On variable gain control benefit and the why’s of filter selection - some examples for those newly looking into getting into Night Vision Astronomy
A great feature to control noise and scintillation while using narrowband Ha filters is variable gain control. Offered on several Night Vision devices, it is a desired feature but not an essential feature and here is some reasons why:
Turning down the gain from max will start smoothing out the view with less scintillation and noise.
You would think it would start getting too dim to make out details as good but it’s really the size notch of your narrowband that improves the contrast on nebulae the most. There are some performance specs on the Intensifier tubes that also can contribute to contrast increases, but filter widths can make the biggest difference in my experience.
The narrower the filter, the more contrast but at the expense of more scintillation and noise.
Turning down the gain a little will lessen scintillation and the contrast from the filter is still there. I generally only use max gain for threshold objects or if I’m using an NVD without the variable gain option.
The faster the f ratio, the brighter the image gets at the expense of image scale. Faster f ratio, smaller image, but brighter view so less noise. NV users will learn how to dial it for aesthetics of view vs amount of gain needed for objects with your particular width of Ha filter you choose.
I think a 7nm Ha is a good place to start for filter notch width.
You learn to balance f ratio with notch selection also. Narrower widths introduce more noise and scintillation so to combat that you have options of reducing f ratio with reducers to brighten the view and decrease noise along with adjusting gain settings to lessen scintillation. If using a barlow and increasing f ratio, you might want a wider notch to decrease scintillation.
Some examples of how this works might be:
7nm filter - everything looks good at native F/5, turn gain down just a hair and lessen scintillation on a bright mag 6 nebula
5nm filter - noise and scintillation increase. Add reducer to increase brightness. Now maybe at F/3 - Image is smaller because of the reducer, but more pleasing with details because contrast has increased by the narrower notch and noise/scintillation decreased because of the brighter image. Dial in gain to taste
12nm filter - you are going for a distant or much smaller object and want to increase image scale so you have used a barlow and now operate at F/10. The longer focal ratio causes a dimmer image so more noise/scintillation introduced but the wider notch of 12nm is simultaneously losing some contrast but allowing a wider signal so its brighter than the 7nm or 5nm and more pleasing to view by again decreasing noise/scintillation introduced by the dimmer image from longer f ratio.
Alternative scenario - you operate at a native F/5 and get a nice contrasty view of the Rosette with your 7nm but want to see some of the star cluster in the middle of it come through better. You switch to a 12nm to see more of the cluster at the expense of a little contrast loss on the nebula.
basically it is figuring out how to blend filter performance, focal ratio performance, and gain performance to get a view that you are most pleased with. Having multiple filters eventually, helps with more options.
The longpass the same. Some examples:
In my Bortle 7 skies, I still want to do some sweeping of star fields. I’ll put in a 640nm that still allows Ha through and can see the billowing gas clouds of the Milky Way while having a pretty bright view for a smooth image and cutting LP a great deal.
Next, maybe I use my mak or barlow my short tube refractor for some closer image scale but now I’m looking for galaxies, HII regions, open clusters, and globulars but still fighting the same LP - I switch to a 610nm to brighten the view a little that has been dimmed from image scale increase and keep a smooth view. I lose a little contrast on HII.
I then decide to just look for galaxies, open clusters, globular clusters only - I switch up to my 685nm filter as Ha is no longer the priority and want some max LP decrease. A little noisier but still pretty smooth because it is a longpass and not a narrowband.
or maybe I want to increase Ha contrast a bit but still get plenty of stars coming through - I switch to my 35nm narrowband.
These are just some examples and theory behind my own methodology. Of course a single narrowband filter in either 3nm, 5nm, 6nm, 7nm, 9nm, 10nm, or 12nm will work at your native scope focal ratio AND reduced focal ratio with reducer AND even increased focal ratio by barlow use. I’ve read posts from various members on narrowband use and have seen all those widths listed in the various observing reports. You can pick just one narrowband Ha filter width to start with and use it in all those configurations.
By no means are all these filters absolutely necessary.
It just increases options. Two or three are pretty much essential though to get the most out of your NVD - if I had to choose only two, it would be my 7nm Ha and my 640nm longpass with my particular scopes. If I’m going to dark skies, I might just use one narrowband only and skip the longpass use and view all non Ha targets with no filter.
I am not suggesting a person needs all these filters but more trying to give examples of how different filters give a variety of options for specific scenarios of objects viewed and blending to get the most pleasing view.
In my experience it is very subjective here on the forums in which is liked the most because we all have a wide variety of telescopes, different observing conditions, varying degrees of performance in image Intensifier tubes, and different ideas of most pleasing blend. After awhile you will find your own favorite combinations and in practice after you’ve picked what you like, it is super easy and becomes second nature.
Someone else here might come along and think I’m completely batty in my methodology too and have their own suggestions on how to benefit from using different type filters. I’m just throwing it out here because I originally wrote this ridiculously long post as a response to someone answering whether they got the variable gain option or not.
When I started using NV, I read a ton from all sources and then combined what I thought made sense with results of my own experiments in filter use, focal reducer use, and gain adjustment.
Eventually everyone does this the longer they use Image Intensifiers for astronomy.
Boy I’m glad I didn’t use this as just a response to a simple answer on the variable gain option.
Edited by Vondragonnoggin, 09 May 2019 - 07:39 PM.