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Favorite Light Pollution Filter for EAA?

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#76 StarCurious

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Posted 30 March 2016 - 03:09 PM

 

...

I am not 100% sure, but I think it was captured with both Deepsky and 495nm Longpass.  I think I still have focusing problems(?), among others ...

attachicon.gifM51 _Stack_50.png

 

Hi Joseph,

 

Yes, it appears that focusing still needs some work.  Don't be discouraged though.  With any new setup it takes time to learn all the sweet spots for focusing, etc.  Once you've gone through the exercise a couple times it will become easier.  For sure use the Bahtinov mask as it will make life much easier.  I make a point of going to one of the brightest stars out at the time as part of my initial mount alignment routine.  While there I get my focus spot on.  Using very narrow filters plus the additional attenuation of the focusing mask can make the signal very low so you need a very bright star to focus with.  Since you are using M51 as your test target, maybe use Arcturus as your focus star as it is relatively nearby in the sky.  It is worth the time to GOTO the star to refocus when you change filters, then GOTO the target object when done.  It seems tedious but will be much easier in the end.

 

You have an interesting complication, that your filter forms part of your spacers to the focal reducer.  This will make it difficult to determine the impact of the filters on exposure time as your f-ratio will be different between configurations.  Is there any way you can set the spacing independent of your filter selection?  Can your filters go on the end of the FR closest to the telescope?  I agree that sometimes due to reflections it is better to put the filter in between the camera and FR, but for your testing it will be much easier to just have the filters on the end of the FR.

 

Keep up the good work, and good luck!

 

cheers,

 

Jim T.

 

Thanks, Jim.

 

I will need to work on focusing skill.  At this stage, I don't know how much the bloating was due to focusing or due to wide bandwidth.  With the 495nm Longpass and Deepsky stacked, only 495nm-530nm and 630nm + should have been passed. In terms of bandwidth, that would have been 25nm wider than a 610nm Longpass, but 80nm wider than IR 685 nm pass .  The trade off is between brightness or number of stars detected versus bloating.  At this point, filters are designed for either visual or long exposure imaging.  With long exposure imaging, one can target a galaxy such as M51 and block everything with shorter wavelengths.  With EAA, one need to detect enough stars, perhaps beyond the IR range, for Live Stack to work.

 

I will move the filter in front of the focal reducer in the next test.

 

One other thing, I don't understand the notion of Gamma and Brightness in Sharpcap. As I shortened the exposure, I had to move Gamma or Brightness to the right (increase).  This is a learning experience for me, and it feels like peeling an onion, one layer at a time.


Edited by StarCurious, 30 March 2016 - 03:10 PM.


#77 JPKellysr

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Posted 30 March 2016 - 03:18 PM

Check out Gamma Correction in Wikipedia.



#78 Lightpath

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Posted 30 March 2016 - 03:32 PM

I love my Astronomik CLS-CCD.


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#79 jimthompson

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Posted 30 March 2016 - 10:55 PM

Hi Joseph,

 

Gamma is an adjustment of how the mid-tones are displayed on the screen.  Imagine you are looking at the histogram of your image.  The default has the black point at the left, the white point at the right, and the mid point (grey point) right in the middle.  Now grab the mid point and slide it down (left towards black).  Now more of the image information is to the right of the mid point than before, resulting in a brighter image.  Thus gamma down = brightness up.

 

The gamma is a very useful setting as it allows you to highlight objects that are not much different in brightness from your background without saturating your image at the black or white end.  With my ZWO camera (an ASI 185) I set the gamma quite low when I use it for EAA.  Values around 0.4 to 0.6 are common.  I also sometimes use low gamma when observing the Sun in Halpha so I can see surface detail and prominences at the same time, or Jupiter when I want to see its moons without saturating the face of the planet.  The gamma can allow you to change how a large dynamic range is displayed on the screen so you can do these types of things.  Sliding your gamma the other way, larger, can be useful for enhancing contrast in the image such as when you are trying to pull more detail out of Jupiters belts, the Moon's surface, or the Sun's surface for example.

 

In SharpCap there are 2 ways to adjust the gamma.  One is directly on the camera, which is the slider you will see on the right along with all the other camera settings.  You will see the impact of this adjustment in the individual frames and the resulting stack.  The second way is to adjust the Grey Level slider on the Histogram tab when you are stacking.  This adjustment will only show up in your stacked image.

 

cheers!

 

Jim T.


