IDAS LPS in lieue of L in LRGB imaging?

[buzz]

The filter suppliers often have Light Pollution Suppression filters in their range but they never group it with their LRGB sets. I realise that they are really for use with Bayer array'd cameras but I was thinking that if they improve image contrast, wouldn't it make sense to use them for the luminance channel too in LRGB imaging?

Apart from a probably focus adjustment for the filter thickness, is there a flaw in my musing?

Has anyone tried it and found it not to work very well?

regards

Chris

[johnrt]

I know a couple of people who do exactly that, they use the IDAS in front of all their LRGB filters with a mono camera and with great success too. I also plan to add one to my set-up to do exactly the same.

[swag72]

I have just got an IDAS as a replacement for my luminance filter. I did a comparison a while back of a 300s sub with the IDAS and without. The general consensus was that the contrast was better with the IDAS. It may cause a colour cast, but nothing that can't be easily fixed in post processing.

[buzz]

I had some more thoughts on the IDAS filter. I started doing what I think the others have done, taking 300s exposures through an L filter and the IDAS. The background level was much lower on the IDAS shot and the stars were a little dimmer, as expected. The finished image noise level was not amazingly different though.

I then realised that I was missing a trick that I uncovered when I was researching exposure.  Signal to noise improves with longer exposures as it does with stacked multiple exposures - I think you have to compare exposures between the two at equivalent SNR.  Using the exposure calculators that take a ratio of sky noise to read noise, I realised that I was doing the IDAS image a disservice. I could expose for double the time and keep the same SNR. I think that is when I will see the real benefit - since the better depth to the image will not require as much stretching.

[sloz1664]

Hi Chris,

I have replaced my Luminence with a LP filter. As you know you need that extra exposure for the detail.

I bin my RGB so don't necessarily need the LP filter for those.

cheers

Steve

[kirkster501]

I have a 2" IDAS.  I have not yet used it with LRGB as yet.  It was mandatory almost with the DSLR but I have not yet made up my mind if it is that necessary with LRGB as yet.  Certainly, luminance is very well protected from LP without the IDAS up to 5 minutes subs.  My subs are shorter - 150s - with RGB so I need to do some more testing.

[MeyGray3833]

Hi!

I use an IDAS LP2 filter on the snout of my camera where it goes into the field flattener. I use it in front of every filter so I can get some decent exposure times from my yellow zone. Would only be able to manage 5 minute subs with out it which is no good to me.

[russellhq]

...I then realised that I was missing a trick that I uncovered when I was researching exposure.  Signal to noise improves with longer exposures as it does with stacked multiple exposures - I think you have to compare exposures between the two at equivalent SNR.  Using the exposure calculators that take a ratio of sky noise to read noise, I realised that I was doing the IDAS image a disservice. I could expose for double the time and keep the same SNR. I think that is when I will see the real benefit - since the better depth to the image will not require as much stretching.

I never thought about it like that, I always assumed exposures of equivalent time would make for a fair comparison. But I'd suggest a fairer comparison would be two exposures of equivalent Signal. The noise is then the noise and you can then compare SNR between the two. The filter should then have a better SNR.

[BlueAstra]

In my old C8 SCT setup I had the IDAS 2" LPF on the 2" nosepiece of my CCD+FW assembly. That worked quite well. In my obs setup I can't fit the LPF that way, but did try it instead of the L filter. However, I found there was a focus difference between the LPF and RGB filter set. Probably because of backlash, I can't get the motor focusser to reliably offset the focus when moving between the RGB set and LPF, but that's another story!

[kirkster501]

Since my previous post I have now found the IDAS to be very useful in front of the RGB filters.  It reduces LP significantly.  

[buzz]

I did some more experimentation - just using the IDAS in the filter wheel, I need to move focus positions between that and the RGB filters (by about 0.2mm) . My OAG is then affected too by the focus. I think I can just find a mid position on the OAG so that it is focus enough for guiding in both focus positions.

Ideally I would like to put the IDAS upstream and keep the original L filter in the filter wheel. To do that I'm going to have to make myself an adaptor, probably within the throat of the filter wheel. I think the extra glass in the optical path is still going to affect the path length, so I may have to think about field flatness adjustment once more.

Perhaps I should just ask Baader to come up with a substitute to their L filter that is par-focal!

Chris

[kirkster501]

Ideally I would like to put the IDAS upstream and keep the original L filter in the filter wheel.

Thats precisely what I do.  All light hitting the sensor first goes through the IDAS 2" filter that sits upstream from the filter wheel.  Then it goes through the LRGB, whichever one is selected.  I have only just bought a Ha filter so no idea about that as yet.  I understand OIII and Su should not have a LPF in front of them but I cannot speak from experience on that one.

[johnrt]

I leave mine in all the time, in front of LRGB, Ha and OIII. Makes not a jot of difference.

[Steve 1962]

My IDAS filter stays in place in front of LRGB, Ha and Oiii filters all of the time too. I've not had any problems with it either.

Looking at the transmission graph for the IDAS, it would degrade Sii by c. 50-60% though.

