BST 25mm or 32mm plossl?

[Starlord19]

I use a Skymax 127 and currently have a set of plossls of various sizes and was thinking about upgrading to the BST explorers. I have a 32mn plossl as my wide view piece, my question is whether or not the 25mm BST will have a similar wide view and whether or not the difference is worth the money? 

[John]

The BST Explorer 25mm has a wider apparent field of view than a 32mm plossl (58 degrees vs 52 degrees) but the 32mm plossl will show a wider true field. If your goal is to see more sky, the £'s will not have bought that.

 

[Starlord19]

Thanks for answering, although now I am even more confused. I saw the BST advertised as having a wider fov and assumed this meant you could see more sky with it, but this is not the case? For example, the 32mm is the only eyepiece that I have in which I can see the whole moon or sun. Would this be possible with the 25mm BST?

[Louis D]

1 hour ago, Starlord19 said:

Thanks for answering, although now I am even more confused. I saw the BST advertised as having a wider fov and assumed this meant you could see more sky with it, but this is not the case? For example, the 32mm is the only eyepiece that I have in which I can see the whole moon or sun. Would this be possible with the 25mm BST?

I think you're confusing apparent field of view (AFOV) with true field of view (TFOV).  The BST is advertising it has a wider AFOV, not a wider TFOV.  You get the TFOV by dividing the AFOV by the magnification.  So, let's take your Skymax 127's 1500mm focal length and work out the TFOV for both eyepieces.  The power of the 32mm, 52 degree AFOV plossl will be 1500/32 = 47X while the power of the 60 degree 25mm BST will be 1500/25 = 60X.  Thus, the TFOV for the plossl will be 52/47 = 1.1 degrees while the TFOV for the BST will be 60/60 = 1.0 degree.  Thus, the plossl will yield an ever so slightly bigger TFOV over the BST.

[Stu]

3 minutes ago, Louis D said:

I think you're confusing apparent field of view (AFOV) with true field of view (TFOV).  The BST is advertising it has a wider AFOV, not a wider TFOV.  You get the TFOV by dividing the AFOV by the magnification.  So, let's take your Skymax 127's 1500mm focal length and work out the TFOV for both eyepieces.  The power of the 32mm, 52 degree AFOV plossl will be 1500/32 = 47X while the power of the 60 degree 25mm BST will be 1500/25 = 60X.  Thus, the TFOV for the plossl will be 52/47 = 1.1 degrees while the TFOV for the BST will be 60/60 = 1.0 degree.  Thus, the plossl will yield an ever so slightly bigger TFOV over the BST.

Quite right, and by definition the OP will still comfortably fit the full moon (which is roughly 0.5 degrees across) in the field of view, but at a higher power of x60.

[Louis D]

10 minutes ago, Stu said:

Quite right, and by definition the OP will still comfortably fit the full moon (which is roughly 0.5 degrees across) in the field of view, but at a higher power of x60.

Of course, Starlord19 could also pick up an 8mm Ethos and still see the whole moon at once at 188X with a 0.53 degree true field of view!

[Stu]

Just now, Louis D said:

Of course, Starlord19 could also pick up an 8mm Ethos and still see the whole moon at once at 188X with a 0.53 degree true field of view!

Indeed, but that wasn't the question ?

[Louis D]

Just now, Stu said:

Indeed, but that wasn't the question ?

I know, but I couldn't resist the urge.

If Starlord19 did want max field of view at higher powers, the 13mm Ethos would be the way to go for a 0.87 degree TFOV.

[Stu]

Just now, Louis D said:

I know, but I couldn't resist the urge.

If Starlord19 did want max field of view at higher powers, the 13mm Ethos would be the way to go for a 0.87 degree TFOV.

If nothing else it shows the effect of increasing afov on the true field of view. Many people choose a higher power eyepiece with a wider afov over a lower power eyepiece with a narrower afov because the result is a smaller exit pupil which helps to darken the sky background. The higher mag leads to a larger image scale who helps with perceived contrast/visibility of DSOs.

I had a 3.7 Ethos SX (110 degree afov) for a while which would fit the whole moon in in my TV85. Pretty spectacular.

[Starlord19]

I was looking at the BSTs and Celestron x-cels as possible upgrades to my solid if unspectacular current EPs. I've seen good reports for both and as a beginner thought that they seemed a sensible step up from standard plossls. I had noticed that the largest they did was 25mm in both ranges whereas plossls seem to go to 40. I have been researching afov & tfov online all afternoon. I definitely had it wrong and now think I understand it much better than before. Thanks to everyone here, I now feel a tiny bit more knowledgeable than this morning!

Mark.

[John]

I'm glad you understand a bit more and sorry if my response confused you initially.