Eyepiece magnification vs FOV on DSOs

[Stu_]

Hi, I've been looking for a new 1.25" eyepiece, for use on DSOs. Is it better to have higher mag with wider FOV or lower mag with narrow FOV? As they can give the same TFOV.

I'm guessing with a higher mag @ 80* or greater, this will allow you to look around the object & see more detail. Or with higher magnification, do you loose detail due to the atmosphere?

I will be using it in a f5 150p newt & a 127 mak.

Cheers

[Stu]

I would consider it as much from an exit pupil perspective as anything else. A 2mm exit pupil is very good for DSO observations.

To directly relate this to your question, generally it is better to go for higher magnification and wider afov as this will give a darker sky background ( because of the smaller exit pupil) and better contrast. Atmospheric seeing is generally more of a problem at high mags eg for planetary observing than for medium magnification. Sky transparency is more important for DSO's and galaxies in particular.

Stu

Sent from my iPhone using Tapatalk

[michael.h.f.wilkinson]

great advice from Stu.

For DSO viewing using 1.25" I would say one of the best deals at the moment is the MaxVision 24mm 68 deg EP. It gives the maximum TFOV for 1.25" format. It works very well in my F/6 scope at an exit pupil of 4mm. It is reported to work well with F/5 scopes as well. This EP stands its ground when compared with the likes of the Nagler 22mm T4 (2.2mm exit pupil in my F/10 SCT, so my main galaxy hunter). The MV 24 has a smaller FOV than the 22T4, but has very good image quality. It would give about 2mm exit pupil in your mak, but 4.8 in your Newtonian (VERY nice for wide field). To get 2mm exit pupil in the Newtonian something like an 11mm Nagler (or my 12T4) is very nice indeed.

[Moonshane]

the two scopes you have demonstrate well the importance of exit pupil and magnification when making eyepiece choices. the attached file might help with this.

eyepieces and scopes.xls

in general with a fast scope (your newtonian at f5) you are better with more magnification and a wider field to achieve higher magnification and a smaller exit pupil (aim for below 5-6mm) with the same field. with the Mak you have a slow scope (f12) and this can yield a very small exit pupil (aim for no less than 0.5-1mm).

so with a 24mm 68 degree wide field eyepiece and a 32mm 50 degree eyepiece you'd get the same field but different results:

f5

24mm = 31x magnification 2.19 degrees  TFOV, and exit pupil of 4.8mm

32mm = 23x - 2.17 TFOV - 6.4mm exit pupil

f12

24mm = 64x magnification 1.06 degrees  TFOV, and exit pupil of 2mm exit pupil

32mm = 48x - 1.04 TFOV - 2.6mm exit pupil

here's my logic of eyepiece choice which may help you with any luck

http://stargazerslounge.com/topic/188794-eyepiece-groups/

up to about 100x, most scopes can cope and the conditions rarely reduce the seeing to such an extent that this is not usable. other than this, the main consideration for me is exit pupil but be aware that almost everything (with few exceptions) will fit into a 1 degree field and most within a half degree field so don't worry about the field too much.

[Stu_]

Thanks for the replies.

That's good to know, I've got a 9mm x-cel LX, which gives me a 1.8mm exit pupil on my 150p, just need a clear night now, to see how I get on.

Will a 3-4 exit pupil give better views? As I'm thinking of a 15-20mm eyepiece, with a wide FOV to go with my 9mm.

Cheers

[michael.h.f.wilkinson]

Thanks for the replies.

That's good to know, I've got a 9mm x-cel LX, which gives me a 1.8mm exit pupil on my 150p, just need a clear night now, to see how I get on.

Will a 3-4 exit pupil give better views? As I'm thinking of a 15-20mm eyepiece, with a wide FOV to go with my 9mm.

