Binoviewers and AFOV

[daslolo]

Hello,

I am a big fan of binoviewers but I don't understand why the view in them is like a keyhole.

And depending on the barlow it can get so bad that the brain can't merge, one image shows up and the other doesn't

Can someone explain and ... how do I get the most out of them?

Also if you have experience with Quark, that would help make a choice.

[Stu]

2 hours ago, daslolo said:

Hello,

I am a big fan of binoviewers but I don't understand why the view in them is like a keyhole.

And depending on the barlow it can get so bad that the brain can't merge, one image shows up and the other doesn't

Can someone explain and ... how do I get the most out of them?

Also if you have experience with Quark, that would help make a choice.

They can take quite a bit of getting used to I will admit, but for high power viewing they really help reduce floater visibility and can make observing much more relaxing.

Unless you can find some with huge prisms, they are always limited to relatively small fields of view compared with a long focal length mono eyepiece. I still prefer a single eyepiece for widefield viewing, keeping binoviewing for high power planetary, lunar and solar work. The style of observing is different in that you view on axis, taking in the field with peripheral vision and moving the scope to see other areas rather than looking around the field of view of a wide field eyepiece.

Making sure the inter-pupillary distance is set exactly right with really help eliminate black outs in one eye. Blink left and right to see that you are getting both channels. I find that Eyeguards, bought or home made help to keep your eyes correctly positioned as the Barlow can push eye relief out a fair way leaving you hovering away from the eyepiece. Focus balance is also very important to getting the image merged, and I find barlowing longer focal lengths better than using natively shorter focal length eyepieces, but a Quark has that already so you are often trying to find longer focal length 32 and 40mm plossl to keep the image scale down.

It can be frustrating, but well worth persevering with. As you can see I like them now!

[mikeDnight]

I can't say ive ever experienced the keyhole effect you mention. I use just a cheap pair of Revelation binoviewers and with a barlow attached to the nose piece I get a very pleasing field of view for lunar and planetary. I tend to use 52° field eyepieces of longish focal length yet still get high power views that are very comfortable. 

[Ricochet]

It sounds to me like your eye positioning isn't correct. A barlow at the front of the binoviewer will increase the eye relief of the eyepieces so you have to consider the distance you hold your head away from the eyepieces as well as getting the binoviewer set to your IPD correctly. 

With respect to the FoV being limited, I've found that due to your eyes not being quite parallel, in the horizontal direction you can only see about 60° with both eyes at the same time. 

[Louis D]

I use a pair of generic 23mm 62 degree aspheric eyepieces and barlow them up to whatever power I need.  No keyhole effect with them, and they're super light and compact.  Once barlowed to 3x or more, they're pretty much sharp to the edge.

Make sure your eyepieces aren't tipping in the holders due to the locking collet tightening unevenly in the eyepiece undercut.  Either use smooth barrels or mash the eyepiece into the holder tightly while tightening the collet.

Decloaked 18.2mm TV DeLites are supposed to be very good in binoviewers if you don't want to go the cheapie route like I did.

[daslolo]

On 28/02/2020 at 02:42, mikeDnight said:

Revelation binoviewers

I cannot find them in the US.

The Celestron and Astromania aren't working for me. But the other day at the store I had the misfortune of trying out a pair of Televue - they are so perfect for my eyes.

 

On 28/02/2020 at 06:55, Louis D said:

Decloaked 18.2mm TV DeLites are supposed to be very good in binoviewers if you don't want to go the cheapie route like I did.

decloaked?

On 27/02/2020 at 23:23, Stu said:

what is this mount? it looks like a DM-4

Denk II on the left, what's on the right?

Edited March 3, 2020 by daslolo

[Stu]

2 hours ago, daslolo said:

I cannot find them in the US.

The Celestron and Astromania aren't working for me. But the other day at the store I had the misfortune of trying out a pair of Televue - they are so perfect for my eyes.

 

decloaked?

what is this mount? it looks like a DM-4

Denk II on the left, what's on the right?

That's the new Rowan AZ100 I've been testing and giving feedback on final development. Now available.

It's actually Baader Zeiss Mark IV on the left, the right are some supercharged TS ones, work very well too.

[Space Hopper]

On 28/02/2020 at 04:09, daslolo said:

Hello,

I am a big fan of binoviewers but I don't understand why the view in them is like a keyhole.

And depending on the barlow it can get so bad that the brain can't merge, one image shows up and the other doesn't

Can someone explain and ... how do I get the most out of them?

Also if you have experience with Quark, that would help make a choice.

Whats your current scope and kit setup ?

