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Barlow/binoviewer test results


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Further to my query here regarding the effect of binoviewers adding focal length in the optical train on barlows.

I decided to do some testing using my mak and various binoviewer/barlow configurations on a house roof. The roof in question is situated a fair distance away and also handily fitted with solar panels which have lines running downwards for measuring purposes.

I wanted to see the effect myself in a more simple way with a direct visual result for immediate comparison.

So I tested a 1.6x & 2x barlow and a tele-extender as well as without the binoviewer and the results are interesting. Eyepiece used a 25mm plossl.

1. First off when I put my eyepiece only in the mak and then compared with binoviewer/eyepiece I found that the binoviewer seems to have a 1.15 barlowing effect! I double checked this and got the same result, curious.

2. Now it's 2x tele-extender/bino's/eyepiece test  and as expected it is exactly 2x magnification

3. Then it's 1.6 barlow/bino's/eyepiece test and this gave 3.2 magnification which is not what I thought it would be but is a very good number from my perspective.

4. Finally it's 2x barlow/bino's/eyepiece test and this is 4x magnification and again is as expected.

How these configurations translate into my magnifications for my 127/1500 mak is 1. x 69 / 2. x120 / 3 x192 /4. x 240

There is a clear sky tonight albeit with a full moon and I am looking forward to having a bash with these configurations  on Mars.

Steve

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a maksutov's f/ratio increases with back focus distance.  Adding a binoviewer adds a substantial optical back focus length.  In your case, it looks like it added 15% to your focal length.

This is not unusual.  it isn't the binoviewer that added the magnification, it's the back focus light path length.  If you added an extension tube, it would go up even more.

Very approximately, the focal length goes up by 10% for every 2 additional inches of back focus in that scope, so it looks like you added about 3" of additional back focus over the diagonal.

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7 minutes ago, Don Pensack said:

a maksutov's f/ratio increases with back focus distance.  Adding a binoviewer adds a substantial optical back focus length.  In your case, it looks like it added 15% to your focal length.

This is not unusual.  it isn't the binoviewer that added the magnification, it's the back focus light path length.  If you added an extension tube, it would go up even more.

Very approximately, the focal length goes up by 10% for every 2 additional inches of back focus in that scope, so it looks like you added about 3" of additional back focus over the diagonal.

thanks for that Don, I didn't actually know that.

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22 minutes ago, Don Pensack said:

Very approximately, the focal length goes up by 10% for every 2 additional inches of back focus in that scope, so it looks like you added about 3" of additional back focus over the diagonal

Hi @Don Pensack,

Out of interest, how did you calculate that approximation? I have a C8 and have also been trying to work out the effect of various barlows and a Binoviewer. 

Given that you have to adjust the focus when adding a barlow (not sure if it’s inward focus or not) and that this in turn will alter the focal length of the scope (due to the mirror moving?), does that mean that a 2x barlow will not actually give a 2x magnification when used in a C8? I think I’ve also observed a difference between a nosepiece barlow vs the same magnification normal barlow, which I wasn’t really expecting, but maybe one of them is poorly specified.

Regards,

Tim

 

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59 minutes ago, AstroTim said:

Hi @Don Pensack,

Out of interest, how did you calculate that approximation? I have a C8 and have also been trying to work out the effect of various barlows and a Binoviewer. 

Given that you have to adjust the focus when adding a barlow (not sure if it’s inward focus or not) and that this in turn will alter the focal length of the scope (due to the mirror moving?), does that mean that a 2x barlow will not actually give a 2x magnification when used in a C8? I think I’ve also observed a difference between a nosepiece barlow vs the same magnification normal barlow, which I wasn’t really expecting, but maybe one of them is poorly specified.

Regards,

Tim

 

If you can reach focus without moving the mirror by sliding the Barlow in and out until reaches focus with the eyepiece, then any magnification change is entirely due to the Barlow.  This assumes the Barlow requires out-focus and not in-focus.  This is generally true for longer Barlows while the opposite tends to happen for shorty type Barlows.

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Thanks @Louis D. I’ll give that a go. I hadn’t really noticed that one barlow had required an inward focus change and the other an outward one, but think I do recall that being the case.

I guess any movement of the mirror is going to change the focal length and therefore the resulting magnification. That makes it difficult to determine what magnification you are going to get with a given combo of barlow, bino, and eyepiece. I was trying to find a barlow + bino + eyepiece combination that would give me around x160 as that’s about the max I can typically use. I know a x2 long barlow is slightly too much, but I’m hoping a x2 nosepiece barlow (is this the same as a shorty?) might just work, as it will require more inward focus, shortening the focal length.

Tim.

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8 hours ago, bomberbaz said:

. First off when I put my eyepiece only in the mak and then compared with binoviewer/eyepiece I found that the binoviewer seems to have a 1.15 barlowing effect! I double checked this and got the same result, curious

Hi Steve,

Yes, I went through this learning process a couple of years ago...have a read of this old but interesting thread from the US (it may have also appeared on Cloudy Nights)...

http://www.astrosurf.com/sliop/bino/Binoviewer test.htm

The reviewer looked at a number of binoviewers, and late on in the review (it's quite long but an interesting read), he looked at the fact that his observations seemed to suggest that different models added a certain amount of extra magnification just by virtue of their own internal  optical path..at the time I found this review, I had just bought an old but excellent Celestron Baader German built set of 60 degree angled binoviewers, and these proved to add almost exactly 15% to the native magnification, with no Barlow added.

