Night vision - M64 from the city

[Highburymark]

Took my F/7.4 Tak FC-100 4” frac out last night to see if I could pick up any spring targets in the SW/W/NW sky with my night vision monocular. Used a TV Delite 18mm eyepiece (and Baader IR-pass 685nm filter) to give 41x. This is particularly slow for a night vision set up - a relatively high power eyepiece in a fairly slow telescope, so a very long way from the brightness I can get with a 55/67mm TV plossl and F/3.3 Tak Epsilon 130d, (equivalent of F/1.5-ish, for faint nebulae). But I was looking for objects that need scale. As ever, I was observing from my garden in North London - the very worst light pollution possible, with a floodlit school at the end of the garden bathing the night sky in local light too.
It was a difficult night in that seeing was poor, but transmission must have been excellent because I managed to pick up a very nice M53, a globular I’ve struggled with before. From here I moved on to a superb M3, the core unresolved, but outer stars showing very nicely for short periods, before the seeing mashed up the view again. I didn’t have much time, so I moved on to M81/82, which were showing impressively higher up towards the zenith, but I was most pleased with M64 - Black Eye galaxy, an object I don’t think I’ve seen from home before. Not enough detail to distinguish the telltale dust lanes, but a satisfying way to end the session. Shows once again that though fast scopes work best for NV, it’s possible to achieve great results with slower systems - even from Bortle 8-9 locations.

[Highburymark]

An interesting comparison. After enjoying M3 with the F/7.4 Tak FC-100DC and Delite 18.2mm on Monday night, I tried the same object at the same magnification with F/3.3 Tak Epsilon and Delite 11mm on Tuesday evening. Both systems produced around 40x. Same Baader IR pass 685nm filter both nights.

First the maths. The FC100/Delite 18.2  combination was operating (roughly) at the equivalent of a very slow F/12.5, with an aperture of 100mm. The Epsilon/Delite 11mm combination was operating at (roughly) F/7.5, with an aperture of 130mm. No contest surely, for night vision, which thrives on faster systems?

Well, if there was any difference in the brightness of M3 between the frac and reflector, it was very difficult to distinguish. The two views were very similar indeed. Perhaps marginally improved brightness on the second evening. Stars perhaps slightly sharper on the first evening.

I think it illustrates the impact of transmission on night vision. On really bad nights, it can make you question whether the intensifier is working properly. On good nights, it’s like I’m under dark skies, not London N1. Clearly the transmission was poorer on Tuesday.

The other factor is that speed matters less with concentrated targets like bright globulars. Night vision works really well on these objects at higher powers with slower refractors. But on more dispersed targets like nebulae, the speed differences are more obvious - and the superfast Epsilon comes into its own.