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#80 jimthompson

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Posted 30 March 2016 - 10:58 PM

To give everyone an idea of what is achievable using an IR pass filter, I finally got around to posting the captures I made on March 18-19th.  They are from my Lodestar X2c with a 650nm high pass filter, and were taken with a bright waxing gibbous Moon directly overhead both nights.  You can get an idea of how bright the Moon was from my lunar image captures in the same album.

 

https://www.flickr.c...157666048277440

 

cheers!

 

Jim T.


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#81 jimthompson

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Posted 31 March 2016 - 11:29 AM

So here's a question I have for the experts here - 

 

I have a Lumicon DS filter for my C8 for use in the city.  Should I use this for all DSO?  or take it out when i view certain objects?

Hi John,

 

If you are observing from light polluted skies (limiting visual magnitude of +4.5 or worse) then I recommend using your Lumicon DS all the time.  It will improve the contrast on all objects.  If highly recommend testing this for yourself by observing a short list of objects with and without your filter.  It is the best way to understand exactly what the impact of the filter is for your setup and location.

 

cheers,

 

Jim T.



#82 Dwight J

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Posted 31 March 2016 - 01:23 PM

 

So here's a question I have for the experts here - 

 

I have a Lumicon DS filter for my C8 for use in the city.  Should I use this for all DSO?  or take it out when i view certain objects?

Hi John,

 

If you are observing from light polluted skies (limiting visual magnitude of +4.5 or worse) then I recommend using your Lumicon DS all the time.  It will improve the contrast on all objects.  If highly recommend testing this for yourself by observing a short list of objects with and without your filter.  It is the best way to understand exactly what the impact of the filter is for your setup and location.

 

cheers,

 

Jim T.

 

 

I leave my Lumicon DS on all the time, even when I am at a dark sky site.  It improves contrast by suppressing natural sky glow and it acts as a dust shield keeping dust off my sensor.  


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#83 StarMike8SE

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Posted 31 March 2016 - 02:51 PM

I live in a RED zone and use my Astronomik's UHC almost all the time



#84 roelb

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Posted 31 March 2016 - 03:10 PM

To give everyone an idea of what is achievable using an IR pass filter, I finally got around to posting the captures I made on March 18-19th.  They are from my Lodestar X2c with a 650nm high pass filter, and were taken with a bright waxing gibbous Moon directly overhead both nights.  You can get an idea of how bright the Moon was from my lunar image captures in the same album.

 

https://www.flickr.c...157666048277440

 

cheers!

 

Jim T.

Hello Jim,

 

Thanks for the very nice images!

Does you known why 'dust bunnies' are visible in the first DSO group, and not in the second, lower, group (after the moon) of images?

When doing EAA I've remarked that I can see sometimes a 'dust bunnie' and, at the same session, not.

Is this depended on the 'histogram tweaks'?



#85 jimthompson

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Posted 31 March 2016 - 03:14 PM

The first batch of images from March 18th had big dust bunnies that I tried to clean off before the next session on the 19th.  I got most of them but there were still a few small ones remaining on the 19th.  You can minimize the appearance of the bunnies by making your background dark, but this is a trade-off against seeing very dim wispy details of your target.  Better just to get everything clean in the first place!  :)

 

cheers,

 

Jim T.


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#86 StarCurious

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Posted 31 March 2016 - 05:13 PM

Jim,

 

What do you think about the IR pass filters below for galaxies?

 

http://www.astronomi...passfilter.html

 

http://optolong.com/...ght-sky-h-alpha

 

Do you think either of these can be used day time?

 

Thanks,

Joseph



#87 jimthompson

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Posted 01 April 2016 - 02:43 PM

Jim,

 

What do you think about the IR pass filters below for galaxies?

 

http://www.astronomi...passfilter.html

 

http://optolong.com/...ght-sky-h-alpha

 

Do you think either of these can be used day time?

 

Thanks,

Joseph

Hi Joseph,

 

Of the two filters you have indicated, my preference would be for the Optolong brand one.  The reason is it is a high pass filter so all the galaxy emission at wavelengths longer than the cut-off (~640nm) will get through to the camera.  The Astronomik one is a band-pass filter which is only passing between 640 and 840nm, so you would be missing out on the emission above 840nm.

 

I would think that either of these could be used for daytime terrestrial photography.

 

cheers,

 

Jim T.


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#88 ChrisFC

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Posted 03 April 2016 - 08:11 AM

So as Censustaker said wayback the #29 deep red looks like a goer for galaxies ? With its cut-off at ~620nm it comes on just before the Halpha and then infra red. Even better, I've just realized I've got one in a box of bits I bought.

