Jump to content


Photo

min aperture to split double near Jup?

  • This topic is locked This topic is locked
9 replies to this topic

#1 Darren Drake

Darren Drake

    Mercury-Atlas

  • *****
  • Posts: 2807
  • Joined: 09 Oct 2002
  • Loc: Chicagoland

Posted 03 January 2013 - 07:58 PM

Over the next several nights there is a challenging red double near Jupiter. It is BU 87 and the mags are about 5.9 and 8.5 with a split of about 1.9 arcseconds. In good seeing it is not to difficult to split in my bigger scopes. I am wondering what is the smallest aperture refractor or any scope for that matter is needed to split it. I would suspect that it will start getting very difficult in any scope smaller than about a 6 incher. It would also be interesting to see what design scope can be used at the smallest aperture to split it. I will mention this in the other forum and lets see what happens. It is easily seen at this time (1/3/13) as a red star posing as a 5th Jovian satellite.
 

#2 Gord

Gord

    Surveyor 1

  • *****
  • Posts: 1686
  • Joined: 06 Jan 2004
  • Loc: Toronto, ON, Canada

Posted 03 January 2013 - 08:09 PM

Hi Darren,

I think a 6" should have no problems doing it. In the summer I was testing on 90 Hercules. It's details are 1.6" split, 5.3 primary/8.8 secondary.

It was really tough in the 6", but I managed to catch it. Easier in the C8.

Clear skies,
 

#3 fred1871

fred1871

    Viking 1

  • -----
  • Posts: 894
  • Joined: 22 Mar 2009
  • Loc: Australia

Posted 03 January 2013 - 08:26 PM

Gord, that's a really interesting comment, that with 90 Her you found it harder with a 6-inch refractor than with a C8. Some of us have been estimating that for close, uneven doubles there's a similar approximate rule to the planetary one for planetary observing - subtract the secondary diameter from the primary to get an estimate of what size unobstructed telescope you're matching.

A C8 would on that be slightly inferior to a 6-inch refractor. You've found the opposite. Hmmm. Same air steadiness for both observations?

Re Darren's question on BU 87, as I've mentioned in another thread, I've seen it double with 140mm (5.5-inch) aperture. And I didn't think it was right at the limit of that size scope. So a bit less could do it.
 

#4 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 8335
  • Joined: 08 Aug 2007
  • Loc: La Union, PI

Posted 03 January 2013 - 09:49 PM

Fred, the C8 obstructed will have a significant edge at high frequency resolution with a diffraction disc angular size capable of the split despite the first ring (without crunching the numbers.)

In a 6" obstructed scope, it was reasonably difficult but easier than c Ori. The companion sat just outside the first ring. In fact pretty much right at the second minimum for a 6" scope. It would sit close the first maximum of a 100mm scope.

The (Do - Ds) rule of thumb really only applies to mid range contrast. You could /almost/ do a (Do + Ds) rule of thumb at frequencies near the Airy disc size.
 

#5 fred1871

fred1871

    Viking 1

  • -----
  • Posts: 894
  • Joined: 22 Mar 2009
  • Loc: Australia

Posted 03 January 2013 - 10:27 PM

Norme, I'm not inclined to believing a Do + Ds rule - I've had too much time observing doubles with SCTs (C8, C9.25, C14) to go with that one. I've also had (and used) Newtonians where, again, the CO had a noticeable effect, and not for the better.

I'll agree that a CO helps for evenly bright very close pairs, but my experience is otherwise with noticeably uneven close pairs. The diffraction effect of extra light going from disc into rings does make an unhelpful difference with the uneven close ones. And that's despite good quality optics, well-collimated, and decent seeing.

What Gord's experience suggests... well, several possibilities. A better than usual C8, and not-so-great refractor? Different seeing conditions? Or a fairly large CO being less harmful than might be thought? With a C8 it's around 32-33% diameter, so that puts quite a bit of extra light into the rings, which can make for issues with uneven pairs, depending on the location of the rings relative to the secondary star.

My C9.25 (current SCT) does about as well on close uneven pairs as my 5.5-inch refractor. It does better on very close even pairs (aperture wins). And it does better on faint pairs - a 1 mag gain, so aperture wins again by giving the eye more light to see.

I'll hope Gord can provide some thoughts on his experience of observing 90 Her. Maybe I need an Edge-HD Celestron with better optics? - but there's still that large-ish CO. :question:
 

#6 Gord

Gord

    Surveyor 1

  • *****
  • Posts: 1686
  • Joined: 06 Jan 2004
  • Loc: Toronto, ON, Canada

Posted 03 January 2013 - 10:36 PM

Fred,

I'll reply more in the morning but there is a thread over on the doubles forum with more details (you'll have to go back a few pages...). Very good C8, good refractor, same night side by side, bute I think it's the extra brightness in the SCT.

More to come...
 

