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L-eXtreme Filter On A Light-Polluted Telescope?

Astrophotography SCT CMOS DSO Filters Imaging LP Equipment
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#1 MeteorBoy

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Posted 17 October 2021 - 03:06 PM

I live in extremely light-polluted skies.  I’m really hoping that Optolong’s L-eXtreme filter might be the imaging answer for me.  But I have concerns as to how much this filter can actually help me.

 

Here’s my imaging environment…

  • Bortle 10 skies (yes I’m off the chart) giving me a LM of just 3.0-3.5.
  • Telescope is a 20cm SCT at F5.7.  It has a small fixed 15x23 arc/min FOV.
  • Mono camera for mono imaging only.
  • My telescope tracking accuracy yields a max exposure of only 2 secs before trailing occurs (2-sec exposures have always been otherwise sufficient for respectable deep-sky photos with 10 mins of integration).
  • Extreme sky brightness requires my camera to use a gain of 90% for that 2-sec exposure (to detach it from the left side of the histogram).

Here are my L-eXtreme filter concerns…

  • Even with “Precise GoTo” my target is outside the FOV one-half the time.  I then try to locate it by mapping the visible stars (plate-solving not available).  With this filter’s light transmission of just 3.8%, how can I continue to map the mostly gone field stars in order to find my target?
  • Wouldn’t my finished photos look unnatural having a strong emission target sitting in a very low count star-field?

To summarize, if my camera gain is already at 90% and I can’t expose for longer than 2 secs what can I do about finding my target that is missing most field stars for navigation and about the resulting low star-field photos that may be lacking real-world aesthetics?

 

I sure would appreciate some guidance before I invest in something that might not work very well for me.

 

Any thoughts please?



#2 descott12

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Posted 17 October 2021 - 03:14 PM

I am not an imager (only EAA) but I use a small guide scope piggy-backed on my SCT. The guide scope is used for guiding sometimes but I also use it as an e-Finder and I plate solve with it. So I can have any filter I want on the main scope and still plate solve.

 

Your 2 second exposure limitation seems like a much bigger problem...


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#3 elmiko

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Posted 17 October 2021 - 06:16 PM

You might want to post this in the beginner imaging forum.



#4 MeteorBoy

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Posted 17 October 2021 - 07:28 PM

I took your advice and posted it to the beginner's thread.  Thanks.


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#5 JamesTX

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Posted 17 October 2021 - 07:31 PM


Here are my L-eXtreme filter concerns…

  • Even with “Precise GoTo” my target is outside the FOV one-half the time.  I then try to locate it by mapping the visible stars (plate-solving not available).  With this filter’s light transmission of just 3.8%, how can I continue to map the mostly gone field stars in order to find my target?
  • Wouldn’t my finished photos look unnatural having a strong emission target sitting in a very low count star-field?

To summarize, if my camera gain is already at 90% and I can’t expose for longer than 2 secs what can I do about finding my target that is missing most field stars for navigation and about the resulting low star-field photos that may be lacking real-world aesthetics?

 

I sure would appreciate some guidance before I invest in something that might not work very well for me.

 

Any thoughts please?

Why not use a lum filter to get your scope lined up and then swap the filters?

 

In regards to your second question.. what are you trying to achieve?  Star field is a matter of taste.. some people love lots of stars.. some people don't like any.  One could argue anything in narrowband is going to look "unnatural".  In your challenging environment I wouldn't worry about a deep starfield.

 

Being limited to 2" max exposures I think puts you in the realm of "lucky imaging".  I've not tried lucky imaging myself but its a technique often used with planetary nebula.  Something you can research.

 

Side note on L-Extreme.. just in case you are not aware... L-Extreme is intended for color cameras.  You can certainly use it with a mono camera.. but there will be no color data that you can create a color image from.  No way to separate the Ha and O3 signal from data collected by the L-Extreme on a mono cam.  If your aim is grey scale images then you are good.


Edited by JamesTX, 17 October 2021 - 07:31 PM.


#6 jonnybravo0311

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Posted 17 October 2021 - 08:15 PM

Regardless of your sky quality, the L-eXtreme filter is an absolute waste of your money with a mono camera. The L-eXtreme (and all of the Optolong L-* filters) are designed to be used with color cameras.