[Louis D]

9 hours ago, daslolo said:

On 28/02/2020 at 08:55, Louis D said:

Decloaked 18.2mm TV DeLites are supposed to be very good in binoviewers if you don't want to go the cheapie route like I did.

decloaked?

Follow the link and all will be clear.

[daslolo]

On 02/03/2020 at 22:23, Stu said:

It's actually Baader Zeiss Mark IV on the left, the right are some supercharged TS ones, work very well too.

Very similar look to the Denkmeiers, I was told the Mark 5 were very good.

Does the size of the prism matter if you are looking for wide AFOV?

On 03/03/2020 at 00:24, Space Hopper said:

Whats your current scope and kit setup ?

Today it's an Orion 120 with a Quark. The binos I am likely to keep are the Celestron.

 

The current eyepieces are 33mm I think, very simple, open glass. When I add a 2x barlow to reach the level of magnification that I'm looking for, the image is blurry and it's a bit like looking through a keyhole, there is almost no keyhole if I use a par of 15mm eyepieces instead. Bluriness remains, I think it's just a bit hazy today.

I wish I could show you but any attempt to take photos through the eyepiece looks like this...

(I wonder if that line in the middle is a crack in the etalon, it disappears when the focus is right)

On 28/02/2020 at 06:55, Louis D said:

if you don't want to go the cheapie route like I did.

After taking a glance at the prices, I'll join you on the cheapie route so...

 

On 28/02/2020 at 06:55, Louis D said:

I use a pair of generic 23mm 62 degree aspheric eyepieces and barlow them up to whatever power I need.  No keyhole effect with them, and they're super light and compact.  Once barlowed to 3x or more, they're pretty much sharp to the edge.

 

Do you barlow each eyepiece or the nose of bino? Show me your setup.

[Stu]

3 hours ago, daslolo said:

Very similar look to the Denkmeiers, I was told the Mark 5 were very good.

Does the size of the prism matter if you are looking for wide AFOV?

Today it's an Orion 120 with a Quark. The binos I am likely to keep are the Celestron.

 

The current eyepieces are 33mm I think, very simple, open glass. When I add a 2x barlow to reach the level of magnification that I'm looking for, the image is blurry and it's a bit like looking through a keyhole, there is almost no keyhole if I use a par of 15mm eyepieces instead. Bluriness remains, I think it's just a bit hazy today.

I wish I could show you but any attempt to take photos through the eyepiece looks like this...

(I wonder if that line in the middle is a crack in the etalon, it disappears when the focus is right)

After taking a glance at the prices, I'll join you on the cheapie route so...

 

Do you barlow each eyepiece or the nose of bino? Show me your setup.

I never barlowed my Quark as it was always a case of trying to keep the magnification down; I used 32mm and 40mm Celestron Omni Plossls which work well. At higher powers the mag is just too high so I'm not surprised you get a blurred image. The x4.2 Barlow in the Quark is plenty so I really recommend not doing what you are trying to do.  33mm should be fine, but even they may be too much on some days. The lower power 40mm do have more of a drinking straw fov but the sharpness and contrast are better.

The Quark also has a limited internal field stop which means that is the factor which defines the visible field of view, getting larger prism binoviewers will not help.

White light is different, although I'm able to get full diak views with standard binoviewers and always use mine at mid and high power, not for widefield where I prefer mono views. I always Barlow in front of the binoviewer, not in front of each eyepiece. The extra light path does increase the multiplying factor which can be useful. I often use a x2.6 GPC fitted to the binoviewer with my 25mm eyepieces, and even then I might use a Barlow at x2 to x4 depending on extension tubes to get the power up to x200.

White light and Quark Ha are very different situations, so adjust accordingly.

[Louis D]

11 hours ago, daslolo said:

Do you barlow each eyepiece or the nose of bino? Show me your setup.

Sorry, I don't have any pictures of my setup, but I use the nosepiece from a Meade 140 2x Barlow screwed into the front of the binoviewer's nosepiece to reach focus with a 3x magnification factor.

[daslolo]

18 hours ago, Stu said:

At higher powers the mag is just too high so I'm not surprised you get a blurred image

Is this blurry because of atmospheric conditions?

10 hours ago, Louis D said:

I use the nosepiece from a Meade 140 2x Barlow screwed into the front of the binoviewer's nosepiece to reach focus with a 3x magnification factor

how do you get x3 with a x2 barlow?

[Stu]

5 hours ago, daslolo said:

Is this blurry because of atmospheric conditions?

how do you get x3 with a x2 barlow?

Yes, that's generally the case. The Quark's x4.2 Barlow built in means that you need to use longer focal length eyepieces to keep the mag down, and no extra Barlow is needed. The conditions mostly don't support going to much higher powers and the image quality just breaks down.