This is the most truly excellent thread. Thanks to Jim T & all who have contributed.

Chris


Edited by ChrisFC, 03 April 2016 - 08:14 AM.


#89 StarCurious

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Posted 03 April 2016 - 02:12 PM

 

Jim,

 

What do you think about the IR pass filters below for galaxies?

 

http://www.astronomi...passfilter.html

 

http://optolong.com/...ght-sky-h-alpha

 

Do you think either of these can be used day time?

 

Thanks,

Joseph

Hi Joseph,

 

Of the two filters you have indicated, my preference would be for the Optolong brand one.  The reason is it is a high pass filter so all the galaxy emission at wavelengths longer than the cut-off (~640nm) will get through to the camera.  The Astronomik one is a band-pass filter which is only passing between 640 and 840nm, so you would be missing out on the emission above 840nm.

 

I would think that either of these could be used for daytime terrestrial photography.

 

cheers,

 

Jim T.

 

Hi Jim,

Please bear with me for this rather long post. The one thing I didn't highlight in this question was that I used the Celestron 102 SLT - an achromat.

 

I checked my first ever image as posted in #71 again, which was  a 50x12s stack with Deepsky and 495nm Longpass (Yellow).  I had previously ordered an UV/IR cut filter to stack with the Deepsky for DSO's other than galaxies and I didn't think IR cut was needed or appropriate for galaxies.  What the Deepsky/495nm passed was similar to a #29 Deep Red with the addition of 495-530nm.

 

Yesterday, I bumped into this thread:

http://www.cloudynig...r/#entry7138574

which led me to this:

http://www.cloudynig...mats-for-video/

 

and posts by Dragon Man about his experience and work with achromats. It appears that my 495nm Longpass is similar to his #12 Yellow which he stacks with UV/IR cut for DSO's quite successfully for his achromats.  I am quite excited about this and look forward to testing when I receive my UV/IR cut filter.

 

On the question of galaxies, since my tests were with pass bands only slightly wider than #29 Deep Red, I thought that perhaps cutting off the longer end of the IR may help – which was why I thought the Astronomik Proplanet 642 Band Pass might work.  The thinking was a narrow bandwidth (Astronomik calls it spectral window) would reduce the effects of chromatic aberration.

 

Because I have a small aperture (102mm). I need wide enough spectral window to give me enough brightness to be able to focus, to detect enough stars for alignment, and to have short enough exposures for EAA on an Alt-Az mount, yet not so wide that chromatic aberration becomes dominant.

 

I just checked your post #72:

 

%LT

#29 Dark Red = 47.5%
generic 850nm high pass = 9.7%
Baader IR Pass = 40.0%
generic 680nm high pass = 36.7%

 

I calculate and guess that the Astronomik 642 Bandpass should yield a brightness of 47.5-9.7% = 37.8% which falls between the Baader 685nm IR pass and generic 680nm IR pass. In cutting off at 842nm, I may not be losing much brightness, while gaining some brightness in shifting from 680/685nm to 642nm on the left (short) side, and while narrowing the spectral window to 200nm to reduce the effects of chromatic aberration.

 

I am a complete newbie, and my post #71 was my first ever image with my first ever camera. This is my attempt in mitigating chromatic aberration for galaxies.  I may pursue the 642-842nm Bandpass after testing the non-galaxies EAA when I receive my UR/IR cut filter. Please tell me if I am off or I miss some other consideration.

 

Thank you for providing much data, charting and real world experience thoughtful guidance, and for your patience.
Joseph


Edited by StarCurious, 03 April 2016 - 02:23 PM.


#90 jimthompson

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Posted 05 April 2016 - 10:14 AM

Hi Joseph,

 

Okay, I understand now what your problem is.  Looking again at your test image I can see that it might not just be a focusing problem causing the fuzzy image, it might be chromatic aberration like you mentioned.  I have been pretty lucky with the refractors I have purchased as all have done fairly well at focusing IR.  I do however have a couple of security camera lenses that I use and they produce similarly fuzzy images to what you have shown...I can use these lenses only in conjunction with a UV/IR cut filter or narrow bandpass filter.

 

Your thoughts regarding the Astronomik 642 filter seem sound to me.  Your idea of using an IR filter that is a bandpass instead of high-pass in order to sharpen up the focus on your achromat has merit.  If you do indeed purchase the Astronomik filter, please let us know how you do.  I would be very interested if you were able to compare a 640nm high pass (roughly a #29 dark red) to the Astronomik 642 to see if there is a difference in image focus and exposure time.