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#7 DirtyRod

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Posted 17 October 2021 - 08:15 PM

Why not use a lum filter to get your scope lined up and then swap the filters?

.

 

Absolutely agree. I use an L-Extreme filter for some targets but it blocks so much light I often cannot frame the target so I bought a filter drawer and I find the target first and frame it then slide in the filter once I have guiding enabled

 

 

 

Your 2 second exposure limitation seems like a much bigger problem...

Also agree. I spent the last month working on tuning my mount so I could get longer than 10s exposures without trailing. I'm up to 60s now and under15s DSS couldn't even find stars to stack with the L-Extreme filter. I would definitely put some cycles into figuring out what's going on with your mount. 



#8 MeteorBoy

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Posted 17 October 2021 - 08:25 PM

Regardless of your sky quality, the L-eXtreme filter is an absolute waste of your money with a mono camera. The L-eXtreme (and all of the Optolong L-* filters) are designed to be used with color cameras.

Yes, I was aware of that.  I simply didn't want to have to buy a filter AND a new camera to go with it when my existing setup works just fine.  I don't mind mono images as most targets are B&W anyways.



#9 Alex McConahay

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Posted 17 October 2021 - 11:41 PM

>>>>>>My telescope tracking accuracy yields a max exposure of only 2 secs before trailing occurs

>>>>>>when my existing setup works just fine.

 

Those two comments do not fit together. If you can only take pictures for 2 seconds before trailing, something is wrong with the mount or how it is being used. A tracking mount, properly polar aligned, can take more than two second exposures before tracking errors become really objectionable. Even, I think, at that focal length. 

 

I think you need to slow down, and work on getting that mount organized. 

 

SOrry, I cannot help much with the light polluted skies. Perhaps regular narrowband filters would help on some targets. I am not too familiar with the multi band narrow band filters, I should think, though, that while they are designed for color filters, there may be some use for them in monochrome imaging. After all, what they do is blank out some areas of the light spectrum, and let other areas pass through----the areas that nebulae glow in. Why should that be all that much different whether using color or monochrome? I am sure more informed others will jump in here.  

 

 

Alex


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#10 Alex McConahay

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Posted 17 October 2021 - 11:45 PM

>>>>>Wouldn’t my finished photos look unnatural having a strong emission target sitting in a very low count star-field?

 

All narrowband filters (of which the L EXtreme is a subset), limit stars. But stars are so much brighter than nebula, and the elements in the star that put out light are so much brighter than that same glowing gas in the nebula, the stars will actually show through. They will be dimmed. But, nearly all stars reach maximum saturation at the very centers. So, really, stars show through. They are just smaller than they would have been in LRGB imaging. 

 

Alex



#11 MeteorBoy

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Posted 18 October 2021 - 08:15 AM

Hey JamesTX & DirtyRod,

 

To answer your questions...

 

I have a filter wheel and I simply didn't think of temporarily swapping the L-eXtreme filter out while I find and frame my target.  What a great idea!

 

As a crude test, I shot the night-time city-filled lights out my window, then reduced the exposure to 4%.  There weren't many lights to see.  OK, emission nebulas with few stars..., get used to it.

 

I'm familiar with lucky imaging, use it all the time.

 

I know that the L-eXtreme filter is intended for color cameras and that for mono images it works just fine.  I shoot mono because it's better under extreme LP.  I can get to mag 19 in 15 mins of exp.  I sometimes colorize them (like the sun or single frequency emission nebula) and they look great and natural that way.

 

LP actually limits my exp to 5-10 secs.  My 2-sec exposure limit is caused by some irregular teeth on the gears of my ALT/AZ mount.  Not something I think can be fixed so I work around it.

 

Great ideas guys.  Thanks.

 



#12 MeteorBoy

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Posted 18 October 2021 - 08:37 AM

Hey Alex & jonnybravo0311

 

To answer your concerns...

 

As a crude test, I shot the night-time city-filled lights out my window with a DSLR, then reduced the exposure to 4%.  There weren't many lights to see.  OK, emission nebulas with few stars..., get used to it.

 

I know that the L-eXtreme filter is intended for color cameras but for mono images from a mono camera it works just as well.  I shoot mono because it's better under extreme LP.  I can get to mag 19 in 15 mins of exp.  I sometimes colorize them (like the sun or single frequency emission nebula) and they still look great and natural that way.

