Questions on Binoviewer
Posted 09 February 2013 - 07:06 AM
I would like to have some explanations regarding Binoviewer:
- it split by half the amount of light received by each eye: does it mean that a 22" primary mirror looks like a 15" when using Bino?
- does a 1.25" Bino grasp all the light cone from a primary mirror? If so, what is the purpose of a 2" Bino (25mm CA versus 45mm CA)?
- does a Bino reduce the resolving power of a scope?
Thank you for your answer.
Posted 09 February 2013 - 09:52 AM
Edz's most excellent Binoviewer Resource links
To net it out, while each eye only receives half the light, the total light that reachs the observer is about the same, just split in half.
However, the brain and eye work together to provide an effect called "Summation." When the brain processes the light arriving from each eye, it trys to add the result, and because of that, your brian preceives the brightnes as about 70% of the original image. In other words, it would be like a telescope with 70% of the light gathering, or in your case it would be like using an 18 inch (slightly larger) telescope.
First, for a dob, you will almost always require a barlow to reach focus, and this will of couse stretch out the light cone to double the focal ratio of the scope. Still, for large dobs, you can get aperture reduction. This is not really a function of the binoviewer itself, but the OCS (Barlow). Denkmeier now has a 45mm (I think that is the size) OCS designed for this purpose.
The aperture of the binoviewer just limits the size of the field stop (and as a result, the true field size) of the eyepeices you can use.
The problem with larger prism binoviewers though is that they increase the lenght of the light path. What this means is that now a unit that might reach focus with a 2x OCS may require even more barlow to get the light cone out to the eyepiece field stop.
So the condition is that you can use eyepeices with larger field stops to allow wider fields of view, but you have to now use more magnification to reach focus, which in many cases can totally counteract the value of the bigger field stop eyepeices.
Resolving power is not reduced (unless the light cone is being cut because of a too small OCS, but again, there are ways to avoid this). The angular and linear resolutoin of the scope is the same. But as mentioned, the image is reduced in brightness.
For most viewing, you can simply use a bit lower power eyepcceie and get the brightness back. For example, the image looks about as bright when I use 32mm Plossls in my binoviewers as it does when I use a 24mm Wide Field in monovision. So, I give up a bit of image scale on extended objects, but they brightness loss of the binoviewers is offset. The penatlty is the narrower apparent field of the plossls, but I have found it a good tradeoff.
The place where I see the biggest difference is on planets. My own best planetery observing tended to come at about a 1mm exit pupil (1x per mm of aperture). So, in my C14, my best views usually came at about 350x. Past this, and for me personally, the view would get to dim to really allow the best detection of the fainest, lowest contrast detail.
When using the binoviewers though, I find that I run out of light at about 305x. Past this and the image for me just gets to dim to see the most challanging detail.
The good news is that even though I cannot magnify the image as much, when using two eyes, I feel that my visual acuity is so good that I can see more datail at lower power than I could using nomovision at high power.
About 6 or 7 years ago, I tried binoviewing, but did not feel that it added anything to plaentary observing. In reflection though, I realize that at the time, I was using the same powers as I had used in monovision. I just could not get a better view using binoviewers.
This time around, I realized what was happening, so I kept the powers lower and have been rewarded with much brighter views, and I feel as if I have done my best ever planetary observing now using Binoviewers in my C14. But I limit my power to 305x, even on nights of excellent seeing, becase for me personally, the image gets to dim past this power and I have problems with visual noise causing the image to loose fidelity.
One last thing. You can cut down the poles on truss dob to eliminate the need for an OCS, but if you do this, aperture reduction may occur. To avoid this, it would be necessary to go to a bigger diagonal mirror. If you enjoy binoviewing and have a truss dob, this may indeed be the best possible setup. It allows you to dispense with the OCS completely, and use as big an aperture bino as you want. Of course the bigger the CA of the binoviewr, the bigger you have to make the secondary. As long as diagonal mirror stays lower than about 35% of the aperture, it should give very good results.
- bgi likes this
Posted 10 February 2013 - 09:05 AM
I will study all that to have a better understanding on Binoviewer.
Posted 10 February 2013 - 01:46 PM