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More on the 3Nm Astrodon.

ollypenrice

By ollypenrice
in Imaging - Discussion

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ollypenrice

ollypenrice

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All this is interesting. We went for the 3 because we wanted the very crisp edged structures, high contrasts and small stars. The endgame for us (Tom and myself) is combination with braodband, not standalone NB.

While I understand that moonlight is reflected sunlight and while I can't explain the superiority of the 3 over the 7 in moonlight, the evidence is pretty clear to me that it is superior. I'm not aiming to make a habit of moonlit imaging since moon week is sleep week, but the top part of this image was taken fairly close to a nearly full moon. The top quarter is pure moon-time and below that it is blending in with a dark time lower panel. It seemed insane to be trying to image in so much light since I could read my notes outside without a lamp! The top panel was noisier than the bottom and needed more flattening in PI, but it is perfectly workable. A couple more 30 min subs on a dark night will see it put to bed, I think.

7822%20Test-XL.jpg

Olly

PS This is just a rough processing, uncleaned, to show the moonproofness of the 3.

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Gina

Gina

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Apart from the Fraunhofer lines of absorption the spectrum is essentially flat over the range we are considering, so the amount of background due to moonlight will be directly proportional to the bandwidth.  So the 7nm filter will pass just over twice as much background moonllight as the 3nm.

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AndyUK

AndyUK

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A set of these is on my list too... but I too am dithering between the 3nm and 5nm for Ha, (but 3nm for the other two). 

As Steve has noted, the exclusion of the NII band in some DSO's using the 3nm would seem to necessitate then also buying an NII filter (not only painful on the credit card, but also time).  However, seeing how much the moon effects the 7nm Ha (I used the night before last for testing my rig for 1800s exposures - As expected the gradient was atrocious!), and seeing proof of what's possible with the 3nm, then that seems to be a plus the other way.  Yet again another compromise required I think... but I will be upgrading at some stage :smiley:.  Thanks for posting this thread Olly...

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opticalpath

opticalpath

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My own take on this as I currently ponder the purchase of a couple of Astrodon filters is this:-

I can't justify the purchase of an NII but I do want to capture NII data (try comparing the detail in images of M27 taken with NII data included and without to see why)......

That was my thinking .... do I really want to go ultra-narrow and throw away the NII data? Trouble is, I don't know if NII is 'good' or 'bad' for the majority of Ha emission objects: it adds signal, but does that improve the narrow Ha-only image or just fuzz it up? Whatever the answer, I haven't heard anyone saying they regretted choosing the 3nm and I think that's where I'm heading.

Adrian

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ollypenrice

ollypenrice

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Tom and I still have the option of shooting in the 7Nm as well or instead. Not sure what the consequences would be! I don't use good skies for testing and testing in poor ones isn't testing, so we'll have to wait for a real world situation to arise, but doubtless it will.

Thanks for your expertise, Ken. I follow your logic.

Olly

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Jessun

Jessun

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Here are two images using the 3nm Astrodon Ha in full Moon. I sure can't complain. On another note in Sara's ongoing thread there are some funny things going on with these filters on the Atiks at a particular spacing it seems. Odd.

Sadir and Pelican, 2 hours and 2 hours 30 minutes.

/Jesper

gallery_16323_2721_1707958.png

gallery_16323_2721_1229853.png

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johnrt

johnrt

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I'm not sure the the reasoning of a narrower OIII & SII than Ha is sound. Here's my thoughts -

In a narrowband image all the detail and contrast sits amongst the Ha signal, on 99% or targets the OII and SII simply provides a colour hue to an area, there is no actual detail or contrast there the signal very much stays in the chrominance data within the image. Yes, you will stretch the OII & SII harder, as it is weak in signal, but this is no issue as chrominance noise reduction is simple and can be done without degrading image quality in any way.

So all you benefit from with the narrower filter is added protection under moonlight, and no matter how narrow the filter I'll need a lot of convincing that OIII & SII imaging under bright moonlight is a good idea. The signal is just too diffuse and tenuous to stand up to a full moon, unlike Ha.

So that is basically the reason I matched up the 5nm OIII to the 3nm Ha, I of course stand ready to swallow everything I've just said and look rather silly should I be talking a load of old nonsense. :)

We will see the next time I get some imaging time! Watch this space!

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ollypenrice

ollypenrice

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John, I take this point. I think that in aesthetic astrophotography, which is what I do, it is very important to know what you are hoping to obtain from each filter. In most objects all I want from O111 is a little contrast, AKA three dimensionality, bringing to the Ha. There are exceptions. Thor's Helmet and the Cresecent are transformed by O111. The Crescent has an outer shell which, blinked on and off in Photoshop, is astonishing! These objects are, probably, Wolf-Rayet 'planetaries' and doubtless there are others. To give a different example, Ha in emission nebulae is the source of structural detail. We all know this, so we want a high contrast filter. But Ha in galaxies, for the most part, is going to give us a series of diffuse blobs void of structure but not void of interest. For most of us a blob is going to be a blob. Blobs can be good! For an exception to the Ha-Blob-Galaxy rule check out R Jay Gabany's M106... Lord knows how he found those jets because I could only find one of them. I'll be back!

Olly

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