If you increase the distance between a Barlow and the eyepiece then the magnifying factor increases too. It won't be exactly x3 I'm sure, but a x2 Barlow on an eyepiece will give a higher factor when placed on the nose of a binoviewer. Much of the time I have only a rough idea of the magnification I am using for white light solar, I just tune it until the views are optimum for the conditions.

[Louis D]

13 hours ago, daslolo said:

how do you get x3 with a x2 barlow?

Because the magnification factor is dependent on the distance between the Barlow lens elements and the field stop of the eyepiece per the equation below (shameless copied from this CN posting):

M = 1 - L / f_b

Where M is the magnification factor.
L is the distance from the Barlow lens to the field stop of the eyepiece, and
f_b is the focal length of the Barlow lens (a negative value).

Most people back into calculating the f_b of a Barlow by measuring M and L and plugging them into the above equation.

8 hours ago, Stu said:

It won't be exactly x3 I'm sure, but a x2 Barlow on an eyepiece will give a higher factor when placed on the nose of a binoviewer

I did measure my setup's magnification, and it comes out right at 3x with the 140's nosepiece.  It's 2.4x by itself.  This was measured using a 14mm Pentax XL because it has it's field stop right at the shoulder.  Thus, actual magnification will be more or less depending on how much above or below the shoulder a given eyepiece's field stop lies.  Other Barlow nosepieces will have different focal lengths and thus different magnification factors from the Meade 140 both by themselves and when used with a binoviewer.

[Merlin66]

There's an easy and accurate method of measuring a Barlow negative focal length.....

Measure the free diameter of the Barlow lens, draw on a card a circle twice this diameter, hold the card behind the Barlow pointed towards the Sun, when the image fills the circle, the distance from the Barlow lens to the card is the focal length. Works for all negative lens.

 

[daslolo]

Got it, and I found out the eye relief becomes insanely long so if I can find or make a cheap telecentric I'll do that.

6 hours ago, Merlin66 said:

Measure the free diameter of the Barlow lens, draw on a card a circle twice this diameter, hold the card behind the Barlow pointed towards the Sun, when the image fills the circle, the distance from the Barlow lens to the card is the focal length. Works for all negative lens.

 

This is fun, sun away for a few more days so let me try the 200W desk lamp. Thrice the diameter for a 3X barlow?

focal length is about 132mm

 

[Merlin66]

No, always twice the diameter- for any Barlow/ negative lens.

the magnification is dependent on the spacing between the lens and The eyepiece/ CCD.

[Ricochet]

On 05/03/2020 at 03:04, daslolo said:

Does the size of the prism matter if you are looking for wide AFOV?

The prism size limits the TFoV that you can view. This will not affect short/medium length eyepieces but long focal length eyepieces may be vignetted by the prisms, depending on the AFov. 

On 05/03/2020 at 03:04, daslolo said:

The binos I am likely to keep are the Celestron

I think these are the same as the "Starguider" ones I have. From memory I think the maximum unvignetted FoV they can give is given by 20mm 68° eyepieces.

On 05/03/2020 at 03:04, daslolo said:

The current eyepieces are 33mm I think, very simple, open glass. When I add a 2x barlow to reach the level of magnification that I'm looking for, the image is blurry and it's a bit like looking through a keyhole, there is almost no keyhole if I use a par of 15mm eyepieces instead. Bluriness remains, I think it's just a bit hazy today.

Given my previous statement, a 33mm will be vignetted if the AFoV is above approximately 40°. Sounds pretty keyhole like.

Also, if those are the same binoviewers and the supplied barlows are the same, then those barlows are absolutely awful and should be disposed of as soon as a replacement solution has been found.

[daslolo]

I've tried another combination and couldn't even get focus. It's so funny how the 120mm Orion has tons of back focus but not enough front focus (is it a thing? when the focuser is all the way retracted inside the tube). I'd pull out my saw if I were sure to keep it.

What is the best combination of OTA, bino and eyepiece to get a nice wide AFOV through a Quark?

[Stu]

5 hours ago, daslolo said:

What is the best combination of OTA, bino and eyepiece to get a nice wide AFOV through a Quark?

I think I've already posted this, but the limiting factor with a Quark is the internal etalon, so widefield eyepieces don't make any difference after a certain point. 32mm Plossls work as well as anything I think 

The best way to achieve full disk views is with a smaller frac with a short focal length. I used to use a Tak FS60C with my Quark, 355mm focal length, of something like my TS 72mm at 432mm focal length would do the trick.