Edited December 15, 2013 by Steve 1962

[Sp@ce_d]

I leave a P2 in front of the FF too on one of my LRGBHaSIIOIII shooters. Makes for better contrast & reduced LP for me. I was going to get another for the second in the 'Triple shooter' setup but see there's also a D1 now on FLO's site. http://www.firstlightoptics.com/light-pollution-reduction-imaging/idas-d1-light-pollution-suppression-filter.html

Can't seem to find a plot yet for it though. Wondering if anyone else had come across it or done a comparison with the P2 yet?

[Steve 1962]

Just found this....

here: http://icas.to/space/optical-filter/LPS-d1.htm

According to Hutech here: http://www.sciencecenter.net/hutech/idas/filtdata.htm it looks to me as though the P2 allows more Sii through than the D1 .... so my earlier post about Sii transmission was wrong - it also looks as though the curve shown on FLO's website for the P2 is more like the D1 curve.

I'm sticking with the P2.

Steve

[kirkster501]

Interesting that Ha and OIII not affected that much.  Thanks for sharing that.

[Sp@ce_d]

Ah thanks for that looks like another P2 then.

Sent from my iPhone using Tapatalk

[buzz]

Interesting thread - the only problem I have run into is how to mount a P2 in front of my flattener. I have the WO FF68 and WO FFIV.  I'm going to have to make some kind a SCT- T mount adaptor that accepts a 2" filter. The 2" noisepiece of the FFIV has an SCT thread, as does the alternative FF68 rear adaptor. I think a trip to AstroPhad in order again.

[Steve 1962]

 the only problem I have run into is how to mount a P2 in front of my flattener. I have the WO FF68 and WO FFIV.  I'm going to have to make some kind a SCT- T mount adaptor that accepts a 2" filter. The 2" noisepiece of the FFIV has an SCT thread, as does the alternative FF68 rear adaptor.

Chris - my IDAS is screwed straight into the 2" nose piece of my WO RFIV.

Steve

[buzz]

Hi Steve - I could do the same but I have an all-screw coupling policy!  I got Astrophad to make me a feather touch to SCT adaptor.  I just bought a Baader Neo filter - hopefully it has the same glass thickness as the RGB filters and I won't need to shift focus after a filter change.

What I'm thinking of is getting another one of these adaptors made up but putting in a 2" filter thread on the hole you can see there.

[Steve 1962]

 I got Astrophad to make me a feather touch to SCT adaptor

Ah ha! I see - thanks Chris

You have a PM

Steve

[kirkster501]

Hi Steve - I could do the same but I have an all-screw coupling policy...

Quite.  I always think compression and lock-screws are trusting to chance.  Sooner or later luck will run out. Screw-in if at all possible.

[James]

The filter suppliers often have Light Pollution Suppression filters in their range but they never group it with their LRGB sets. I realise that they are really for use with Bayer array'd cameras but I was thinking that if they improve image contrast, wouldn't it make sense to use them for the luminance channel too in LRGB imaging?

Apart from a probably focus adjustment for the filter thickness, is there a flaw in my musing?

....

I've used an IDAS filter on my setup for some time now. It's a UV/IR filter in it's own right it's good for luminance. I tend to switch it out when I do Red, green, blue but that's just preference on my part - I tend to do longer subs for luminance and shorter for RGB so appreciate the LP cut in my luminance. A focus adjustment is the only thing I require when I witch between it and my Baader RGB filters but it's minor and I'd check the focus anyway.

I know some leave it in when doing narrowband as well and I doubt that hurts but it isn't necessary as narrowband filters cut out everything else anyway!

IDAS have recently discontinued the P2 version of their LP filters and replaced them with their new D1 filters. The primary difference, as also pointed out above is that they've gone for a better colour balance for DSLR users by cutting into the SII emissions. For LRGB use in a DSLR this will give a better colour balance (and there's rarely that much SII anyway). For a cooled Mono CCD user doing LRGB the difference shouldn't be that much again as the SII signal isn't going to be huge.

For narrowband use however leaving the new D1 in your setup all the time will obviously impact SII collection so should you go down the narrowband road you'll want to find a way to not have to leave the filter in all the time

As Steve has pointed out above you should be able to screw the filter into the front thread of the FFIV, that's where mine sat for a while..?

James

[Sp@ce_d]

....snip

IDAS have recently discontinued the P2 version of their LP filters and replaced them with their new D1 filters. The primary difference, as also pointed out above is that they've gone for a better colour balance for DSLR users by cutting into the SII emissions. For LRGB use in a DSLR this will give a better colour balance (and there's rarely that much SII anyway). For a cooled Mono CCD user doing LRGB the difference shouldn't be that much again as the SII signal isn't going to be huge.

For narrowband use however leaving the new D1 in your setup all the time will obviously impact SII collection so should you go down the narrowband road you'll want to find a way to not have to leave the filter in all the time

As Steve has pointed out above you should be able to screw the filter into the front thread of the FFIV, that's where mine sat for a while..?

James

:eek: .. oh no no..  I was going to order another P2 after xmas. Any stock left?