'd certainly in your situation choose nights of no moon to attempt 90 Her. Enough magnification is needed as well - very unequal pairs require more power than the separation suggests if you base it on equal pairs. Notice that VanJan used 250x on his 200mm reflector to get a split. Not surprising.

PJ's note about SW Burnham is interesting. Though back then, Burnham measured the pair at 1.9", a little easier than now - there has been slow change in this long period binary. No chance of an orbit yet, from the numbers I've seen.

So, despite the RoT suggesting 170mm needed for seeing 90 Her as a double, a bit less aperture might do it IF the seeing is steady enough, and with magnification over 200x.

#7 fred1871

fred1871

    Viking 1

  • -----
  • Posts: 893
  • Joined: 22 Mar 2009
  • Loc: Australia

Posted 30 October 2012 - 08:46 PM

Some further thoughts. I recently observed Theta Gruis, also on the Haas list of unequal pairs. It's easier than 90 Her, as the delta-m is 2.15 magnitudes, compared to 3.5 for 90 Her. Separation is similar, 1.5" for Theta Gru, 1.6" for 90 Her.

I'd seen this one nicely split some years back at 180x with a 7-inch refractor. This time, even with Theta high overhead, it needed 250x for a neat separation with a 140mm (5.5-inch) refractor. The companion was seen at less power, but not cleanly separated.

90 Her will be tougher. So I'm thinking that separating it with 140mm, if that is possible?, will take very good air steadiness and good visual acuity by the observer. And quite high magnification.

#8 WRAK

WRAK

    Apollo

  • -----
  • Posts: 1172
  • Joined: 18 Feb 2012
  • Loc: Vienna, Austria, Europe

Posted 31 October 2012 - 05:59 AM

... Your telescope loss will be less than the 3 magnitudes of naked-eye loss ...

Fred, this is certainly true - I think I can see stars up to +11,4mag with averted vision and this means a loss of "only" about 2 magnitudes compared with the theoretical limit of +13,4mag.
If I add 2 mags to 90 Her this would give a +7.3/10.8mag double and I assume that the same delta-m gets increasingly difficult with fainter pairs.
I will certainly try again, but have to wait for next year because Hercules has meanwhile left my field of view.
Wilfried

#9 Sasa

Sasa

    Viking 1

  • *****
  • Posts: 862
  • Joined: 03 Nov 2010
  • Loc: Ricany, Czech Republic

Posted 31 October 2012 - 09:41 AM

I had no luck with 130mm refractor (Vixen 130ED SS) this summer.

#10 Cotts

Cotts

    Just Wondering

  • *****
  • Posts: 4899
  • Joined: 10 Oct 2005
  • Loc: Toronto, Ontario

Posted 31 October 2012 - 12:30 PM

Wilfred, I was successful on two nights at Okie Tex with my 150mm AP refractor. I posted at the time (Sept 9+/-) in this forum......

Dave

#11 WRAK

WRAK

    Apollo

  • -----
  • Posts: 1172
  • Joined: 18 Feb 2012
  • Loc: Vienna, Austria, Europe

Posted 02 November 2012 - 02:52 AM

Translating the 200mm reflector with 29% CO into 142mm refractor I try a resumee: 150mm aperture certainly big enough for split, 130mm and below most certainly not. 140mm should be possible given very good conditions.
Wilfried






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics