Telescope Reviews: Baader Mark V and fast Newtonians

The Baader Mark V binoviewer remains rather unknown, and owners of this excellent device, including myself, seem not very present in this forum. I will try to make up this fault, by sharing my experience with my f/4 Newtonian.

The following is derived from several months of intense star testing, with a very good 16" f/4 Newtonian. I must add that I am very demanding about optical quality, and I generally see flaws that many observers will never notice.
I am not interested in low powers.

There are four optical problems if one wishes to use a binoviewer in a fast telescope :
1- the binoviewer requires some in-focus clearance, and it is sometimes impossible to get the focus, even with some Barlow lenses
2- the prisms of the binoviewer introduce some chromatic aberration, and this CA increases as the f/d ratio decreases
3- the prisms also add spherical aberration, although CA is predominant
4- fast Newtonians have a big amount of coma, and users generally want to avoid it, as it is possible in monoviewing mode thanks to the Paracorr.

I know from several reliable testimonials that most binoviewers fail to give perfect images if the telescope is about f/4 or faster. This includes the DenkII with its Newtonian powerswitch. How about the Mark V ?

The Mark V has four optional optical correctors :
- the 1.25x glasspath corrector. It's a cemented doublet that is designed to compensate CA and SA of the prisms. It has an external 28.5 mm thread (1.25" filter thread), and it can be screwed in front of the binoviewer. It is possible to unscrew the retainer ring and to reverse the doublet in its cell. This feature is implemented to allow the use of this corrector with the Baader Maxbright binoviewer, which doesn't have the 1.25" filter thread. The user has just to verify the right position of the lens, as explained in the notice and on the corrector itself.
The coatings are excellent, and the lens is virtually transparent. I have measured the magnification, and I have found 1.23x, in good agreement with the specifications. Unfortunately, the in-focus travel is so large that it is absolutely impossible to use this corrector with a Newtonian telescope.

- the 1.7x glasspath corrector. A part from the magnification, it is very similar to the 1.25x model. I have found that the real magnification is 1.46x, rather different than the labelled magnification. Like the 1.25x corrector, it is impossible to use it in a Newtonian telescope.

- the 2.6x glasspath corrector. It's a rather different system. It's an air-spaced doublet, that cannot be turned in its cell. To be used with the Mark V, this corrector must be introduced in the 2" nosepiece and maintained by the supplied plastic ring, then this nosepiece is attached to the binoviewer. Screwing this corrector directly into the binoviewer would put the lens in the wrong position.
Contrary to the previous ones, the coatings are mediocre : the lens exhibits a strong blue reflection, and looking through the lens shows a marked yellow cast. Compared to a single eyepiece, the focuser must be pushed back by at least 55 mm, thus many Newtonian telescopes are not suited for this corrector. The real magnification is about 2.9x. Optically, the star test is good, but I have not been convinced by lunar observations : the image appears too dim, the edges could be better, and there is a lot of stray light.

- the 2" Newton 1.7x coma-correcting compensator. This big component is especially designed for Newtonian telescopes. With a Nagler 13T6, the focuser must be pushed back by about 40 mm compared to the mono eyepiece configuration (the exact distance varies slightly with the type of eyepiece). The magnification is exactly 1.7x, and the coatings are very good.
At the eyepiece the image is excellent. The coma is well corrected in a fast scope : with a pair of Panoptic 24, stars remains almost pinpoints even at the field stop, without field curvature, or apparent vignetting. At very high powers the image is still excellent, and exploring the moon with a pair of Nagler is a wonderful experience. The only drawback is that the moon shows sometimes a bright and disturbing reflection with short focal length eyepieces. I have found that this unwanted light can be suppressed by inserting a cardboard ring into the filter thread of the binoviewer, in order to reduce the aperture from 28 mm to about 15 mm.

My conclusion is that any Newtonian owner who wants the Baader Mark V binoviewer should consider the Newton 1.7x coma-correcting compensator, if the telescope has enough back focus. The image is close to perfection even in a f/4 scope.

However, it is not perfect. The star test is slightly altered, and there is a small chromatic aberration. Generally, atmospheric turbulence in large scopes does not allow the observer to see such tiny alterations, but from time to time the seeing improves, and when the Airy disc is visible chromatic aberration becomes apparent : the brightest stars are surrounded by a violet halo, the moon may exhibit delicate purple fringing even near the centre of the field of view, and Saturn and its ring have a very faint atmosphere.

So I have decided, out of curiosity and out of challenge, to search for a really perfect configuration for my Mark V.

The first idea that comes to mind is to use a Paracorr. There is a difficulty here : the focal plane must lie at a specific distance behind the Paracorr in order to provide good off-axis performance. This is technically possible, but the only solution is to use the complex following arrangement : Paracorr photo without the tunable top + Baader T2 extension tube 7.5 mm (#25C) + Baader 2.6x glasspath corrector (inserted in the extension tube) + Baader T2 quick changer (#06, supplied with the binoviewer) + Mark V. The total magnification is 3.3x, and the focuser must be pushed back by only 25 mm. Coma is totally removed from the image, but there is a small reddish fringing, and the moon appears rather dim, with a lot of stray light. Finally, this is not a good solution.

A very common solution is to use a Barlow lens in front of the binoviewer. Unfortunately, many Barlow lenses are not designed for light cones as steep as f/4, even if observers generally fail to notice this fact. I have myself tried the Baader FFC, which is one of the very best Barlow lenses, and I have discovered to my cost that it exhibits spherical aberration at f/4. However, there is absolutely no chromatic aberration, and the image of the moon given by the FFC/Mark V combination is bright and has a lot of contrast. The coma is not removed, so the image softens very slowly towards the edge. The total magnification is 3.6x, and the focuser must be pushed back by 25 mm.

Finally, I have found the following configuration to be the solution I was looking for :
Powermate 2x without the black barrel + Powermate 2x T-Ring adapter (PTR 2200) + Baader T2 quick changer (#06, supplied with the binoviewer) + Baader 1.25x or 1.7x glasspath corrector (screwed into the binoviewer) + Mark V. This configuration has many advantages :
- The powermate 2x does not alter the diffraction pattern in very fast telescopes : the star tests with and without the powermate are nearly identical
- The powermate 2x does not require a lot of back focus
- The powermate 2x provides a partial correction of the coma. This fact is obvious when comparing the Powermate to the FFC for example.
- The Powermate does not produce diverging rays like other Barlow lenses, so I believe that the risk of stray light is greatly reduced
- The powermate 2x has excellent coatings
- The powermate T-Ring adapter shortens the total length of the configuration
- The glasspath corrector cancels the chromatic aberration due to the prisms, that is still present at f/8
- The glasspath corrector avoids the use of the cardboard ring to suppress the disturbing reflection on the moon.
The final image is excellent : the star test is the best among all of the configurations I have tried, there is no chromatic aberration, the contrast is very good, and the sharpness barely deteriorates at the field edge. With the 1.25x glasspath corrector, the total magnification is 2.42x and the focuser must be pushed back by about 28 mm compared to the mono eyepiece configuration.
With the 1.7x glasspath corrector, the total magnification is 2.86x and the focuser must be pushed back by about 24 mm.
Another and less expensive solution would be the 2.5x powermate and its specific T-Ring adapter, but I haven't tried it.

I cannot honestly say that more details are visible with the configuration above compared to the 2" Newton 1.7x coma-correcting compensator. But once you have seen CA in the image, you see CA very often, and this can become disturbing.

Clear skies