A little confused about FOV and magnification

[Orac]

I’m looking to upgrade my current set. I recently purchased the BST Starguider 25 mm. Now I am looking for a low power eyepiece. But when I use astronomy.tools, add my scope (Skymax 127) and compare my 25mm with say a Vixen NPL 30mm. I get the same FOV. The Vixen gives 50x magnification and the BST 60x. I understand that this is because of the FOV of the eyepieces but what of the projected image? Is the image of the Vixen a bit more bright/more detail then the BST? Still a bit confused. 

[John]

The level of brightness and detail is determined by the aperture of your scope I believe.

These two eyepieces will show a 1 degree true field of view with the 127mm mak (ie: the same amount of sky is visible). The background sky might be slightly darker with the 25mm BST because of the higher magnification that it delivers.

To see more sky, you would need something like a 32mm plossl with a 52 degree apparent field or a 24mm super wide with a 68 degree apparent field. These will give you marginally more true field  - 1.1 degrees. Not a big gain really but as good as the 1.25 inch format can deliver.

I don't know if that answers your queries ?

 

[Geoff Lister]

19 minutes ago, John said:

To see more sky, you would need something like a 32mm plossl with a 52 degree apparent field

I nearly always start an observing session with my Celestron 32mm Plossl with most of my OTAs, including my 127mm Mak. It's great for finding objects before going for higher magnifications.

Geoff

[Orac]

Thank you John. Yes, I think I do. So am I correct in thinking that the Vixen will show a little more of the fuzzies then the BST? 

Thank you for the tip of the 32 mm plossl. I will look into that. 

[John]

4 minutes ago, Orac said:

 So am I correct in thinking that the Vixen will show a little more of the fuzzies then the BST? 

 

Not really. They both show the same amount of sky so will show fuzzies the same way. The slightly higher magnification of the BST might help pick the fainter ones up against a slightly darker background sky.

 

[Orac]

Now that surprises me. But it makes sense when I come to think about it. 

[Stu]

24 minutes ago, Orac said:

Now that surprises me. But it makes sense when I come to think about it. 

Visibility of deep sky objects is a fairly complex topic, I won't pretend to understand more than the basics. Simply put, increasing the magnification dims the sky background, but also dims extended objects like faint galaxies. However, your eye can detect the contrast of larger objects more easily so the slightly higher mag will probably show things a little better under any sort of light pollution.

Big scopes work by allowing you to magnify objects more whilst maintaining their surface brightness. The larger size is easier to detect.

[Louis D]

Fuzzies will show as brighter on a per unit basis because your exit pupil is larger with the lower powered eyepiece.  Think of it like a focal reducer concentrating the same light cone into a smaller image circle.  This effect can make nebula filters like OIII types more effective by making the nebula brighter to the eye.  You lose some resolution of fine details, but gain in contrast.

If you want a truly wider field of view with your 127 Mak, you can go the route I went.  Get a Mak to SCT adapter, a 2" SCT visual back, a 2" diagonal, and a widest field 2" eyepiece.  Surprisingly, vignetting isn't really noticeable.  The only thing I notice is when bright objects pass the edge of the Mak's rear port, they cast an oval reflection of some sort.  Don't try a 2" SCT diagonal with that little Mak.  There isn't enough clearance with the focus knob to work.  Of course, you'll spend nearly as much as you spent on the scope if you go this route.  I don't know if they're selling it in the UK, but here in the states, Sky-Watcher is selling their 127 Mak with SCT threads, a 2" visual back, a 2" diagonal, and a starter 2" eyepiece.

[John]

7 minutes ago, Louis D said:

Fuzzies will show as brighter on a per unit basis because your exit pupil is larger with the lower powered eyepiece.  Think of it like a focal reducer concentrating the same light cone into a smaller image circle.  This effect can make nebula filters like OIII types more effective by making the nebula brighter to the eye.  You lose some resolution of fine details, but gain in contrast....

 

I'm not saying that this is incorrect Louis but it does seem, on the surface at least, to contradict what I posted and I think what Stu has posted.

