Magnification through binoviewers

[Mark in Macc]

A quick question to the binoviewer brigade. I've been trying out some WO binoviewers with the supplied 20 mm eyepieces and am convinced magnification is higher when used with the binoviewers. I swapped to a single 20 mm eyepiece at one point and the image definitely seemed smaller. I assumed that binoviewers wouldn't affect magnification, or is this an optical illusion?

[Mark at Beaufort]

I understand that when you use binoviewers the distance from the main lens/mirror increases so hence a longer focal length. This results in a higher magnification. This is my understanding so your opinion is correct.

You may receive a more technical response to your query but certainly when I use my 9mm EPs on my binoviewers, with a 1.6x barlow, its more like a 4mm EP.

Mark

[Peter Drew]

Assuming you are using the binoviewers on your C9.25, you will have moved the primary mirror forward with the focuser to achieve focus. Doing thi increases the effective focal length of the telescope and hence the higher magnification.

[Mark in Macc]

Thanks guys, that now makes sense. It seems to make quite a difference though. The 20 mm EPs in the Binoviewer were similar to my 12 mm EP without. Does that sound reasonable? Those 2 EPs give me 120 and 195 respectively without the BV.

[Mark in Macc]

Just out of interest, what would be the effect on reflectors and refractors?

[SkyGibbon]

Just out of interest, what would be the effect on reflectors and refractors?

Usually with reflectors and refractors there are in-focus problems. Adding a 2x barlow is a must. There are a few mods that can help a bit. One is to move the primay forward, either by cutting the tube or getting longer collimation bolts and springs moving the mirror up the ota. The mod I did with my 10^ dob is I attached four unused filter cells (glass out of course) stacked to my binoviewer inlet and was able to use the 1.6 barlow that came with my unit. It added a tiny bit of extra mag by moving the barlow the extra distance with the filter cells. It still has less mag than the 2x barlow I could just drop in and use. I also have a TV 3x I use. On all barlows, I unscrew the element from the barlow, and attach it directly to the binos. A low profile focuser may help a bit, but I am nervouse about the drawtube getting in the lightpath.

[Peter Drew]

A 2X Barlow is 2X only when it is inserted into the telescope and an eyepiece placed in its holder. If you place a binoviewer into the Barlow the distance between the Barlow and the eyepiece is considerably increased producing a higher than expected magnification. The final magnification of a Barlow/eyepiece combination is a function of their separation.

[htj]

As several people have mentioned having the focus plane far back (as with a binoviewer) on a cassegrain, means that mirror has to be moved forward which increases the focal length. However this also decreases the effective aperture of the system as some of the light from the primary will not hit the secondary. Furthermore this will decrease the contrast of the system, as the central obstruction will have a relatively larger size.

If you are using a 2" diagonal, I would suggest trying a 1.25" diagonal or one of the Baader T2 prims/diagonals, which adds very little length to the light path (but will only work if you binoviewer has a T2 thread for mounting).

There is no similar focal-length changing effect on newtons and refractors since their focal length is decided by the primary mirror or front elements, and not a combination of two mirrors.

The 20 mm -> 12 mm effect sounds like too much. The image with binoviewers is often perceived as larger and wider, than a corresponding single eyepiece, so juding this can be tricky.

[Peter Drew]

You still have the greater Barlow to eyepiece distance if one is used on a reflector/refractor which easily accounts for the increased magnification. Unless you are using a telecentric Barlow such as a Powermate.

[Mark in Macc]

Thanks for all the replies, I can always count on SGL members to give good advice and decent technical responses. It does make sense now. Just to clarify, the increased magnification wasn't a problem, it was more of a surprise. I have borrowed the binos to try out and was thinking that I should try some EPs with a shorter focal length than the 20s it came with. I probably wouldn't buy anything below 15 mm now with my SCT. In fact, I have ordered another 25 mm TV Plossl to pair with my existing one. It is difficult to say how much difference it makes as the views through a bino are totally different. My 20=12 estimate was probably an exaggeration, but I would say it is at least 20=15. As for the contrast, the views of the moon and Jupiter were sensational, the best I have ever seen. I certainly didn't sense a problem there.

[Space Dragon]

As you've discovered, high power views are not a problem with binoviewers, low power/wide field views are the more difficult to achieve. By the way, it's usually best to barlow for higher powers rather than using shorter focal length EPs, as they will be more sensitive to mis-collimation in the bino and between themselves.

It's for this reason most people use binoviewers for planets and binoculars for DSO. With an SCT the best way of getting a wide field is to use a large prism bino and keep the light path as short as possible, as HTJ mentions above, this means the Baader system, which, for the Mark V works out very expensive.

There are alternatives but none work as efficiently, if you stick a reducer in the path, it will reduce the aperture of an SCT.

It's great using both eyes though......can't beat it.

[Mark in Macc]

The WO system does come with a 1.6 x Barlow nose-piece, I might try that with the 25 mm eps to give approx a 16 mm equivalent. Barlowing the 20 mm will be too much. I must admit though, I don't like the look of the Barlow they supply with the binos, it does look rather nasty in comparison to my TV 2x Barlow. Worth a try though. I have my ES 30 mm 82 deg for wider field views, shame it doesn't give stereoscopic views though.