Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Speed vs Aperture with NV

NV
  • Please log in to reply
16 replies to this topic

#1 GOLGO13

GOLGO13

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,309
  • Joined: 05 Nov 2005
  • Loc: West Virginia

Posted 07 August 2020 - 07:32 PM

Throughout reading comments in the NV group, I see discussions of speed and aperture. What I am unsure of is what helps the most. From my personal experience it seems like aperture helps a lot. But then I wonder if that's just be getting a bit more image scale because of a longer focal length. For instance, I see a pretty obvious improvement going from my 6 inch F4 to my 10 inch F4.7. And I'm not saying the 6 inch F4 is bad because it's actually quite good. But the 10 inch does better on almost every object (as long as it fits in FOV). 

 

The times I notice the speed benefit is when using something like my 4 inch F7.7 which is an excellent scope, but doesn't do too well on NV nebulas...I have not tried it too much otherwise.  That being said, some of my best views with the NV has been with the 8 inch F10 on Globulars and smaller DSOs. It worked so good I was super glad I didn't sell that scope as I was trying to make up for the NV purchases. I have not tried the 8 inch F10 on many larger nebulas, but I want to give it a bit of a try with maybe my 12nm HA filter. 

 

I think NV is a tricky dance of speed, aperture, and framing objects.

 

A bit of discussion on framing objects and why I think having many scopes really helps. Really large nebulas need a wide field of view. Even the 6 inch F4 needs some serious reduction to fit some of the larger nebulas in the view. However, seeing some detail on objects requires the 8 inch F10 for globulars and the 10 inch F4.7 for getting close enough views of the Eagle Nebula and the Horse Head Nebula. So really, my personal opinion is having multiple scopes is pretty  important.


  • chemisted likes this

#2 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 27,011
  • Joined: 01 Feb 2006

Posted 07 August 2020 - 08:08 PM

The difference between f/4 and f/4.9 is not that great and my own opinion is that sometimes having more image scale is more desirable than having a smaller, faster aperture with very small image scale.  The thing though is that every subject is unique and sometimes speed is the most important thing and sometimes aperture is the most important thing.

 

I use a Barlow in my 12" all the time for clusters and globulars, but not so much for nebula, though I use it at f/4.9 for many nebula. 

 

But let's look at the Pillars of Creation.   The Eagle Nebula is much brighter in my 6" f/2.8, but the scale is so small that I oniy see the pillars as a kind of tiny, unresolved dark smudge.  In the 12" f/4.9, the pillars are more than 3 times larger, and while the view is noisier, the pillars are very well resolved.

 

I am not so obsessed with speed.  I have seen nebula at f/2.8 that is almost at the limit of the boundaries shown for dim nebula in Sky Safari H-a overlay.  I have seen the nebula that M29 sits in under dark sky with the 6" f/2.8 very easily, but I have also seen it from my red zone in the 12" at f/4.9.  Noisy, but I could see it.   And that is the thing about speed.  You get the benefit of a less noisy view, but I have found that even faint detail will still be detectable, And I am often happy to accept the higher noise level to get the bigger image scale.

 

 

Everyone should find out what works best for them on a given subject, but modern low noise tubes can still work quite well at slower focal ratios and the benefit of image scale is often more important than the increase in noise.  I have seen the Pillars of Creation from a red zone using a 12" f/4.9 scope at f/4.9.   That, in my book, is pretty awesome. Had I only tried it with my 6" f/2.8, I would have never really seen it.


Edited by Eddgie, 08 August 2020 - 08:49 AM.

  • GeezerGazer and chemisted like this

#3 GOLGO13

GOLGO13

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,309
  • Joined: 05 Nov 2005
  • Loc: West Virginia

Posted 07 August 2020 - 08:27 PM

I agree Eddgie, the Eagle is a perfect example where the larger dob does better. I get the same result in the smaller scope.

I was very surprised how much I liked the 8 inch F10 though. Pretty sweet. I am going to give it some more tries to see how it goes.

While I find my 10 inch is my favorite scope so far for NV, I think a good combo is the 6 inch F4 and the 8 inch F10. I think it would be a tough call what I would bring to a dark sky.

Edited by GOLGO13, 07 August 2020 - 08:27 PM.