Without getting into too much complexity, is there a way to explore this a little further to provide more clarity for Orac ?

Thanks

[Orac]

Basic rule of thumb: you notice fuzzies more in a bigger FOV eyepiece?

[Louis D]

10 minutes ago, John said:

I'm not saying that this is incorrect Louis but it does seem, on the surface at least, to contradict what I posted and I think what Stu has posted.

Without getting into too much complexity, is there a way to explore this a little further to provide more clarity for Orac ?

Thanks

Think about taking it to the other extreme where you push the magnification way up.  You often times end up looking right through the nebula because you've spread out the light so much.  It then becomes detectable only by sweeping past it to detect the edges.  Decreasing the magnification concentrates the available light flux into a smaller area to be projected onto your retina by the eyepiece.  Compare it to what happens when you use a magnifying glass to concentrate the image of the sun into a smaller spot.  Same light flux, smaller area, brighter per unit intensity.  The downside of this at night is of course that the sky background gets brighter at the same rate.  There may or may not be any change in relative contrast between the sky background and the nebula.  This is where the aggressive OIII or UHC filter comes into play, blocking the brighter skyglow, but passing the brighter nebula.  Once the exit pupil matches your eye's pupil, decreasing the magnification further won't make the nebula any brighter, though.

[Ricochet]

4 minutes ago, John said:

I'm not saying that this is incorrect Louis but it does seem, on the surface at least, to contradict what I posted and I think what Stu has posted.

Without getting into too much complexity, is there a way to explore this a little further to provide more clarity for Orac ?

Thanks

You are all correct. Increasing the eyepiece focal length increases the exit pupil which increases the brightness of extended objects such as "fuzzies". However, the sky is also an extended object so the sky brightness increases by the same amount and the level of contrast does not change. 

If you have no filter then increasing the magnification makes the object a little bit larger and easier to see. Aiming for an exit pupil of around 2mm, which is a magnification of half your telescope's aperture (in mm), is often quoted as the "ideal". 

If the object is a nebula then you can use a filter, i.e. uhc or oIII, to dim the background more than the nebula. In this case you might want the longer focal length eyepiece to increase the nebula brightness without also increasing the background brightness. 

With regards to the initial FoV question, you can approximate the True Field of View (TFoV) that a telescope-eyepiece combination will give by dividing the Apparent Field of View (AFoV) of the eyepiece by the magnification the combination provides. That is: 

TFoV ~ AFoV/magnification 

2 minutes ago, Orac said:

Basic rule of thumb: you notice fuzzies more in a bigger FOV eyepiece?

Not necessarily. If a faint object is larger than the FoV then you won't see it easily because you won't see the contrast between the object and background sky. However for some objects a bright star in the FoV will make it more difficult observe an object. 

[Louis D]

2 minutes ago, Orac said:

Basic rule of thumb: you notice fuzzies more in a bigger FOV eyepiece?

You can detect larger faint fuzzies more effectively once the edges have been well framed by the FOV so you can see the contrast between the nebula and the sky background.

[John]

4 minutes ago, Orac said:

Basic rule of thumb: you notice fuzzies more in a bigger FOV eyepiece?

If the magnification is higher then, under my skies, I do find small and medium sized faint deep sky objects easier to spot. Louis is right to point out though that using too much magnification can cause you to "look right through"  large, extended deep sky objects.

So I guess the answer is "it depends"

[Stu]

I did say it was complicated

I think we are all adding additional info and are all correct.

The point about exit pupil and filtered views is important though. For unfiltered views then getting the balance of image scale and brightness right is just that, a balance. However, for filtered views using UHC and OIII filters for instance, using a larger exit pupil is a good thing as it increases the brightness of the object whilst the filter keeps the sky background under control, result is better contrast. I’m just repeating what LouisD said really, but I think it does bear repeating as it is a complex topic.

[Orac]

19 hours ago, Stu said:

I did say it was complicated

Thank you all for chipping in. I can start to grasp why people have so many EPs. I thought four would be more then enough. There are so many things to factor in (let us not forget budget). Many of the long treads in the eye piece forum actually start to make sense now.