#7 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 8335
  • Joined: 08 Aug 2007
  • Loc: La Union, PI

Posted 03 January 2013 - 11:08 PM

Thanks, Fred. It's an interesting question. I am not trying to say a split for close, unequal pairs is easy even cooled, collimated, and in steady seeing. Not even in my own limited experience under those very conditions. In fact, I often wonder why they are, indeed, difficult.

So why is that? Is it the spacing of the ring(s) relative to the dimmer companion's angular separation? If spacing plays a role, a C8 will have a first ring well inside (and out of the way) from the companion. For example, c Ori appeared to be an occasional, distinct brightening of the first ring (150mm aperture) while this particular double was clearly a distinct (yet somewhat difficult) spurious disc.

Is the issue the brightness of the rings? If so, then it seems only stars sitting on or very near the bright ring would be difficult. If an 8" puts up a very bright first ring, then it might obscure the companion. But, I'd doubt the ring is that bright and is sufficiently removed (about the third minimum) from the companion's position to allow resolution. In any case, a 6" obstructed scope can do it (at the second minimum), certainly an 8" can.

Or, is the reason for the difficulty separating close, unequal pairs due to the OPD and interference of the primary. After all, this companion sits right at the second min (of a 6" scope.) This is a point of destructive interference. One might be forgiven for asking if the primary's OPD is interfering with the companion's image as well (since waves are converging at the same point, some of them out of phase.) Dunno, but if so that would apply equally to both obstructed and unobstructed scopes - making observations difficult between the bright rings.

In any case, I am sure you are correct. Your observation in a 140mm scope and indication it is doable in a smaller scope seems reasonable. Of course, it's gets more difficult with decreasing aperture down to about 100mm, or so. Not sure what the limit would be, but I'd guess something between 100 and 140mm.

Can it be done at the Raleigh limit of a 72mm scope? Dunno. I am just not sure what makes these pairs so difficult.
 

#8 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44369
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 04 January 2013 - 07:56 AM

I'll agree that a CO helps for evenly bright very close pairs, but my experience is otherwise with noticeably uneven close pairs. The diffraction effect of extra light going from disc into rings does make an unhelpful difference with the uneven close ones. And that's despite good quality optics, well-collimated, and decent seeing.



:waytogo:

In my experience, obstructed scopes hang with a good refractor on equal magnitude doubles but have a more difficult time of it with unequal doubles, part of it is the scattered light, part of it is the greater energy in the rings.

Jon
 

#9 Gord

Gord

    Surveyor 1

  • *****
  • Posts: 1686
  • Joined: 06 Jan 2004
  • Loc: Toronto, ON, Canada

Posted 04 January 2013 - 12:34 PM

So, I described my experience with 90 Her in this thread over on the doubles forum:

Need help with 90 Her

I actually did the split with 3 scopes, but not all on the same night. Two C8's (Edge HD and regular) and the 6" IStar. Both C8's are recent (late 2010 and newer). The comparisons were between the SCT's and the refractor, beside each other on the same night.

The C8's definitely did it easier, but not as pretty (clean) as the refractor in the moments of good seeing that the refractor finally did it. Seeing was always variable ranging from so-so to seconds of decent. All scopes showed fuzzy stars in the not good seeing with the airy disk blending to the first ring. This impacted the SCT's more than the refractor as there is more light in the rings of the SCT's and I think partly to do with the extra brightness in the SCT (more on that in a moment). But the SCT's just drew out the companion more often.

I think the brightness thing has something to do with it. It seems these modern Celestron's have incredible coatings on them because they are just *so* bright! The refractor was very noticeably dimmer. The other thing coupled with that is the high magnifications used. I had the most success in all scopes north of 300x. I gave more separation due to the larger scale and seemed to handle the seeing better (in terms of letting the airy disk stand out).

In terms of the pretty view, the refractor had it (cleaner stars) with I find to be the case often since the rings are less obvious. It just didn't show things as easily or as often.

Note that these are the first SCT's that I've ever been satisfied with. Optical quality and consistency seems to be higher than in the past. I replaced the standard C8 above with the Edge. The standard was just a hair better on axis I felt. The previous owner of the C8 Edge has had 5 C8's over the years and felt this Edge was the best he has ever had. He replaced it with a C9.25 Edge. I favor the Edge's due to the better field performance, but it seems the regular ones are getting the special treatment as well. If I didn't care about the off axis performance as much, the new standard ones offer great on axis views.

Clear skies,
 

#10 KWB

KWB

    Hubble

  • *****
  • Posts: 16312
  • Joined: 30 Sep 2006
  • Loc: Westminster,Co Elev.5400 feet

Posted 04 January 2013 - 01:18 PM

This thread was initially started in the Observing area of our forums,Double Stars to be precise,so to avoid duplicity we need to direct our attentions and all our postings to that original thread.

Here's a link Double Stars thread
 






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics