Telescope Reviews: Merge problems w/ less expensive BV's? Solution:

When you need to "practice" to merge images, you are training your eyes to strain (whether mild or severe) to accomodate misalignment of the images. This probably means the merging of the images is not within tollerance for your eyesight. Incredibly, some people can accept an extremely wide error, yet many, if not most others, have a low tollerance for error.

There are allowable tollerances for collimation. The tollerances get much tighter as magnification increases. These tollerances are published in any number of optics handbooks, particularly in the sections dealing with binocular adjustment. A 7x or 8x binocular with 2 to 3 arcmin miscollimation won't look bad at all. However, in a 25x binocular with even 50 to 60 arcseconds of miscollimation it may be difficult to merge images. You can guess what is required of binoviewers that are used at magnifications of 75x, 100x, 150x and more.

Do you know how to let your eyes relax so you can measure miscollimation? Are you familiar with some double stars so you can test collimation on a double star of known separation? This gives you a benchmark separation from which you can determine the severity of miscollimation. Generally, for testing binoviewers at very high power the target stars chosen must be much closer, on the order of 10-20 arcseconds.

An excellent double to test collimation is Nu (v) Draco. It has a separation of 62". It's like having a 1 arcmin measuring device in your image. If your low power binocular collimation is within 2 or 3 times the separation of Nu Draco, you may want to just leave it alone. BUT if you are using a binoviewer at say 75x, and the alignment error you see when observing nu Draco is even only half as wide as the separation of the pair, you've got some problems. If you want to use that same binoviewer at 150x, the error correction needs to be closer than one quarter of the separation in nu Draco.

Generally there are three collimation errors to be concerned about. For all collimation misalignments, the allowable error in image displacement is smaller as magnification increases. Actually the results are measured in apparent visual error, the angular error multiplied by the magnification.

Vertical Alignment, a serious error, is when one image is higher than the other image. The eyes have no muscles to accommodate for vertical error. The allowable divergence is only 4 arcmin at 7x to 10x, only 3 arcmin at 12x to 15x and 2 arcminutes at 15x to 20x.

Note that these allowable tollerance ranges for Vertical error are almost equal. They are apparent error of 28-40, 36-45 and 30-40. You can get an average from this that the allowable apparent error for Vertical misalignment is 35-40 arcminutes apparent. Personally, I cannot tollerate even 2 to 3 arcminutes of vertical error even at magnifications of 10x and only 1 arcmin at 16x, So my tollerance level for vertical error is closer to 20 arcmin apparent error.

Binoviewing is not any different. The allowable tollerances do not change. By use of high magnification, the need for alignment to be more critical is increased. As an example, 20 arcmin is 1200 arcseconds. So, at my tollerance level, a binoviewer that is measured to be out of alignment by only 12 arcseconds vertical error will show as not merged when used at any powers over 100x. Below 100x, my eyes would accomodate and it will do fine.

The other errors of misalignment are:

Horizontal Convergence, the least problematic error, is when the image in the right eyepiece is to the left of the image in the left eyepiece. You might think of this as cross-eyed. The most separation can be tolerated in horizontal convergence, 10 arcmin at 7x to 10x (70-100), 8 arcmin at 12x to 15x (96-120) and 6 arcmin at 15x to 20x (90-120). Most eyes can readily accommodate this error, but these limits I find a little too wide. I could not tollerate even half that much.

Horizontal Divergence, another serious error, is when the image in the right eyepiece is to the right of the image in the left eyepiece. The images are spread apart, an error that the eyes cannot accommodate without strain. The allowable error for horizontal divergence is 6 arcmin at 7x to 10x (42-60) and only 4 arcmin at 12x to 15x (48-60) and 3 arcmin at 15x to 20x (45-60).

If your binoviewer does not fall within these tollerance ranges, you will NOT get that instant merging feeling when you view. Your eyes may be able to accomodate, but not without working to do so.

Keep in mind, the magnification of your binoviewer has a significant influence on how you see this error. Your eyes see the apparent error, or the angular error multiplied by the magnification. For instance a 1 arcminute error in a 25x binocular will appear to have a magnified apparent size of 25 arcminutes. That same 1 arcminute error in a 150x binoviewer would have an error of 150 arcminutes. At that power, the images would look like they are in two different states.

For reference 1 inch at 100 yards is just slightly less than 1 arcminute.

I've measured the error in my Stellervue B3. And for those that might suggest all those little methods I've heard, there is no eliminating the error by rotating eyepieces or pressing on one side or the other. I've rotated half a dozen pairs of eyepieces that did absolutely nothing to change the error. I've pressed on the eyepieces and that's fine, if I want to hold it that way I can correct it, but as soon as the pressure is released, it goes right back to the exact same error.

My SVB3 is out by about 10-15 arcseconds. It's a combination of Horizontal Divergence and Vertical Error. This is often the case. Now that error seems small, binocular manufacturers could only hope to achieve such closely merged images. Few binoculars that I own are merged to within that close. Even in a binoviewer, that amount of error can barely be detected at 50x to 75x. BUT at 200x it's more than my eyes can accomodate. My SVB3 will not handle 200x, but it's fine at 125x-150x.

My Denk Big Easy has a smaller error, but is not without error. However, at all the magnifications I've used it, it merges almost instantly. I haven't used it at anything over 200x. The SVB3 can be seen to take a few seconds at magnifications ~150x.

This in not a matter of "practice". This is eyestrain. It may be minor, but it's eyestrain. If you need to "practice" to merge images in a binoviewer, then the unit is not within tollerances for your eyes.

edz

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