7mm vs 9mm Delites for BV'ing

[vineyard]

Hello,

Hope everyone is safe & well.  I had a really nice session just being hypnotised by the moon yesterday through a BV.  Used it with a pair of 18.2mm Delites.  And unsurprisingly found myself wanting to be able to get closer in as well.  So I'm thinking of going for either 9mm or 7mm Delites (I have some intermediate focal length ones that I can sell for part-financing of that!).  I'm sure both of those EPs are individually excellent.  But wanted to see if anyone had tried both sets in BVs and whether there were any form factor differences?  Would it be better to go for 7mm to get closer in quicker, or stay to 9mm as that then allows for an eventual future even shorter focal length EP as well as a 2x step from the 18.2mm?

Cheers,

Vin

(PS - I should have said my main scope for BV'ing is 4" 880mm f/l)

Edited April 8, 2020 by vineyard

[Don Pensack]

9mm.  You will find it is a great magnification and comfortable exit pupil.

If the OCA on your binoviewer applies a 2X magnification factor, then a pair of 13mm might be better as a second choice after the 18.2mm, or, perhaps, 11mm.

But if your binoviewers do not have an amplifying OCA, then 9mm for sure.

[Timebandit]

 

 

Hello. Hope I do not put a spanner in the works. But if you can live with tighter eye relief. Then you could save yourself a considerable sum of money. You do not have to get expensive eyepieces to get cracking views with binoviewers and a pair of TV Delites are not cheap.  I have found a decent pair of plossl or Orthos work just as well as more expensive eyepieces , something like TV plossl or Baader Orthos which I have used with excellent results.

 

Hope this helps 

 

 

 

[vineyard]

Thanks both @Don Pensack and @Timebandit.

Don, I don't think my BVs have OCA or magnifiers in the nose. (I got them from Denis, they're excellent).  Thanks for the 9mm recommendation.  Just out of curiosity are there particular advantages of them over the 7mm, or is it that the step from 18.2mm to 9mm is a better one (leaving the space for more powerful EPs in the future)?

Timebandit, that's a good point.  I don't mind tight ER if needed (I tend to take my glasses off for viewing).  And the cost saving would certainly be great!  The main Q I'd have about Plossls and Orthos would be the FOV - would that be too narrow (or would it not be as noticeable b/c of stereoscopic vision)?

Yes the Delites would be pricey, but luckily I have a 15mm & 13mm Delite in great condition that I bought from a very careful prior owner - and since they're so close in range, that's what made me think about selling those and going a bit further down the EP focal range (plus the desire to be able to see the moon in more detail!).

Stay safe both & thanks again for the tips,

Vin

[Steve Clay]

I've  just bought a BST bino viewer from sky's the limit. Are zoom Ep's an option. I already have an ovl 7-21.5  and it's not a massive problem to get another.

[Louis D]

1 hour ago, Steve Clay said:

I've  just bought a BST bino viewer from sky's the limit. Are zoom Ep's an option. I already have an ovl 7-21.5  and it's not a massive problem to get another.

Absolutely!  I use a pair of Celestron Regal zooms in mine.  They allow you to quickly dial up the power when conditions allow or dial it back for when conditions deteriorate.  As far as getting them at the same power, I turn them simultaneously and then fine adjust one to match the other in power after refocusing.  It's pretty amazing how easily your brain figures out the moment when the two views are identical and you can stop zooming.

[Stu]

I would tend to avoid going too short on eye relief as I can just make things that bit more tricky to get yourself if the right position. I’ve used 12.5mm Orthos quite successfully and possibly even shorter, I can’t recall. I actually prefer barlowing longer focal lengths to get to the magnification I want, you could for instance get some 18mm Orthos and use a Barlow to get to higher powers. Just another option.

@Louis D that is interesting, always thought it must be very tricky to do, but it sounds like our brains are smarter that I thought! Certainly very handy if it works, I found the OVL type Zooms are pretty good.

[Louis D]

4 hours ago, Stu said:

@Louis D that is interesting, always thought it must be very tricky to do, but it sounds like our brains are smarter that I thought! Certainly very handy if it works, I found the OVL type Zooms are pretty good.

Not only that, I've found that my brain can compensate for slight differences in power between the two eyes and merge the images successfully.  This whole thing about finding matched sets of eyepieces is completely overblown as it relates to magnification.  What's more important is making sure they focus at the same height relative to the tops of the eyepieces.  You physically can't compensate for one eyepiece being 5mm closer to one eye than the other eyepiece.  You can't get to both exit pupils simultaneously, so you see only part of the image in the one too far away.  I've run into this buying a second eyepiece years later for binoviewing and either the field stop location has moved up or down relative to the shoulder or the eye lens recession has changed relative to the first eyepiece.

[Stu]

4 minutes ago, Louis D said:

I've run into this buying a second eyepiece years later for binoviewing and either the field stop location has moved up or down relative to the shoulder or the eye lens recession has changed relative to the first eyepiece.

I had exactly the same with a pair of 18mm Baader Genuine Orthos, bought separately on the used market, one was a different height to the other which surprised me.

[Don Pensack]

On 10/04/2020 at 12:09, vineyard said:

Thanks both @Don Pensack and @Timebandit.

Don, I don't think my BVs have OCA or magnifiers in the nose. (I got them from Denis, they're excellent).  Thanks for the 9mm recommendation.  Just out of curiosity are there particular advantages of them over the 7mm, or is it that the step from 18.2mm to 9mm is a better one (leaving the space for more powerful EPs in the future)?

Timebandit, that's a good point.  I don't mind tight ER if needed (I tend to take my glasses off for viewing).  And the cost saving would certainly be great!  The main Q I'd have about Plossls and Orthos would be the FOV - would that be too narrow (or would it not be as noticeable b/c of stereoscopic vision)?

Yes the Delites would be pricey, but luckily I have a 15mm & 13mm Delite in great condition that I bought from a very careful prior owner - and since they're so close in range, that's what made me think about selling those and going a bit further down the EP focal range (plus the desire to be able to see the moon in more detail!).

Stay safe both & thanks again for the tips,

Vin

Because it's a 4" with an 880mm focal length and ~100x is a great magnification in a 4" scope, 9mm made more sense for a (relatively) high power eyepiece you would use all the time.

You can, at some point in the future, go as short as a 4mm eyepiece, but I think 9mm is a logical step from a 13mm and would be used all the time.

In my own 4", I have a magnification run of 102x, 143x, 179x, and 238x, but the usage declines with increasing magnification from 102x, which is a 1mm exit pupil.

I find in both my scopes that a 1mm exit pupil is one of those "just right" magnifications, because it is not high enough for floaters to interfere with lunar observation, but high enough for some high-resolution images.

The reason I asked about an OCA in your binoviewer is that many binoviewers have a 2X OCA in the bottom, so every magnification is doubled, which would turn a 9mm into a 4.5mm, and I don't think you'd use a 4.5mm focal length often.

[vineyard]

Brilliant thanks Don. 9mm it is!  Cheers & stay safe, Vin