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Observing >> Deep Sky Observing

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Dave MitskyModerator
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Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Bill Weir]
      #5535585 - 11/23/12 02:03 PM

Quote:

..a location where the SQM reading only gets to 21 to 21.31, ie my back yard.




Bill,

I wonder how many of us would sell our souls to have a nearby 21.3 SQM site, let alone in our backyards.


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Sarkikos
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Loc: Suburban Maryland, USA
Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Dave Mitsky]
      #5535659 - 11/23/12 02:44 PM

Yeah, only 21 to 21.31. I'd have to travel six hours for that.


Mike


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GlennLeDrew
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Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Sarkikos]
      #5535999 - 11/23/12 06:23 PM

There is probably no one 'ideal' exit pupil, for B33 or any object. The relevant factors are object size, contrast against the sky and aperture, all of which play a role at all times. For any given object:

- A darker sky allows a larger exit pupil. This is because with more contrast the object need not be magnified as much.

- A larger aperture allows a larger exit pupil. This is because for unit magnification the image is brighter.

A light bucket could permit the largest exit pupil, while a small scope may necessitate a smallish pupil. In the latter case, the lesser light grasp necessitates a higher magnification per unit aperture. The overall dimmer view will necessitate more thorough dark adaption and sheilding from ambient light.


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KidOrion
professor emeritus


Reged: 07/07/07

Loc: Carbondale, IL
Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Dave Mitsky]
      #5536187 - 11/23/12 08:07 PM

Quote:

Quote:

..a location where the SQM reading only gets to 21 to 21.31, ie my back yard.




Bill,

I wonder how many of us would sell our souls to have a nearby 21.3 SQM site, let alone in our backyards.




*raises hand, whistles*


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Carol L

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Reged: 07/05/04

Loc: Tomahawk, WI 45N//89W
Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: KidOrion]
      #5536502 - 11/24/12 12:14 AM

The discussion on exit pupils has got me wondering something about my two 26mm ep's.

The first one is a 1.25" Meade Super Plossl with a 50° AFOV and a 42.6' TFOV.
The other one is the 2" QX that came with my 16" Lightbridge - it has a 70° AFOV and a 59.7' TFOV.
Both give 70x magnification with the f/4.5 16LB.

According to the exit pupil formula, a 26mm ep has an exit pupil of 5.77 - but i'm wondering if the difference in TFOVs (and eyepiece size) should also be taken into consideration when choosing an eyepiece for the Horse Head. It seems logical to me that a wider TFOV ep would allow more light in, regardless of the exit pupil formula - or am i overthinking this?


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David Knisely
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Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: ensign]
      #5536571 - 11/24/12 01:49 AM

Quote:

I'll throw my $.02 in: having searched for the HH using a 10", I finally did manage to _detect_ it with averted vision on a very clear night under a very dark sky using an Orion H Beta filter and a Nagler type 4 17mm. I think the 3.6 mm exit pupil may have helped.

Does anyone have thoughts on an ideal exit pupil for viewing the Horsehead?




I generally like exit pupils from about 5 mm to 2.5 mm for viewing the Horsehead. 4 mm tends to be something of a "sweet spot", but it depends on what you want to view. I got a great view one night in my 14 inch f/4.6 Newtonian at 52x (6.8mm exit pupil) using the H-Beta filter, when I could see both the Horsehead and the Flame Nebula at the same time in my 36mm Hyperion Aspheric Eyepiece (about an 80' arc true field). I liked the view of the Horsehead itself better at 79x (4.5 mm exit pupil) or 104x (3.4 mm exit pupil), but it was nice to see the two nebular complexes at the same time with lots of room to spare. Clear skies to you.


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Tony Flanders
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Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Carol L]
      #5536658 - 11/24/12 05:24 AM

Quote:

The discussion on exit pupils has got me wondering something about my two 26mm ep's.

The first one is a 1.25" Meade Super Plossl with a 50° AFOV and a 42.6' TFOV.
The other one is the 2" QX that came with my 16" Lightbridge - it has a 70° AFOV and a 59.7' TFOV.
Both give 70x magnification with the f/4.5 16LB.

According to the exit pupil formula, a 26mm ep has an exit pupil of 5.77 - but i'm wondering if the difference in TFOVs (and eyepiece size) should also be taken into consideration when choosing an eyepiece for the Horse Head. It seems logical to me that a wider TFOV ep would allow more light in, regardless of the exit pupil formula - or am i overthinking this?




