Getting a ZWO 2600MC-PRO - choosing the right filters

[Sqren]

Hello! 

I ordered my first dedicated astrocamera a few days back, the ZWO ASI 2600MC-PRO. So while I'm waiting to get my hands on it, I figured I'd have to look into which filters to get for this camera. I have already settled on the Optolong L-eXtreme for the narrower stuff as I've seen some pretty amazing work done with this combo. 

But my problem is that I live in a bortle 8-9-location and have to deal with alot of light pollution. So which filter do I get for when I want to image some more broadband targets? Is there some good allround LP-filter I can use for most targets, and combine with some data taken with the L-eXtreme?

I have a lot to learn here, so I appreciate all the help I can get 🙂

/Aleksander

[vlaiv]

https://www.firstlightoptics.com/optolong-filters/optolong-l-pro-light-pollution-broadband-filter.html

if you want to stay with Optolong, or maybe one of these:

https://sciencecenter.net/hutech/idas/lps/plots/
(hover mouse over table on the right to see different charts - filter / LP source combination).

I personally use Hutech IDAS LPS P2 and it does make a difference in SQM18.5 skies.

[david_taurus83]

The IDAS are very good filters. The D2 is supposed to be very good at reducing LED light pollution. I have one myself but still have orange lights on my street, also filters those out well.

They have also released the IDAS NBX which looks like their version of the Optolong L extreme

[AnakChan]

IMHO, broadband in Bortle 8-9 isn't going to be useful. In Bortle 5-6, maybe a different story.

Here's a test I did last year in my Bortle 5-6 skies. The setup is as such, Takahashi µ250CRS (2500mm/F10), CentralDS A7S cooled to -18C ambient, ISO2000, 5 min exposures. Most are with IDAS filters but with one STC filter :-

1) HEUIB-II is probably the widest band but as you can see it's washed out in just 5 mins
2) LPS-D1 was the replacement of the LPS-P2 which is a wide-band LPS filter
3) LPS-D2 was designed to cut out LED (although I'm not so keen on the green cast)
4) LPS-V4 which "back in the day" was somewhat a "narrowband" but by today's standards isn't very "narrow"
5) NB2-PM is an Ha/Oiii dual narrowband with approx 19nm & 21nm respectively
6) NB3-PM is a Oiii/Sii dual narrowband to compliment the NB2-PM. The NB3-PM has been replaced by the NB4-PM. The NB4-PM is a little narrower.
7) STC DuoNarrowband - IMHO, STC was the one who kicked off dual narrowband trend back in 2018. After that we see a plethora of dual/triple/quad narrowband filters. The STC DuoNarrowband is either 11 or 12nm

You can see that even D1/D2 are kinda struggling under Bortle 5-6 as wideband LPS filters. I could shoot shorter exposures naturally. But if you're shooting under Bortle 8-9, IMHO you're better off just shooting NB, or (if you follow Cuiv, The Lazy Geek on Youtube) shoot many really shot 30 sec exposures and build your image that way. With my sensor, it's not practical for me to shoot thousands of short subs to process.

 

[vlaiv]

5 minutes ago, AnakChan said:

IMHO, broadband in Bortle 8-9 isn't going to be useful. In Bortle 5-6, maybe a different story.

What is not going to be useful? Filter or imaging? Neither is correct btw.

Only difference between heavy LP and light LP is amount of time you need to image. Given enough time, you can make good image even in Bortle 8 skies.

Filter only helps this - by reducing time needed.

As for your comparison - did you SNR tests on any of these images?

Image washed out is really not a problem - one can always remove DC offset created by LP. We can even remove gradients created by uneven LP. What is problematic is shot noise associated with LP signal. This impacts final SNR, but final SNR depends on number of stacked subs.

[AnakChan]

IMHO broadband filters are not going to be useful in Bortle 8-9 skies. If you're going to shorten the exposure to build an image, then you may as well ditch the broadband LP filter altogether and shoot without it.  As mentioned in my post, one could even shoot over a thousand 30sec subs to build that image (that addresses the SNR you're referring to). But the OPs proposed camera is the ASI2600MC which is 50MB per sub. Integrating 1000x 50MB subs would be challenging nevertheless.

It's true post processing could potentially address issues (gradients, etc.) however I would assume one would want to capture as many clean subs as they can to reduce the amount of post processing work.

[vlaiv]

4 minutes ago, AnakChan said:

IMHO broadband filters are not going to be useful in Bortle 8-9 skies. If you're going to shorten the exposure to build an image, then you may as well ditch the broadband LP filter altogether and shoot without it.  As mentioned in my post, one could even shoot over a thousand 30sec subs to build that image (that addresses the SNR you're referring to). But the OPs proposed camera is the ASI2600MC which is 50MB per sub. Integrating 1000x 50MB subs would be challenging nevertheless.