 

LP actually limits my exp to 5-10 secs.  My 2-sec exposure limit is caused by some irregular teeth on the gears of my ALT/AZ mount.  Not something I think can be fixed.  I work around it by stacking more frames until I get to 10-15 mins of integration.  So yes, my existing setup indeed works fine with a 2-sec exp limit.  I'm one of those that still appreciates it when my glass is half-full.

 

For centering, I was informed by others that what I could do was to swap out my L-eXtreme filter from the wheel, find and center my target, then return to the L-eXtreme filter.  I had not thought of that!

 

Many thanks, guys.



#13 jonnybravo0311

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Posted 18 October 2021 - 09:27 AM

I'm a mono shooter... so I understand what's involved. I'm just confused as to why you want the L-eXtreme filter. Why not just use a dedicated Ha filter? That way if you do decide at some point to create a color image, well, you could pick up a dedicated O3 filter and create an HOO image. You _cannot_ do that with the L-eXtreme because you have no way to separate out the Ha and O3 signals it passes.

 

If you've got a filter wheel already in the mix, then to answer your other question about finding/framing... the suggestion of just moving to a different position where either no filter, or a much broader filter (like a luminance) is, works perfectly. It's how I do my plate solving... I change to my L filter, take short exposures, slew and center, then once I'm all framed up, change my filter back to whatever one I want to image with.



#14 WadeH237

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Posted 18 October 2021 - 10:36 AM

Since you are using a mono camera, you do not want to use an L-eXtreme.  They are made specifically for one-shot-color cameras.

 

You will want a single wavelength, narrow band filter.  If you only have a budget for one, then get a good Ha filter.  You can do your framing with your luminance filter.  If you get a narrow band filter that is parfocal with your luminance, then you can also focus with the luminance filter.  If you have automated focusing, you can also determine focus offsets for each of your filters, relative to the luminance filter.  Then you can always use luminance filter for focus, even your filters are not parfocal, and the software will adjust focus to match whichever filter you select from the filter wheel.



#15 dx_ron

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Posted 18 October 2021 - 01:26 PM

One note about plate-solving and focusing with narrow filters:

 

You don't need to do the capture for plate-solving (or captures for focusing) with the same gain and binning settings you use for capturing Light frames. Go ahead and crank the gain way up and bin however your software/camera will let you. (some software might make you manually reset those back to normal - all I know is that Ekos can use high gain plus binning for either alignment or focusing without it affecting the capture sequence settings)


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#16 WadeH237

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Posted 18 October 2021 - 01:53 PM

As a crude test, I shot the night-time city-filled lights out my window with a DSLR, then reduced the exposure to 4%.  There weren't many lights to see.

This isn't really a fair test.

 

The reason is that stars emit light in the entire spectrum, so they are actually pretty bright in Ha and OIII.  Yes, they will be dimmer through a narrow band filter, but not down to 4% - not even close.

 

To give a reference, my wide field setup is an ASI2600MC Pro, one shot color, camera.  I'm using an 80mm refractor with a reducer/flattener running at F/4.8 (384mm focal length).  Without a narrow band filter, I was using 5 second exposures, binned 2x2 for my plate solve exposures.  When I put in my L-Extreme filter, I did some testing, expecting to have to increase the exposure length, but it still solved fine, even through the narrow band filter.  For focusing, I take even shorter exposures (I think down to 1 second).  I was surprised to find that my autofocus routine worked even through the L-Extreme without changing anything.

 

To be fair, my skies are fairly dark (Bortle 3).  If I had significant light pollution, I might need to tweak some things, but not ridiculously so.



#17 sbharrat

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Posted 18 October 2021 - 02:54 PM

One note about plate-solving and focusing with narrow filters:

 

You don't need to do the capture for plate-solving (or captures for focusing) with the same gain and binning settings you use for capturing Light frames. Go ahead and crank the gain way up and bin however your software/camera will let you. (some software might make you manually reset those back to normal - all I know is that Ekos can use high gain plus binning for either alignment or focusing without it affecting the capture sequence settings)

Same thing with NINA... Options -> Plate Solving options apply to plate solving only and don't affect the regular imaging...

 

platesolve.jpg


Edited by sbharrat, 18 October 2021 - 02:55 PM.



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