#4 nimitz69

nimitz69

    Mercury-Atlas

  • *****
  • Posts: 2,617
  • Joined: 21 Apr 2017
  • Loc: A barrier island 18 miles south of Cocoa Beach

Posted 08 August 2020 - 07:12 AM

My experience with NV is a little different that a lot of folks here as I only have a 14” f/4.6 Dob for visual. My 80mm APO refractor is permanently setup in my observatory for AP and I only got to use my NV with it once at a Club outreach event before it went into the observatory.
Therefore my NV observing is at the extremes; 1x - 8x from my ENVIS - 200mm DSLR and then up to 165x through the Dob. Things like the ring nebula are pretty amazing at 165x. I know I’m giving up that middle range by not having a small, fast wide field scope and I should really look into setting one up but giving that for large parts of teh year I basically can’t do any observing i’ve Not done anything. Maybe starting next summer since I’ll be retired for good I might do it since staying up late during teh week will no longer be an issue and there may be more opportunities to observe.

Edited by nimitz69, 08 August 2020 - 07:14 AM.


#5 Jeff Morgan

Jeff Morgan

    Voyager 1

  • *****
  • Posts: 12,251
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 08 August 2020 - 11:42 AM

I think NV is a tricky dance of speed, aperture, and framing objects.

 

The tricky part for me is the filter choice for a balanced view.

 

As far as scopes, that is easier. Obviously one uses the scope one set up for the evening, but given a choice and a specific target the biggest one that can frame the object is the best choice, even at half the speed.

 

Comparing objects I have shot with multiple scopes, the larger aperture always shows more detail. Attached is an example at equal speeds, filter, and ISO: f/7 native, afocal with the 55 Plossl. IC 5146, the Cocoon nebula. One scope is 130mm aperture, the other is 400mm.

 

To "equalize" them I enlarged the shot through the smaller refractor to 300% and did a screen grab. The detail is better in the larger scope. It shows even better on objects with fine filaments, like the Eastern Veil. Unfortunately, those examples are at different exposures and filters, so not quite as "apples to apples" as this example.

Attached Thumbnails

  • Cocoon Nebula Z16 afocal.jpeg
  • Cocoon 300x.jpg


#6 M44

M44

    Viking 1

  • *****
  • Posts: 658
  • Joined: 24 Mar 2007
  • Loc: Southern California

Posted 08 August 2020 - 11:49 AM

When I started NV observing, I prepared a spread-sheet with mags, speed, exit pupil, fov etc. for my 3 telescopes.

 

Now I stopped all that and just go on observing with one of the three scopes and enjoy what they present. 

I like the views with all 3. 90mm goes wider, 130mm sharper and 14.5 dob shows brighter. This is plenty, now I don't even think of bigger or faster scope. 


  • Jeff Morgan, GeezerGazer and AllStarez like this

#7 Starman81

Starman81

    Soyuz

  • *****
  • Posts: 3,808
  • Joined: 06 Mar 2008
  • Loc: Metro Detroit, MI, USA

Posted 08 August 2020 - 02:55 PM

I was very surprised how much I liked the 8 inch F10 though. Pretty sweet. I am going to give it some more tries to see how it goes.

While I find my 10 inch is my favorite scope so far for NV, I think a good combo is the 6 inch F4 and the 8 inch F10. I think it would be a tough call what I would bring to a dark sky.

 

I have an 8" f/10 SCT and also a larger dob (11" f/4.4), so very much like your situation. I believe I have gone some similar thought processes about how they relate, so I'll share...

 

The larger dob can also be made to run at f/10, which as you mentioned is very good for globular clusters, and when you use the barlow to get up to f/10 (for a fair comparison), you will have more magnification at the same speed.

 

So a standard 26mm NVD in an 8" f/10 (f/l of 2032mm) gets you ~78x. The same NVD in a 10" f/10 (for the sake of argument we are going to assume you barlowed exactly to get to f/10) will yield ~98x.

 

So, at a given speed, increased aperture yields increased image scale (i.e magnification).  So

 

8" @f/10 - 78x

10" @f/10 - 98x

16" @f/10 -  156x

.

.

.

I'm sure this understanding is old hat for many, but I thought I'd spell it out anyways.

 

I think what makes the 8 f/10 SCT  a compelling option at times over a larger dob is 1) its easy to deploy 2) already at a good focal ratio for stellar/non-nebula targets without any barlowing 3) can be double-mounted with a wide field refractor for larger/nebular objects.

 

If your larger dob is able to be focally reduced enough, you might be able to get enough TFOV to fit a lot of large, nebular objects as well as being able to barlow-up for stellar targets. 



