When Smaller Is Better

Venus is a notorious tease. Her brilliance in the deepening twilight sky is a lure to any beginning astronomer, yet nearly all come away disappointed after centering the planet in the eyepiece. No details are to be had (at least none to an untrained eye without the aid of a filter) other than the changing phase of a featureless cue ball. 

The exception is when we view Venus in UV light. Thanks to an as yet unknown compound in the Venusian atmosphere that absorbs ultraviolet light the planet shows structure in its omnipresent cloud deck. As we know, UV is not something our eyes can discern. The best we can do visually is to apply a violet filter such as a Wratten #47 that reduces the glare a teases out a tiny bit of that structure as low contrast shadings. Most common of these (and with a scientific rationale for their existence) are the "cusp caps" seen at either or both poles.

All is not lost, however. The amateur who has outfitted his rig for planetary imaging can take advantage of the fact that the camera's sensor can record in UV. Some cameras are better than others in terms of their sensitivity in the UV portion of the spectrum, but even if you do not happen to have one of the more UV friendly models the odds are good that with the addition of an ultraviolet filter you can capture some details. A little over a year ago I purchased an Astrodon UVenus filter to pair with my ZWO ASI178MC (color) camera to see if I could capture the cloud patterns, and was delighted to have success earlier this year.

In researching the best approach to obtaining a good image of Venus in UV one of the tips that I ran across was to avoid lenses in the optical path. The anti-reflection coatings on them can apparently cut down on the UV transmission, making the image dimmer (and consequently requiring a lower frames-per-second rate that introduces atmospheric smearing). It has also been noted that the corrector plate on an SCT is not designed to provide correction in the UV range, so that design may be a bit handicapped when attempting UV capture.¹

Another factor I have noticed is that while you can detect cloud markings when Venus has become a large but slender crescent on either side of an inferior conjunction, the results seem far more interesting when the phase is somewhere greater than 35%. Enough of the disk is presented to allow one to often capture cups caps/collars as well as streaks in the middle of the planet that a crescent won't show.

My recent session a couple weeks ago was under average seeing with only fair transparency. Our sister planet was sporting a 70% phase and a diameter of only 16" as the planet continues to pull away from us following last August's inferior conjunction. Having to forego the 2.5x Barlow means it is a small image that one gets to work with, but it was clear even on the laptop screen during capture that the cusp caps were visible. Post processing can afford some help in enlarging the image (such as adding drizzle in AutoStakkert3!, leveraging the resizing functionality in Registax, or even using a custom resizing application like Topaz's GigaPixel product).

Since I had the time I decided to do a second capture, but this time with the Tele Vue 2.5x Powermate Barlow in the imaging train. I could immediately see the impact in terms of a dimer image, dropping my fps from 32 down to 9. This of course allows more time for atmospheric blurring to occur, reducing the number of frames of steady seeing that can be harvested. 

So while the image was certainly larger, the attenuation of UV light by the Barlow gave an image with far less clarity. Yes, the cusp caps are there, but the details in the smaller image, even after undergoing enlargement, were far superior. The verdict was clear - leave the Barlow in the box and work with whatever size we have to get the best UV image of the elusive Venusian cloud details.

¹ https://www.thefreelibrary.com/Imaging+Venus+in+the+ultraviolet%3a+a+new+development.-a0357147028