It's true post processing could potentially address issues (gradients, etc.) however I would assume one would want to capture as many clean subs as they can to reduce the amount of post processing work.

Broadband filters are designed for that specific purpose - at least LPS ones.

It is all about SNR in astrophotography and LPS filters improve SNR of your image. Simple as that.

Use of broad band filters is actually questionable in lower levels of LP than in higher levels. This is because LPS filters remove signal from both LP and from target. In order for LPS filter to be successful it needs to remove much more LP than signal since we want to remove noise associated with LP - which is square root of LP signal level.

if LP is weak - not enough of it will be removed to offset the fact that we are also removing part of signal from the target.

Another important thing is that LP signal needs to be "bunched up" in certain part of spectrum (multiple parts of spectrum are fine as long as it is bunched up). Above holds true for broadband targets like galaxies, clusters, reflection type nebulae and similar.

Emission type nebulae fare much better with narrowband type of filter - UHC type filter for example.

Here is example of light pollution spectrum:

We can see that bulk of light pollution here comes in ~550nm to ~620nm range and if we can remove that - we will remove much of LP signal.

Here is spectral response of Hutech IDAS LPS P2 filter:

Notice the match between above sample of light pollution spectrum and blocking part between about 530nm to about 620nm. There are two small band passes in this range and that is to offset color change that such filter makes.

Most galaxies and clusters are made out of star light, and also, reflection type nebulae - reflect star light. Here is example of stellar spectra:

stellar spectra are rather "uniform" in 400-700nm range - there is no major "bunching up" of signal, unlike light pollution. This means that we only take out part of the signal that is proportional to width of blocking part in filter signature.

So name of the game with broad band filters is to remove bulk of LP signal while leaving the most of stellar spectrum intact. If LP signal is weak - we will remove enough of target signal and end up lowering SNR in our images.

To see this - just examine case where there is no LP at all. Using filter will not reduce noise from LP since there is no LP signal, but will reduce target signal - lower target signal means lower SNR. This is extreme case. Other end of extreme is heavy LP in just few lines like Sodium lines. Somewhere between the two there is tipping point when use of broad band LPS filter becomes feasible.

[Sqren]

Thank you! Very interesting discussion 🙂

Seems like I'll be looking into some IDAS-filters then. The D2 seems like a good option, as I have alot of LED street lights etc. around me. Doesn't seem like the P4 is doing the best job with white LEDs. 

It also seems like I'll have to start packing up my setup and travel to dark sites some more. Is it really that much of a difference? And what do you do at dark sites then? Do you capture data without any filter at all? Or do you use like a UV/IR-filter or something? 

 

[vlaiv]

3 minutes ago, Sqren said:

Thank you! Very interesting discussion 🙂

Seems like I'll be looking into some IDAS-filters then. The D2 seems like a good option, as I have alot of LED street lights etc. around me. Doesn't seem like the P4 is doing the best job with white LEDs. 

It also seems like I'll have to start packing up my setup and travel to dark sites some more. Is it really that much of a difference? And what do you do at dark sites then? Do you capture data without any filter at all? Or do you use like a UV/IR-filter or something? 

 

I should be moving to a new house some time next year (was planned for this year but covid-19 and all that prevented us) and will change my sky from SQM18.5 to SQM20.8 (red zone / bordering on white to green zone, or Bortle 8 to Bortle 4).

I did some calculations and for most interesting targets, improvement will be around x6 - meaning same SNR can be achieved at that location in one hour that I now need 6 hours of exposure.

That is serious change and tells you how important dark skies are. I won't be using filters at that location because I believe they will hurt my images more than help.

You should use UV/IR cut filter if you have refractor as there will be significant bloating in UV and IR part of spectrum. With reflectors you don't need one for luminance in monochromatic imaging but I would use one for OSC as it will help with color information (easier to calibrate color).

My estimate is that IDAS LPS P2 reduces sky brightness by about 1 magnitude here where I am now - this is just estimate not actual measurement - so SQM 19.5 instead of SQM 18.5

[DaveS]

Dark sky makes more of a difference than any kind of filter. When I moved from SQI 18.25 (-ish) London to 21.66 Dorset the difference was spectacular, galaxies suddenly became doable and worthwhile.

[Nikolas74]

I am reading this topic with a lot of interest because I am on the same path of choosing the correct filter for my new Zwo Asi2600Mc pro which is on the way.... Since I live in Athens with Bortle 8-9, for sure I will buy the Optolong L extreme for, lets say, short of a narrowband imaging(I use to image with an Atik 383+mono) so I know already that I am not going to get the same Nebula details that I would like to...

but I think that I need also a Light Pollution filter that will act as a luminance filter that I can put on top of my narrowband images...and also be able to shoot some Galaxies. 

Do you think that is possible to do that?