Logic says that the eyepiece with the smaller field of view should help you see the Horsehead.

As you say, the one with the wider field of view takes in more light -- from the surrounding nebula and stars. Both eyepieces take in the same amount of light from the area right around the Horsehead, which is tiny and takes up only the part of the field that's common to both eyepieces.

So the wider field of view means that the fraction of light devoted to the object you actually want to see is smaller. In theory, the wider field of view might reduce your dark adaptation and make the Horsehead harder to see. But in practice I have never seen such an effect. Not unless a very bright star happens to stray into the field of view of the eyepiece with the wider field of view.


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stmguy
sage


Reged: 10/11/12

Loc: Western NH
Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Tony Flanders]
      #5536729 - 11/24/12 07:45 AM

I know sometimes a Barlow helps with contrast, anyone have any thoughts on this for viewing the Horse head ?
Norm


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Sarkikos
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Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: stmguy]
      #5536937 - 11/24/12 09:57 AM

Maybe better to keep as much glass out of the optical train as possible when you're looking for something as faint as IC 434 and B33. Just use a simple glass eyepiece with a relatively narrow AFOV that will give you an optimum exit pupil. Makes sense to me.

Probably the most important factor is having skies that are dark enough and transparent enough.

Mike


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BillFerris
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Loc: Flagstaff, Arizona, USA
Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Carol L]
      #5537027 - 11/24/12 10:59 AM

Quote:

According to the exit pupil formula, a 26mm ep has an exit pupil of 5.77 - but i'm wondering if the difference in TFOVs (and eyepiece size) should also be taken into consideration when choosing an eyepiece for the Horse Head. It seems logical to me that a wider TFOV ep would allow more light in, regardless of the exit pupil formula - or am i overthinking this?




As long as the object you wish to observe is fully contained within the field of view, different eyepieces used in the same telescope will deliver the same intensity light packet from that object. In other words, the object will have the same apparent integrated brightness, regardless of the exit pupils and true fields produced by the eyepieces.

Since B33 is a dark nebula, it does not have an integrated magnitude, per se. The Horsehead is lower in surface brightness than the surrounding sky and even darker than faint IC 434, the nebula against which B33 is seen. At about 4' by 4' in size, a magnification of 75X will give B33 an apparent size of 5 degrees in the eyepiece. This is more than large enough for the eye to discern the Horsehead as an extended object. 30X magnification will present it as 2 degrees in apparent size, which is still large enough for the dark-adapted eye to discern and is probably a more useful magnification for small apertures.

With 1.9 magnitude Alnitak just one-half degree to the north, a modest true field of view offers the advantage of preventing the bright star from limiting your dark adaptation. I've found a true field of about 45 arcminutes works quite well to frame the B33 & IC 434 complex for viewing. I used a Meade 13.8mm SWA eyepiece with my old 10 inch Meade Starfinder and a 22mm Nagler T4 with the 18 inch Obsession.

Bill in Flag


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Carol L

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Reged: 07/05/04

Loc: Tomahawk, WI 45N//89W
Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Tony Flanders]
      #5537083 - 11/24/12 11:28 AM

Thanks for the explanations, Tony and Bill.
I'll definitely experiment with my ep's this winter to see which is best on the Horse Head.


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Dave MitskyModerator
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Re: Bound and determined : Horsehead thru a ten..BUT.. [Re: Carol L]
      #5537178 - 11/24/12 12:31 PM

While I've seen B33 at small exit pupils, it has been much easier in my experience to make out its shape when using eyepieces producing exit pupils of 4 to 5 millimeters. Employing eyepieces with smaller apparent fields of view such as Plössls does seem to make the task easier.

Dave Mitsky


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Starman1
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Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Dave Mitsky]
      #5537506 - 11/24/12 04:45 PM

1) I searched for it for 11 years with an 8" but never saw it. That, from a high altitude site in a Blue LP Zone.
2) Saw it on first try, without filter, with a 12.5". I was surprised how large it was.
3) An H-Beta filter definitely helps, by accenting the nebula IC434 and suppressing star light.
4) My favorite view is at 140X (42' field, 2.3mm exit pupil) because the contrast is excellent. I was surprised by how LARGE the HH was--perhaps 1/5 the width of the entire FOV. I hadn't seen it before, I think, because I was looking for something much much smaller.
5) Carol's drawing is EXCELLENT.