 

cheers,

 

Jim T.



#91 StarCurious

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Posted 06 April 2016 - 12:10 AM

Hi Jim.

 

I will test the UV/IR Cut with 495nm Longpass with the ~200nm spectral window.  If results are satisfactory, validating the narrower spectral window hypothesis for my scope, I will be ready to order the Astronomik 642nm BP.  I will share the outcome of the my tests. This may take some time, probably in May.

 

Thanks,

Joseph



#92 ChrisFC

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Posted 11 April 2016 - 07:46 AM

So I've got (what I think) are my favourite filters (at least for now). If I'm not limited by space where is the best place to put a filter(s) ?

 

Between

- the camera and focal reducer and camera, or

- the focal reducer and scope ?



#93 jimthompson

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Posted 11 April 2016 - 01:54 PM

So I've got (what I think) are my favourite filters (at least for now). If I'm not limited by space where is the best place to put a filter(s) ?

 

Between

- the camera and focal reducer and camera, or

- the focal reducer and scope ?

Hi Chris,

 

For the most part it does not matter where you put the filter in your assembly, however depending on your situation it can matter somewhat in practice.  Below are a few observations I've made over the years regarding filter placement.  You can pick and choose depending on whether or not they apply to you.

 

1. Often the easiest place to locate a filter is on the end of your camera-focal reducer assembly.  Putting the filter here in theory gives the best filter performance because the light passing through the filter here is as close to perpendicular to the filter as it can be, exactly perpendicular being the ideal case.  You would probably never see a difference in performance with scope focal ratio (ie. filter in front vs. behind FR) unless you are using a very narrow band pass filter and/or a very fast setup (< f/3).

2. Sometimes with the filter on the end of the camera-FR assembly I have encountered very noticeable and distracting reflections.  The reflection appears to be between filter glass and the focal reducer glass.  It does not always happen, and will depend on the particular filter/FR/telescope combination.

3. When you are using a focal reducer you will find that your setup is more sensitive to vignetting.  For example adding a filter to the end of the camera-nosepiece assembly may not result in any vignetting, but when you have a FR in between there may be vignetting.  The chances that the filter will cause vignetting when placed on the end (towards telescope) increases the more focal reduction you use.

4. Placing the filter between the camera and FR can help avoid the first three issues above, but you will need to account for the extra spacing.  Sometimes this can push your focal reduction too far and you start to get vignetting from the FR and/or coma.  Other times you can use this to your advantage and incorporate the length of the filter in your spacing.  My experience has been that in most cases the best result comes from placing the filter between camera and FR.

5.  The issue of vignetting can be for the most part eliminated by using strictly 2" filters and focal reducers.  It costs more to get 2" versions of everything, but the improvement in overall performance is worth it in my opinion.  There will still be the same issues of getting your FR spacing correct to minimize coma, but you will at least not have to worry about vignetting.

 

cheers,

 

Jim T.


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#94 astronet1

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Posted 24 April 2016 - 01:48 PM

A lot of good information in this thread.

I just bought an unmounted 31mm Astronomik UHC filter though I am live viewing from a grey zone

I'm having an optician (not as brave with the dremel as Jim) cut it to down to two different filters to fit in a mechanical CS mount for 38mm ccd boards. This way the camera has internal switching of IR filter over UHC.

One of the filters is for a remote live view machine. The other is for a 360 camera that see's so well so fast I need to limit incoming light to make the milkyway pop out out more.

I had done some research on what the milky way was mostly comprised of and it looked like the UHC filter would be my best bet. We'll see how it goes.


 



#95 astronet1

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Posted 24 April 2016 - 06:46 PM

Also,

From what I've gathered here. If I have star bloat where my stars are round it's because of an excess of IR light(?) - and if I get an IR bandpass filter over 840nm, I won't be missing any deep space objects by cutting off above that IR spectrum and it will still help with star bloat? Correct?



#96 JPKellysr

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Posted 24 April 2016 - 07:07 PM

An IR pass filter will only let you see IR. It is great for galaxies in light polluted skies. An IR/UV cut will remove IR & UV.

#97 JPKellysr

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Posted 24 April 2016 - 07:08 PM

An IR pass filter will only let you see IR. It is great for galaxies in light polluted skies. An IR/UV cut will remove IR & UV.

#98 astronet1

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Posted 24 April 2016 - 08:54 PM

sorry yes I meant a IR-cut filter that cuts wavelengths over 840NM. Is there any point in viewing IR light (700 - 1900nm) on a CCD over 840nm when it comes to deep space objects?