#8 TopQ

TopQ

    Vostok 1

  • *****
  • Posts: 155
  • Joined: 05 Nov 2008

Posted 08 August 2020 - 03:13 PM

For nebulae, I think the main reason why people prefer a larger scope (even if is not as fast) over a smaller fast scope is because the bigger instrument allows you enjoy many more objects, almost

exponentially.  For example, if you try to observe small objects (size wise) with the small telescope, the views are just not as rewarding due to the image scale. Kind of like seeing the ring nebula at low power. You can of use a Barlow lens but then you are using a small slow scope with the penalty that your small scope does not offer the resolution that a larger instrument gives you. Also the view is going to be a lot noisier.

At least for me, the best way to use NV is to couple the image scale produced by the telescope with the object that I want to see. This means having multiple fast scopes. Small scopes for the huge objects and bigger scopes for the smaller objects. Of course having multiple fast scopes is not always possible but it really is the best solution to take advantage of NV to its fullest. Again if this is not possible then the second best option is having a larger telescope, even if is not the fastest, over having only a small fast scope. 

For example I do have a 82 mm F/3.7 scope that is great for very big objects like the Veil, the North America Nebulae (120' × 100') or the California nebula (2.5° long), but completely blows for things like the Crescent nebula (NGC 6888 18′ × 12′) or even the Crab nebulae (M1 420″ × 290) due to image scale. When I use my 30" F 2.7,  the Crab nebulae looks like a picture you see in a magazine, detail wise. The Crescent is also very impressive. Also, the Horse Head is almost better than in a picture. The detail in the horse mane and nose is tremendous. Now put the 30" on the Veil and you are completely lost on the nebulae, yes there is a lot of detail in what you see but you lose the splendor of the object. Kind of like seeing the Orion nebulae at low power compared to a high power view of the trapezium only.

Again this is mainly for nebulae. For globular clusters, things are a little different since, even the smaller scopes coupled with a Barlow lens, can deliver good images with little scintillation.

In my case these are the scopes I use mainly for NV:

Modified Meade Eclipse 82 mm F/3.7

Comet Catcher 5.5" F 3.6

Meade 12.5" F/4.8

Spica 30" F2.7

 

Bruno

 

 

 

 


  • Mike Lockwood and AllStarez like this

#9 GOLGO13

GOLGO13

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,309
  • Joined: 05 Nov 2005
  • Loc: West Virginia

Posted 08 August 2020 - 03:33 PM

Oohhh. Some of my best views in the past was a 30 inch f3.3. Wish I had the NV device back then.
  • AllStarez likes this

#10 bobhen

bobhen

    Skylab

  • *****
  • Posts: 4,458
  • Joined: 25 Jun 2005

Posted 09 August 2020 - 07:16 AM

 

I think what makes the 8 f/10 SCT  a compelling option at times over a larger dob is 1) its easy to deploy 2) already at a good focal ratio for stellar/non-nebula targets without any barlowing 3) can be double-mounted with a wide field refractor for larger/nebular objects.

 

See my image below.

 

You have to look at an image intensifier not as an eyepiece used visually where aperture is about light gathering, but more as a camera in the way it interacts with a telescope.

 

Aperture is not as important with a camera as are speed and FL, or, in other words, field of view and image scale. Aperture can offer resolution advantages but seeing limits the resolution of all telescopes to 10” or less in 99% of all locations in the US on the vast majority of nights – especially nights with clear transparent skies that are conducive to deep sky observing. But deep sky observing is really about light gathering and contrast anyway.

 

An intensifier is “the great equalizer” and takes care of light grasp more so than the aperture differences between telescopes. In other words, the image brightness of the view presented in a 10” F4 versus a 6” F4 will be close to the same. What’s different is the image scale. So it’s easier to see more detail in the 10 because the image is larger not because it is immensely brighter.

 

It’s a tough call on what to take to a dark sky because image brightness is not really a factor with an intensifier. With an intensifier’s light gathering capability, a 50mm guide scope and that 10” F4 will both show most deep sky objects but the image scale will be vastly different. So one needs to take a few telescopes. 

 

A camera lens or guide scope, a fast 4” refractor or fast 6” Newtonian and something larger like an 8 to 11” SCT or 10 to 12 inch Dobsonian gives one different fields of view and images scales and all are still exceptionally to reasonably portable. The intensifier takes care of light grasp and filters take care of contrast.

 

My 102mm F5 refractor, which can be reduced to around F3.5 and my C8 which can be reduced to around F7 and Barlowed to around F20 – twin mounted.

 

Bob

Attached Thumbnails

  • IMG_0130 (1) 2.jpg

Edited by bobhen, 09 August 2020 - 07:22 AM.