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starrancher
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Reged: 06/09/09

Loc: Northern Arizona
Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Starman1]
      #5537538 - 11/24/12 05:02 PM

Quote:

1) I searched for it for 11 years with an 8" but never saw it. That, from a high altitude site in a Blue LP Zone.
2) Saw it on first try, without filter, with a 12.5". I was surprised how large it was.
3) An H-Beta filter definitely helps, by accenting the nebula IC434 and suppressing star light.
4) My favorite view is at 140X (42' field, 2.3mm exit pupil) because the contrast is excellent. I was surprised by how LARGE the HH was--perhaps 1/5 the width of the entire FOV. I hadn't seen it before, I think, because I was looking for something much much smaller.
5) Carol's drawing is EXCELLENT.




Excellent tips and explanation on how to increase the possibilities . Between this post and Carols drawing , this is great help in what to look for and what to expect .


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Carol L

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Reged: 07/05/04

Loc: Tomahawk, WI 45N//89W
Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Dave Mitsky]
      #5538216 - 11/25/12 02:32 AM

Quote:

While I've seen B33 at small exit pupils, it has been much easier in my experience to make out its shape when using eyepieces producing exit pupils of 4 to 5 millimeters. Employing eyepieces with smaller apparent fields of view such as Plössls does seem to make the task easier.

Dave Mitsky




Thanks for the extra info, Dave.
The sketch was done using a 32mm Plossl that yields an exit pupil of 7.11 - way too much according to what i've read here. I'll try a 20mm Plossl ASAP - the exit pupil is 4.44.


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Carol L

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Loc: Tomahawk, WI 45N//89W
Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Starman1]
      #5538225 - 11/25/12 02:54 AM

Quote:

4) My favorite view is at 140X (42' field, 2.3mm exit pupil) because the contrast is excellent. I was surprised by how LARGE the HH was--perhaps 1/5 the width of the entire FOV. I hadn't seen it before, I think, because I was looking for something much much smaller.
5) Carol's drawing is EXCELLENT.




Thanks Don! What scope/ep do you use to get your favorite view? I've got a number of eyepieces and plan on experimenting with them ASAP to see which one is best on the HH. And i agree with you regarding the size of the nebula - it's much bigger than i'd thought it would be.


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Carol L

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Loc: Tomahawk, WI 45N//89W
Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: starrancher]
      #5538231 - 11/25/12 03:09 AM

Quote:

Between this post and Carols drawing , this is great help in what to look for and what to expect .





Thanks Dave, this thread has been a wealth of info for me too - it's been quite an educational discussion!


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Starman1
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Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Carol L]
      #5538458 - 11/25/12 10:04 AM

Quote:

Quote:

4) My favorite view is at 140X (42' field, 2.3mm exit pupil) because the contrast is excellent. I was surprised by how LARGE the HH was--perhaps 1/5 the width of the entire FOV. I hadn't seen it before, I think, because I was looking for something much much smaller.
5) Carol's drawing is EXCELLENT.




Thanks Don! What scope/ep do you use to get your favorite view? I've got a number of eyepieces and plan on experimenting with them ASAP to see which one is best on the HH. And i agree with you regarding the size of the nebula - it's much bigger than i'd thought it would be.



My favorite view is the one with the best contrast. Usually, that's my 12.5" with a 13 Ethos. It was very nice through a 24" f/4.5 last year, though.
I guess the point is, if you can see it easily, and conditions are good, it's a nice object to view, and sketch!


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tcmzodiac
professor emeritus


Reged: 11/11/11

Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: Starman1]
      #5538497 - 11/25/12 10:36 AM

Im very grateful for the quality of discussion. Many thanks, I am learning a bunch! Now...if only the Evil Orb will kindly leave the stage.....

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Starman1
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Re: Bound and determined : Horsehead thru a ten..BUT.. new [Re: tcmzodiac]
      #5538517 - 11/25/12 10:49 AM

Quote:

Im very grateful for the quality of discussion. Many thanks, I am learning a bunch! Now...if only the Evil Orb will kindly leave the stage.....



Many years ago, I had a t-shirt that said:
"Fight Light Pollution. Nuke the Moon!"


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