The other afternoon I was playing about with my current Revelation binoviewers, a pair of Morpheus 17.5mm's and both 1.6x and 2.25x Barlows. My findings were very similar to yours, except that my higher power Baader 2.25x Barlow seems to add over 4x to the magnification, more like 4.5x?

If my working is right, this means that in my scope (1040mm focal length), my binoviewed, barlowed Morpheus pair were giving me about 270x, a really nice high power magnification, with the excellent Morpheus 76degree field. (This is with the Barlow threaded into the front of the binoviewer, but behind the diagonal prism (Baader T2) ).

If the Barlow was threaded into the front of the prism diagonal, it would deliver even more magnification, probably too much, but I will try it at some point.

The rather poor images below give you an idea of what that x270 looked like on my daytime test object..our local village church tower and a pair of Jackdaws, (shown next to the tower lightening conductor rod), about 150metres from the scope..(note the image is poor as I handheld my mobile phone to the eyepiece and it was VERY difficult to get clear focus and correct alignment of the phone camera to the eyepiece through one barrel of the binoviewer!) It was also starting to rain..

All interesting stuff, thanks for posting Steve😊👍

Dave

IMG_20200929_170759115~2.jpg

IMG_20200929_170759115.jpg

Edited by F15Rules
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Hello,

I have a Zeiss binoviewer with a I believe 1.2x corrector in front of a Zeiss Baader  diagonal. Does this mean if I use a Televue x2 pwermate that I will have 3.2x magnification at the eyepieces ? I have often wondered about this.

 

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1 hour ago, F15Rules said:

 

Hi Steve,

Yes, I went through this learning process a couple of years ago...have a read of this old but interesting thread from the US (it may have also appeared on Cloudy Nights)...

http://www.astrosurf.com/sliop/bino/Binoviewer test.htm

The reviewer looked at a number of binoviewers, and late on in the review (it's quite long but an interesting read), he looked at the fact that his observations seemed to suggest that different models added a certain amount of extra magnification just by virtue of their own internal  optical path..at the time I found this review, I had just bought an old but excellent Celestron Baader German built set of 60 degree angled binoviewers, and these proved to add almost exactly 15% to the native magnification, with no Barlow added.

The other afternoon I was playing about with my current Revelation binoviewers, a pair of Morpheus 17.5mm's and both 1.6x and 2.25x Barlows. My findings were very similar to yours, except that my higher power Baader 2.25x Barlow seems to add over 4x to the magnification, more like 4.5x?

If my working is right, this means that in my scope (1040mm focal length), my binoviewed, barlowed Morpheus pair were giving me about 270x, a really nice high power magnification, with the excellent Morpheus 76degree field. (This is with the Barlow threaded into the front of the binoviewer, but behind the diagonal prism (Baader T2) ).

If the Barlow was threaded into the front of the prism diagonal, it would deliver even more magnification, probably too much, but I will try it at some point.

The rather poor images below give you an idea of what that x270 looked like on my daytime test object..our local village church tower and a pair of Jackdaws, (shown next to the tower lightening conductor rod), about 150metres from the scope..(note the image is poor as I handheld my mobile phone to the eyepiece and it was VERY difficult to get clear focus and correct alignment of the phone camera to the eyepiece through one barrel of the binoviewer!) It was also starting to rain..

All interesting stuff, thanks for posting Steve😊👍

Dave

IMG_20200929_170759115~2.jpg

IMG_20200929_170759115.jpg

Great post and love the jackdaws. glad to hear someone else has been trying out testing on a similar basis.

I think the amount of magnification added on bino's will vary on design and depend on the extra distance/length added to the focal train. 

And yes agreed, the further down the focal train, the greater effect the barlow will have although I didn't realise this until quite recently.

I am glad I shelled out on the 1.25 tele extender, it's the ES version.  Really does throw up a lot of extra options when used in situations like this.

Steve

 

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12 hours ago, AstroTim said:

Hi @Don Pensack,

Out of interest, how did you calculate that approximation? I have a C8 and have also been trying to work out the effect of various barlows and a Binoviewer. 

Given that you have to adjust the focus when adding a barlow (not sure if it’s inward focus or not) and that this in turn will alter the focal length of the scope (due to the mirror moving?), does that mean that a 2x barlow will not actually give a 2x magnification when used in a C8? I think I’ve also observed a difference between a nosepiece barlow vs the same magnification normal barlow, which I wasn’t really expecting, but maybe one of them is poorly specified.

Regards,

Tim

 

The C8 adds 31mm to the focal length for every addition 10mm of back focus distance.

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On 02/10/2020 at 00:33, HAsun said:

Hello,

I have a Zeiss binoviewer with a I believe 1.2x corrector in front of a Zeiss Baader  diagonal. Does this mean if I use a Televue x2 pwermate that I will have 3.2x magnification at the eyepieces ? I have often wondered about this.

 

I reckon it would be at least that or maybe a little more, try a test like i did, it was fun running it.

steve

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