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IDAS LPS-P2 versus LPS-D1

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#1 Alan Erickson

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Posted 06 December 2013 - 05:00 PM

Just as I was about to take the plunge and buy an IDAS LPS-P2, I see they have discontinued it in the round threaded format. The LPS-D1 is being promoted as its replacement, citing better color balance.

This is a head-scratcher. The D1 reduces red transmission beyond the H-alpha emission line, specifically reducing transmission around the SII line. Yes, it takes away some red bias, but it doesn't appear to make for a completely color balanced image. Software white balancing is still required, so why not give me all the red signal I paid for when I modified my camera? I may not want my images to look red, but I do want the higher signal-to-noise in the red, in my RGB images.

The D1 might promote contrast in the H-alpha details, by blocking much of the neighboring SII details. Fine, but I still want the P2 filter. If I want to promote H-alpha details, I will shoot through an H-alpha filter separately and manage it in post-processing.

I guess it comes down to wanting a choice.

I am attaching a couple images. One is chart I cobbled together, overlaying the response curves for four filters: the stock Canon IR-UV cut filter, the Baader replacement, the LPS-P2, and the LPS-D1. The LPS filters differ on the far red, with the D1 cutting out much of what the Baader filter added.

The P2 curve nicely extends out to the range of the Baader filter; a good match.

The other image is taken from this DigiBorg blog posting:
http://digiborg.word.../lps-d1-lps-p2/

That blog compares daytime images shot with a modified Canon using the P2 and D1 filters, with automatic white balance. I further white-balanced those images in software (and even the D1 image needed it).

So my ask is, give me all that red in the P2 filter, promoting SNR, and I'll balance it in software.

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#2 Alan Erickson

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Posted 06 December 2013 - 05:06 PM

Here is the white balance image I mentioned above.

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#3 Lightning

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Posted 07 December 2013 - 07:20 PM

S-II emission is _faint_. Try finding an S-II image taken with a DSLR (that hasn't been mono'd) and you'll get why it doesn't matter.

Cheers,
Cam

#4 microstar

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Posted 07 December 2013 - 08:05 PM

I just ordered a P2 myself, only to find that it has been phased out for the D1. I too wondered about the loss of the SII, but having tried SII with an OSC (QHY8) I decided it wasn't a big concern as it's really hard to get SII with a color camera. I've attached an image done with my old QHY8 of the Tulip Nebula. Even with more integration time (and more stretching) the SII was pretty weak - probably no more than 5% of the Ha on most objects. I decided that it wasn't much of a sacrifice.
...Keith

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#5 Samir Kharusi

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Posted 08 December 2013 - 01:15 AM

SII is very weak, hence it is probably unimportant whether it is sacrificed or not. You get much more signal from H-beta (generally around a third of the accompanying Ha strength and consequently much brighter than SII). Problem with all these filters that attempt to give a so-so white balance is that artificial light pollution these days is becoming too broad-spectrum to filter out effectively. See how much the LPS-P2 passes of the strong emission lines from a modern energy-saver bulb and be discouraged (second line from bottom):
Posted Image

What we really need as a nebula filter currently for all one-shot color cameras is a duo-tone filter that passes simultaneously a very narrow band (say < 10nm) at Ha and a second narrow band that passes Hb and OIII only (say 20nm wide). The closest I could come to this was an Astronomik UHC combined with a rectangular passband UV/IR Blocker (like many modded cameras retain, or the Baader version, or Hutech's UIBAR):
Posted Image

The vertical lines indicate (left to right) Hb, OIII weak, OIII stronger, Ha and SII. As more and more street lights are updated (LEDs?!) it'll probably be a toss up whether one should use any light pollution filter at all when shooting galaxies. But get yourself an Astronomik UHC for those nebulae if your skyfog is brighter than Visual Limiting Mag 4.5. More on light pollution levels and what filters to go for here and here.

#6 microstar

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Posted 08 December 2013 - 03:43 PM

What we really need as a nebula filter currently for all one-shot color cameras is a duo-tone filter that passes simultaneously a very narrow band (say < 10nm) at Ha and a second narrow band that passes Hb and OIII only (say 20nm wide).


I'd argue for something slightly different. Narrow <10nm passband in both Ha and OIII - the H beta doesn't add anything that isn't already in the Ha red, so all it does is mess up the OIII and prevent you from getting good moonlight rejection in the OIII.

However, such a filter might be under development. I've corresponded with one supplier who emailed me that he is working on such a filter for OSC narrowband, and if you look at this DigiBorg blog ( http://digiborg.word...22/idas-lps-d1/ ) at the bottom of the page Ted writes: "Also IDAS is planning to introduce quite new type of RGB and narrowband filter system next year. Please keep your eye on IDAS products. Enjoy astro imaging more with the coming new IDAS filters". I queried him on whether this new narrowband filter would be for OSC/DSLR cameras and he indicated affirmatively. If you tightened up the OIII (and eliminated the Hbeta) from the Hutech LPS V4 you would have something very close to what's described above.

I'm keeping my fingers crossed that one of these filter producers will shortly be marketing an option for simultaneous Ha and OIII data collection with a single dual-band filter.
...Keith

#7 Samir Kharusi

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Posted 09 December 2013 - 12:10 AM

I corresponded with one of the filter fabricators on this topic about 5 or 7 years back, and while he said he saw no difficulty in the design, he was not convinced that there was enough demand to warrant production. Hopefully as people become more aware of the issues, we will finally get one. One can argue either way for inclusion or exclusion of Hb. I found Hb surprisingly bright in the Orion Nebula and may help in prettifying things, but slicing it off will help in rejecting more light pollution, Moon or even artificial light:
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The horizontal arrows indicate the FWHM bandpasses of the Astronomik UHC + UV/IR Blocker I used to capture the far right image of Orion in a VisLimMag 3.5 sky. Nevertheless there is a strong emission line to the left of OIII in the energy-saver spectra below (I have no clue what it is from, phosphers?) that could be hugely annoying. Compare the spreads for the 8nm Baader OIII and the UHC with the energy-saver light (lower set):
Posted Image






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