  • Starman81 likes this

#11 GOLGO13

GOLGO13

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,309
  • Joined: 05 Nov 2005
  • Loc: West Virginia

Posted 09 August 2020 - 09:47 PM

OK...I gave the 8 inch F10 a good go on the Swan, Lagoon, Trifed and Eagle. I used it at native F10, .7 reduced and probably .5 reduced (not sure what you get when combining two .7 reductions).

 

Not horrible, but yes, not as good as I remember with the 10 inch dob in general.

 

The swan was still pretty decent. The Eagle was just slightly too dim to make out good detail in the pillars. It's there but not great. Trifed was decent. Lagoon was a pretty interesting view as it was quite zoomed in. Somewhat cool details because it was zoomed in.

 

Pretty decent night tonight sky wise. 

 

Kind of interesting results, but I will probably keep using the 8 inch F10 mostly for small nebulas and globulars etc. If I had more reduction capabilities, it could have been a bit of a different story. But I certainly had some decent reduction going.



#12 Jeff Morgan

Jeff Morgan

    Voyager 1

  • *****
  • Posts: 12,251
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 10 August 2020 - 03:03 AM

Kind of interesting results, but I will probably keep using the 8 inch F10 mostly for small nebulas and globulars etc. If I had more reduction capabilities, it could have been a bit of a different story. But I certainly had some decent reduction going.

 

Remember while extended sources prefer speed, point sources of light (stars, star clusters) are less affected by speed.

 

I have gotten great cluster views at f/13 even in small apertures (Meade ETX 90). Larger apertures can go even slower.



#13 GOLGO13

GOLGO13

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,309
  • Joined: 05 Nov 2005
  • Loc: West Virginia

Posted 10 August 2020 - 07:16 AM

No doubt Jeff. The 8 in F10 is great for non nebula objects or small nebulas.

#14 hoof

hoof

    Surveyor 1

  • *****
  • Posts: 1,767
  • Joined: 07 Apr 2005
  • Loc: Monroe, WA

Posted 13 August 2020 - 01:21 PM

Bobhen is right, which is why my primary viewing option is a 15" F/4.1 scope, with an attached (as a finder) 80mm F/3.75 finderscope.  I have a 56mm (soon to be TV 67mm) plossl for the big scope and a 40mm plossl for the finderscope.  Both take the TNVC adapter, both have narrowband h-a filters, so I can switch easily between 28x and 7.5x, and even 1x (got the 2" filter adapter for the TNVC adapter too, along with another 1.25" h-a filter).  Since the 80mm and the 15" are aligned, they are always looking at the same thing.  The filter adapter lets me quickly get the NV device in 1x h-a mode, useful for looking for interesting stuff, quickly lining up the 'scope through the telrad (which works with a h-alpha filter!), then quickly switch to the 'scopes (either)

 

Very nice to be able to look at wide sweeps, then switch "mags" for detail view, with similar effective brightnesses.



#15 Astrojedi

Astrojedi

    Aurora

  • *****
  • Posts: 4,766
  • Joined: 27 May 2015
  • Loc: SoCal

Posted 18 August 2020 - 12:39 PM

Both speed and aperture are equally important with NV. Speed for extended objects and aperture for smaller fainter objects. If aperture is held constant than faster speed produces the side effect of a larger fov (assuming your optical system can handle it without loss of light).


Edited by Astrojedi, 18 August 2020 - 12:40 PM.


#16 Jeff Morgan

Jeff Morgan

    Voyager 1

  • *****
  • Posts: 12,251
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 18 August 2020 - 06:51 PM

Both speed and aperture are equally important with NV. Speed for extended objects and aperture for smaller fainter objects. If aperture is held constant than faster speed produces the side effect of a larger fov (assuming your optical system can handle it without loss of light).

 

All of the scopes I have had in my possession (or had on loan) for the last four years have had a useful NV niche.

 

Some more niches than others.

 

None of them have covered all the bases.



#17 Astrojedi

Astrojedi

    Aurora

  • *****
  • Posts: 4,766
  • Joined: 27 May 2015
  • Loc: SoCal

Posted 19 August 2020 - 10:55 AM

All of the scopes I have had in my possession (or had on loan) for the last four years have had a useful NV niche.

 

Some more niches than others.

 

None of them have covered all the bases.

Not disagreeing with that. I am responding to the OP’s question. Just making the point that speed and aperture are both important. My 8” F4 just cannot match the brightness, detail and depth produced by my 14” F3.5 even if they are running at the same effective f ratio. The pillars of creation in the 14” look three dimensional.

 

Ideally if you can get both it will always be better than one or the other. But that is not always possible. Which is why I agree with the op.


Edited by Astrojedi, 19 August 2020 - 11:08 AM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: NV



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics