3nm Filters HA and [NII]

[avandonk]

Why I bought the ten position filter wheel was the availability of a Nitrogen [NII] 3nm filter. The continuum 5nm red filter also looked useful for reducing NB flare etc around bright stars.

Here is an explanation how a 5nm or wider HA filter covers both HA and [NII]. The detail in HA images can be due to Nitrogen at 658.4nm [NII].

http://dg-imaging.astrodon.com/gallery/display.cfm?imgID=148


For more info on Astrodon NB filters see here.

http://www.astrodon.com/products/filters/narrowband/

When I ordered the Astrodon filters the 3nm [NII] and continuum were not in stock. I will get them when they are available.

Should make for interesting NB images when there is little or no SiII 671.6nm.

Any thoughts on this?

Below is M27 with four NB filters from Astrodons site.

Bert

[gregbradley]

I think that's a good call Bert. I use 5nm Ha and with my CDK17 I find I get moon effects still. Also a bright spot but thats the CDK style optics.

3nm Ha no doubt would go a long way to rectifying that. But as the article points out you would need the N11 to make up for the lost signal.

If you are imaging from a light polluted location 3nm Ha makes a lot of sense with fast optics that make up for the increased exposure time needed with 3nm.

Greg.

[Poita]

The NII certainly gives a dramatic view.
Time for me to do some reading.

[Bassnut]

The NII instead of SII is an interesting idea. SII was often woefull with moon up. I don't quite get how you will use the continuum filters.

I tried continuum filter subs to subtract stars, but the imaging time spent taking extra exposures for them was tedious, the time would have been better spent on other image data and using PS to remove stars an overall better result (less noise for a given total imaging time)

[avandonk]

Fred I want to experiment to reduce flair etc due to the optic and filters from bright stars such as in the images below. This can only be done by real modulated subtraction of data from the NB image with the data from the red continuum image. When there is no faint nebular detail present in the continuum image this should be relatively straightforward. We hope?

The aim is to reduce the secondary reflections and flair etc due to the optic and filters which mainly affects the very bright stars. Not to eliminate all stars. It was an idea that came to my fevered mind when I was reading about the continuum 5nm NB filter. In theory it should work for the colour data as well. This is only true if the optic has very little chromatic aberration.

A good area for this method would be suppressing the very bright stars near the Horsehead.


Bert

[gregbradley]

Interesting Bert. I look forward to seeing the results. This is all new to me.

Generally speaking though Bert, you should find your filters are very resistive to flaring and halos anyway with their antireflection coatings.

I believe Astrodon now blacken the edges of their filters after pressure on the net. If not you should do that too. I noticed a small change when I did that but on some scopes it seems to make a large difference.

I am not sure if those scopes are the faster types (quite possibly) in which case you should be aware of that on yours. I think you already mentioned these were factory blackened.

Greg.

[avandonk]

Greg this idea was more to suppress the effects of the optic on very bright stars. No matter how good an anti reflection coating is or how well an optic is made, a very small amount from from a very bright star will be in the order of all the faint stuff we are trying to image. This is even more true with modern front illuminated very sensitive sensors.

The formula is

very small number X brightness of very bright star = measurable signal ;)

All of experimental science is about signal to noise. In any experiment the major aim should be to minimise noise first. Not just collect data. It is pointless when the data you are after is below the noise you are collecting.


Bert

[avandonk]

I forgot to say it also could be useful for minimising diffraction spikes from secondary mirror holders.

Bert

[Poita]

It certainly is a huge undertaking (your filters are worth more than my entire gear) and I am learning heaps from the discussion.
I'm really looking forward to seeing the results.

[avandonk]

Thanks Peter the cost keeps me awake at night. This is good for an astrophotographer.

All jokes aside this renewal of equipment was not taken lightly. It is about the same as the cost of a new car. It will keep my mind and body occupied till I croak. It is all about priorities. I am retired and own my house lock stock and barrel. This will be left to my daughter and grandaughter when I fall of the perch. All the astro gear will go to a good home.

Meanwhile I will battle on to think of new ways to do things. I used to work for CSIRO and our group was a world leader in it's field.

The results of all this equipment should be interesting.

Bert

[Poita]

Oh I would much rather drive an old car, and have that setup, than a new car and a pair of binoculars.

I agree it is all about priorities. The amount of gear I have is probably just as extravagant compared to my salary, but my reasoning is the same. I don't drive new cars, I service mine myself and I don't really drink, or go out and my dozen items of clothing all have to last for years. I don't own a TV.
This allows me to pursue a hobby that sharpens my mind and instills curiosity and exploration in myself and my kids. It is money well spent.

I do enjoy living vicarously through these threads and seeing what is possible with more esoteric gear. I am planning on hitting some of the same targets as you so I have reference images to work with, and it is always interesting to see the difference between low-end and high end gear.

I remind myself sometimes that I am getting better planetary images than the best observatories were 30 years ago, and I don't even know what I'm doing!

We live in lucky times for astronomy as a hobby.

[avandonk]

Peter I was in my backyard with a home made mount manually tracking for imaging in 1977. One mistake and you ruin the exposure. i used to hyper the film at work. A DSLR can do now in tens of seconds do what film did in hours.

We do live in interesting times.

Bert

[Poita]

Interestingly enough, I was doing the exact same thing ten years later in 1987 hypering the Fuji film at work, and manually guiding at night, though I did purchase the mount. Not a lot changed in those ten years, then next ten and the ten after that though...