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North America nebula in 80mm?

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#26 Carol L

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Posted 04 July 2014 - 08:30 PM

Thanks Carol, great sketch! Would LOVE to see this in my set-up, same as yours!


You're welcome, Bob - hope you get a chance soon. :)

#27 Carol L

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Posted 04 July 2014 - 08:40 PM

Two questions if I may, Carol: any particular reason why you use Plossls instead of wider angle eyepieces for these wide-field views? And, the 7-8mm exit pupil gives you no problems?
P.S. am still waiting for clear skies.


TBH, I'm not quite sure if I had any wide angle ep's at the time. The targets fit nicely in the chosen Plossls though, and to my recollection I had no problem using them.

#28 ggalilei

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Posted 05 July 2014 - 02:41 AM

Well, I finally caught a pretty decent view of this nebula. The "jet stream" part of it that follows an arc of stars is quite bright and easy to see. Other parts of the nebula are more elusive to my eye, particularly the dark "gulf" region. The OIII filter helps bringing out the nebula, but I don't enjoy the overall dimming effect on the view. Extended period of dark adaptation helped too, but most of all it's the growing familiarity with the nebula that is aiding me in viewing it more and more clearly. Initially it was easier in the wider field of view of the 35mm Pan with the OIII filter, and also somewhat visible in the 10x25 binoculars. The whole Veil nebula also fitted nicely in the 35mm Pan. I spent the most pleasant two hours out there, taking advantage of a clear and dark night. Thanks to all for your tips!

#29 REC

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Posted 05 July 2014 - 10:25 AM

Good for you, what scope?

#30 skysurfer

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Posted 05 July 2014 - 11:31 AM

With a dark sky no optical device at all is required, only a UHC or OIII filter placed before one of your eyes. I did this once.

#31 ggalilei

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Posted 05 July 2014 - 12:22 PM

I don't think my eye would make it all by itself, I seem to have a "tired" eyesight. But it's a great idea for younger eyes to try! Or one could try to place the filter in front of the finder scope.
In answer to the previous question: I used the SV80ST, fl = 480mm.

#32 Carol L

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Posted 06 July 2014 - 06:39 PM

Great, glad you got to see them! :)

#33 ggalilei

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Posted 07 July 2014 - 10:06 AM

Now I'm debating whether it would be worth purchasing the TV 55mm Plossl to get a 3.3 degree field of view in my TSA 102 (fl = 816mm) and presumably increase the brightness of the view. I'm trying to figure out what other uses I would have for it, and what limitations the larger exit pupil would introduce. Maybe I'll ask on the eyepiece section.

#34 Jon Isaacs

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Posted 07 July 2014 - 10:40 AM

Now I'm debating whether it would be worth purchasing the TV 55mm Plossl to get a 3.3 degree field of view in my TSA 102 (fl = 816mm) and presumably increase the brightness of the view. I'm trying to figure out what other uses I would have for it, and what limitations the larger exit pupil would introduce. Maybe I'll ask on the eyepiece section.


To know whether that would be worthwhile, you would really need to know the diameter of your dark adapted pupil. A 40mm eyepiece provides a 5mm exit pupil which could well be optimal.

Jon

#35 ggalilei

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Posted 07 July 2014 - 10:52 AM

I understand that the larger exit pupil is at least a "waste" of light, but could it be an acceptable compromise? Each eyepiece has its advantages and disadvantages. I don't understand how bad the large exit pupil problem is. Is it mainly decreased brightness, or what else?

#36 Jon Isaacs

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Posted 07 July 2014 - 12:22 PM

I understand that the larger exit pupil is at least a "waste" of light, but could it be an acceptable compromise? Each eyepiece has its advantages and disadvantages. I don't understand how bad the large exit pupil problem is. Is it mainly decreased brightness, or what else?


Tonio:

In a refractor, there is no loss of brightness if the exit pupil is larger than your dilated pupil, you are giving up AFoV and magnification, both of which are useful. A 40mm SWA provides the same TFoV as a 55mm Plossl so if you are looking to maximize the TFoV and image brightness, that would likely be the better choice.

Jon

#37 Chuck Hards

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Posted 07 July 2014 - 12:43 PM

As important as a dark sky, maybe more so, is transparency. I've seen NGC 7000 naked-eye several times from our club observatory, which is located at the border of a yellow/orange zone. Hardly pristine, dark skies. But the times it was obvious, the air was absolutely free of dust and the humidity was very low.

#38 ggalilei

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Posted 07 July 2014 - 03:59 PM

Well, that is indeed the crux of the matter! I thought I'd give up brightness (which could be acceptable to get a wider FOV), but if in fact I'd be giving up FOV, then the whole idea is fruitless! Thanks Jon,
Tonino

#39 Jon Isaacs

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Posted 08 July 2014 - 06:36 AM

Well, that is indeed the crux of the matter! I thought I'd give up brightness (which could be acceptable to get a wider FOV), but if in fact I'd be giving up FOV, then the whole idea is fruitless! Thanks Jon,
Tonino


Tonino:

Actually I made an error in what I had written. What I should have said was something like:

If you dark adapted pupil is 7mm and you are using an F/6 scope, then in comparison to the 55mm Plossl, the 41mm SWA will provide the same image brightness, the same TFoV but 34% greater AFoV and magnification.

Another way to think about it with the 55mm Plossl that your 80mm F/6 would effectively be an 61mm F/7.9. The magnification would be about 9x so what you would have is a 9x60 finder... not a major step up from a 9x50 finder..

