Whitepeak Observatory, Tacoma, WA

Comparing Minus Violet filters to #58 wratten for Mitigation of Chromatic effects in Achromats; Double Stars


Modern mid f/l achromats are notorious for injecting chroma into the interspace of double stars, making for a need for a higher power to establish a clear & clean separation than would be necessary while using a better corrected system. There is a way around this effect however.

Isolating, as much as possible, the spectrum range within which a moderate f/l achromat is capable of delivering .80strehl is the goal in selective filtration of a raw target image. By comparing the strehl curves of a garden variety modern achromat to the transmission curves of a typical minus violet filter vs. a #58 wratten colored filter yields interesting predictions:

One can immediately see how much more effective the #58 is at isolating the optimal wavelengths of an achromat compared to the broader band coverage of the minus violet filter. As the relationship above predicts, I have found that the #58 filter far surpasses the MV filter in it's effect upon image clarity and thus the scale of magnification necessary to achieve a given level of separation between the components of doubles generally.

So last evening, thwarted by extremely poor seeing regarding the project i had wanted to work on, i instead took the opportunity to try and quantify the effect of these filters on a couple of doubles having contrasting characteristics in more detail, in order to get an idea of the scope of the advantage of using a #58 over a minus violet filter for splitting doubles. I knew the #58 was very effective, but i wondered how much?

The two doubles I chose for the test were a dimmer, closer pair, STF 1523 (xi ursa majoris), at 1.7" 4.3/4.8 and Castor, 4.3" 2/2.9, an eight times brighter, widely separated double. I used an Orion V-block, and the following wratten standard filters: #11, 12, 58. Comparisons were made at length between all filters at a wide variety of magnifications on both pairs. The relationship would seem to bear out the prediction the above graphic implies:

Castor: Using the #58 filter affforded clear duplicity at 47x (25mm kellner, 202" apparant separation) where using the MV filter the magnification required was about 60% higher, or 70x (16.8mm abbe, 301" apparant separation) The #11 and #12 filters also gave improvement over the Minus violet but it was not great enough to measure using a magnification differential.

The second double, Xi Ursa Majoris at 1.7", was considerably more difficult and required higher magnifications. With the #58 filter clear duplicity was achieved at 153x (7.7mm abbe, 260" apparant separation) and using the MV filter required ~50% more magnification (236x) for a comparable split, (5mm abbe, 401" apparant separation). #11 and 12 filters ofered increasing clarity, improving on the MV filter but falling well short of the effect of the #58.

On a previous night, i was able to ascertain duplicity of Zeta Cancri at very high power but only while using the #58--without this help the double would simply not separate.

As factors such as seeing, individual vision variences, etc are either shared or would be proportional between these representative observations, i think the gross relationship could be considered an true representation of the gross effect of concentrating an image within the most highly corrected wavelengths of an achromat. It seems plain that MV filters, of which the Orion V-Block can be considered representative, (as the variences in transmission characteristics between it and other brands are negligible) are of very limited utility in reducing chroma from a point source image compared to the #58 filter's effect; better than a white light view, yes, but not measurably so.

Summing up, it would seem that the #58 offers perhaps a 40-50% imporovement in the apparant visual separation angle required for a clean, clear split for both very bright and mdeerately bright doubles of wide to moderate separation. The MV filter comes in last with the #11 and 12 filters 2nd and third respectively. White light rendered the poorest definition of all of course. The #58's advantage, however, could be expected to dissappear when target intensity becomes low enough to make the #58's ~35% loss in transmission (at 550nm) a factor. I have found this point to occur around 6th magnitude.

I'd welcome anyone else's experiences that have tried something similiar with their achromat, pro or con.


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