 



#99 jimthompson

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Posted 25 April 2016 - 11:51 AM

Also,

From what I've gathered here. If I have star bloat where my stars are round it's because of an excess of IR light(?) - and if I get an IR bandpass filter over 840nm, I won't be missing any deep space objects by cutting off above that IR spectrum and it will still help with star bloat? Correct?

Star bloat can be caused in two different ways:

 

1.  Saturation in the camera sensor; and/or

2.  Chromatic aberration in your optics.

 

The first cause is often overlooked but is an important contributor to star bloat.  Camera sensors don't all respond the same to saturated signal, ie. too much charge for a pixel well to hold.  Sensors deal with an overflowing charge well in different ways but in general the overflowing charge leaks into neighboring pixels.  This results in stars tending to fill more than one pixel on the detector, with more pixels being filled the longer the integration.  Adding any sort of filter, not just an IR cut filter helps to reduce the intensity of stars and thus reduce the amount of charge overflow...stars look smaller.

 

The second cause is the one most people are familiar with.  The problem comes from re-purposing telescopes and lenses meant for visual observing, using them instead with cameras that have a much wider spectral response than our eyes.  These optics are designed to do a good job of focusing light within a limited range of wavelengths, normally from 400nm Blue to 700nm Red.  Any light from outside this range of wavelengths cannot be focused at the same time as all the other wavelengths, making the overall image appear fuzzy.  This inability to focus light of all wavelengths to the same point is called chromatic aberration, and is particular to refractors and camera lenses.  There are a few tricks that can help to reduce the effect of chromatic aberration:

 

- Use an LP filter.  As mentioned above using an LP filter will help to reduce the sensor saturation and bleed from stars, but it also helps with focus.  Most LP filters cut out the Blue end of the spectrum, leaving only Green and Red (and sometimes infrared).  With the absence of blue the range of wavelengths you are trying to focus is smaller, giving you a better chance of getting the wavelengths of interest focused.

- Use a narrowband filter (Halpha, O-III, etc.).  For the same reason as using an LP filter, using a narrowband filter greatly reduces the range of wavelengths over which you are trying to focus.  Even if your narrowband filter was entirely in the IR range, you would still be able to reach sharp focus because you are only seeing the light from a narrow range of wavelengths.

- Use a UV/IR cut filter.  This is the option that most people use.  It works for the same reason the above two work; you are reducing the range of wavelengths you are trying to focus at the same point.

 

Depending on the severity of your chromatic aberration, or what it is you are looking at, one of the above three choices will work best.  For example a reflecting type or SCT type telescope has no chromatic aberration because of the lack of refracting elements (an SCT has a tiny amount due to corrector plate).  You should be able to focus the full spectrum in your camera without a problem.  An APO refractor or ED doublet refractor has a little bit of chromatic aberration, but using your normal LP filter is usually enough to deal with it.  An achromatic refractor or camera lens can have a lot of chromatic aberration, and to get a good focus you may need to use a filter that really cuts the range of wavelengths down (ie. UV/IR cut or narrow band LP).  These recommendations are a generalization, you will have to try different treatments yourself in conjunction with your telescope and camera to be sure what works best.  To further complicate things, what you want to look at also impacts your solution.  If you are observing emission nebulae, you can remove all of the IR band without much concern for affecting the view.  If you are observing galaxies however, cutting the infrared will have a large impact on the view, so using a UV/IR cut should only be the solution if the chromatic aberration is very bad.

 

cheers,

 

Jim T.


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#100 StarCurious

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Posted 15 May 2016 - 02:02 PM

Jim,

 

Just to say I haven't forgotten this thread.  I have ordered a Lumicon #29 Dark Red filter (I think nominally 600nm Longpass) and an Astronomik Proplant 642 BP which passes between 642nm and 842nm.  I plan to use these with my ASI 224 MC for EAA with galaxies.  The #29 Dark Red should detect more alignment stars, while the 642 BP with 200nm width of band pass should yield less star bloat or chromatic aberration with my achromat Celestron 102 SLT, and when the moon is out in force. I have been struggling to decide which one to buy and eventually decided to buy both, to give me options for different objects and viewing conditions.

 

I think the H-Alpha filters have too narrow band pass and will require exposure times that are too long for my Alt-Az mount.

 

Cheers,

Joseph


Edited by StarCurious, 15 May 2016 - 02:04 PM.

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