Jon

#40 ggalilei

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Posted 08 July 2014 - 07:42 AM

Jon, you made no error, that's what you had said. It's I who jumped to incorrect conclusions. I appreciate the clarification. Now what I have to do is try to sketch the "optical geometry" so that I can understand it. Thanks!

P.S. OK, I believe I understand it now. The exit pupil is the image of the objective lens and all the light caught by it; if you don't catch it all, you're effectively using a smaller aperture scope of the same focal length. Until now I naively thought the exit pupil was an image of the object you were trying to observe.

#41 rb1082

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Posted 08 July 2014 - 03:23 PM

Seems like some have seen NA without a filter in light polluted skies. I'm having a hard time finding it. I go to Deneb, then head down (toward the horizon) and down and east. Using a standard issue 8" dob with 25mm plossl that came with it, no filters. Is this a issue with me looking at the wrong spot? Any tips on how to get there from Deneb would be appreciate. Feel free to move this to Beginners if more appropriate, just figured since you're already on the topic, I'd ask here.

Also, would any of the intro level planatery color or a moon filter help?

#42 ggalilei

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Posted 08 July 2014 - 05:02 PM

The NA doesn't jump at you, so just going towards the horizon from Deneb may not be good enough to find it. Either by naked eye or (if you are in a light polluted area) using a finder scope move the telescope slightly from Deneb to Xi Gygni, a couple of degrees away. The nebula starts a short distance away from Xi and extends towards Deneb. Familiarize yourself with the brightest stars in that area from a chart: for instance, stars 56 and 57 are the brightest ones and point to the NA nebula. You should see those in the finder scope or the telescope at low power. Use as low power as possible in your telescope. All in all, it is not an easy target, it will take patience.
P.S. I think that any of the filters you mention would make it worse, so try without any filter, unless you have a "nebula" filter.

#43 17.5Dob

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Posted 08 July 2014 - 06:16 PM

Here's the NA taken a couple of weeks ago, with a 70mm FOCAL LENGTH camera lens, an un-modified dSLR, in a deep red zone, without an LP filter. You can use it for a finder chart.

Posted Image

#44 Jon Isaacs

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Posted 08 July 2014 - 06:42 PM

Seems like some have seen NA without a filter in light polluted skies. I'm having a hard time finding it. I go to Deneb, then head down (toward the horizon) and down and east. Using a standard issue 8" dob with 25mm plossl that came with it, no filters. Is this a issue with me looking at the wrong spot? Any tips on how to get there from Deneb would be appreciate. Feel free to move this to Beginners if more appropriate, just figured since you're already on the topic, I'd ask here.

Also, would any of the intro level planatery color or a moon filter help?


The North American Nebula is quite large, about 2 degrees across. With a 25mm Plossl in your XT-8, your True Field of View is about 1.1 degrees, it's too narrow to see the North American.

There may be some who see the NAN from an urban light polluted site without a filter but I doubt very much that it would be from a city as larger as LA or even San Diego. I would consider it a real success if I were able to see the NA from my San Diego backyard with a UHC or O-III filter.

It's large and diffuse and best seen from a dark site where the regions nebulosity can be seen naked eye. From Los Angeles, I would be looking south at Scorpio and Sagittarius..

Jon

#45 ggalilei

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Posted 08 July 2014 - 07:26 PM

"A 40mm eyepiece provides a 5mm exit pupil which could well be optimal." OK, I took your advice the best I could, Jon. I'm not ready to go for the 41mm Pan, but I ordered the WO 40mm SWAN. Hopefully it will be good enough for my f/8 refractor. If not, it's not a great investment. Thanks, Tonino.

#46 rb1082

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Posted 10 July 2014 - 01:10 AM

Thank you very much for the detailed explanation. Appreciate the knowledge sharing. Beautiful pic of the nebula too! Stunning! Can't wait to see it myself on a darker night.

#47 ggalilei

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Posted 10 July 2014 - 01:15 AM

Oh, I finally managed to understand that image: it's a very broad field of view! Deneb is the brightest star and Xi is the orange star at about seven o'clock from Deneb, with the NA nebula looking smallish in between them. Very nice!

#48 mattc_uk

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Posted 10 July 2014 - 09:06 AM

I've seen great views of the Veil nebs through my 250mm reflector and OIII but never really tried the NA neb becuase I was always told its too faint. You've all inspired me to have a go this summer. (I'll have to wait a few more weeks yet as I live in the UK where June/early Jul is a twilight washout thanks to our high latitude)

#49 ggalilei

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Posted 10 July 2014 - 09:32 AM

The hardest part may be to get a field of view wide enough.

#50 blb

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Posted 11 July 2014 - 11:40 AM

My skies are reasonably dark, green zone, about 5.3 or better limiting magnitude. How easy/difficult should it be to get a convincing view of this nebula, and how wide is it? I'm thinking of using an f/6 80mm (triplet), and I don't have a nebula filter.

A good low power eyepiece should give you a wide enough field-of-view to observe NGC 7000, the North America nebula. From a dark sky sight, this nebula is visible naked eye as a small hazy spot. With my 10x50 binoculars verification that this is NGC 7000 is possible. So from a dark sky site you snould have no trouble seeing the NA nebula with your 80mm refractor and your lowest power eyepiece. If you have trouble, wait for a better night or go to a darker site.






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