Cheers

3-4mm or even 5 is very good if you have darker skies (and more extended objects). I have used the Nagler 31mm (3.1mm exit pupil in my SCT) and the 42mm LVW (4.2mm exit pupil) quite a bit in that role. From a dark site Markarian's chain was gorgeous with those EPs. The 20mm and 24mm 68 deg MaxVisions are really excellent EPs, especially given that you pay 1/3rd of the price of these EPs if they fly the Meade SWA branding. The 16 is nice, but the eye relief is not enough for those with glasses (like me). For those without glasses, the 16mm MaxVision is fine for those who observe without glasses

[Stu]

Thanks for the replies.

That's good to know, I've got a 9mm x-cel LX, which gives me a 1.8mm exit pupil on my 150p, just need a clear night now, to see how I get on.

Will a 3-4 exit pupil give better views? As I'm thinking of a 15-20mm eyepiece, with a wide FOV to go with my 9mm.

Cheers

3-4mm will not necessarily be 'better', but would generally be good for wider fields of view for instance for open clusters and larger nebulae under darker skies

Sent from my iPhone using Tapatalk

[AlexB67]

It is quite a complex thing exit pupils and DSOs and contrast perception, an while the rule smaller exit pupil resulting in a the ability pick out DSO is often said to work well around 2mm, and I find to be true often thanks to my polluted skies. The few times I've been able to push my scope under better skies show that some great views can be had with exit pupils of 4 or even greater on galaxies, which results in a washout in my backyard skies. 

There is some interesting stuff on Clark's page and the development of the OMVA concept. I've not had enough experience to really test this out. There have been developments on this concept like the threshold method. it seems though, depending what it is that you want to see varying magnifications can give different results  in terms of picking out specific details.

see for example

http://www.clarkvision.com/articles/visastro/m51-mag/index.html

I have his book, As Cark admitted later  the ODM calculation  is too flawed to be reliable in many cases for best contrast perception in his text. The threshold method  is another such variation on a theme. I use the latter in my own calculator and having tested on a few objects so far feel there is an element of truth in providing reasonable magnification guides for  DSOs.

http://www.uv.es/jrtorres/tools.html

Packages such as sky tools actually use such algortihms with refinements to predict visibility and suggested eyepieces and magnifications to use for a given scope to predict visibility as a guide. On the whole from the starter edition I have of skytools I've found it to underestimate, but it gives an idea, but many times it said no you cant see that, but I did and this is largely because ultimately it is based on average surface brightness, rather than a brightness profile.

It is not always what it seems and low mag wider exit pupils can sometimes be better than higher mags depending on  size of object, even under modest skies sky conditions and what you want to pick out.

In the end of course it is much simpler to stick in a few eyepieces and see what happens, but it is fun all the same to be able to make some predictions and correlate with observations, it is part of the fun in the hobby, for me anyway 

[John]

I tend to go for an ultra wide field of view and higher magnification when viewing deep sky objects and comets (which have similar characteristics) because where I do 99% of my observing has some light pollution. Mind you I've also had more impressive views when using a similar formula under dark skies so there you go

"Low and wide" is very useful to locate faint objects, view them in the context of their star field and to observe the larger ones. "Medium and wide" or even "High and wide" has produced some stunning views for me of planetary nebulae, globular clusters and galaxies. The recent supernova in the galaxy M82 was a case in point where a 100 degree 8mm eyepiece delivering 199x with my 12" dobsonian showed wonderful close up structure and contrast variation in this fascinating galaxy with the supernova gleaming out strongly when it was at it's brightest.

Very non-analytical compared to Alex's approach but it works for me     

[AlexB67]

I agree it very much depends on type of objects, PN so far I my experience pump up the mag, pump up the mag I say, depending on your aperture how far you push that.   Those sort of patterns do drop of the calculations quite well for different types of DSO and aperture sizes. Galaxies depends, they are so varied, it is hard to generalise, size, surface brightness, sky barkground darkening all come into it what works best is hard to predict .  John, I had some great views of M81/M82 as well around the SN days, 100x - 200x were all used.

[John]

....  John, I had some great views of M81/M82 as well around the SN days, 100x - 200x were all used. 

Thats great Alex. It was so good to have a really bright SN is such a fascinating and easy to find galaxy. As well as observing the SN I found I studied M82 much more than I'd ever done before