Quark + Lunt 40mm double stack at 102mm aperture

Oak · Post by **Oak** » Thu Apr 28, 2022 2:23 am

Hi All,

Since this forum is the major source of information for this mod, I wanted to share the details here. I recently acquired a Lunt 40mm scope for quick full disk views to complement my Quark. It’s great. That aside, one of the secondary goals of getting the 40mm was to try to put the air-spaced etalon in the telecentric beam after the Quark for a double stack – similar to the Quark+PST mods here. The mod also needed to be quickly reversible so I can still use the 40mm as a grab-and-go scope. I had a custom etalon enclosure machined to hold the Lunt40 after the Quark and I am pretty happy with the end result. Details and a few images below.

The etalon enclosure:
The Lunt 40mm etalon screws on to the 40 mm scope with M60x1 thread. I had an adapter machined to accept the etalon between standard T2 threads. This was based on some of the adapter designs already on this board and those sold by Beloptik.

Adapter design:

Images below (aluminum black used for internal parts).

The adapter was then placed between a T2-to-Quark adapter from FLO and a blue fireball T2-1.25 inch eyepiece holder and attached to the quark.

Assembled and in place in Astrotech AT102ED:

(AT102ED, Badder 35nm H-alpha, extension tubes, Quark, Lunt 40mm, diagonal, Arcturus binoviewers with 25mm Plossls and bino bandits)

Visually, I am quite pleased with the results, although I have only had this contraption out for a few sessions. Much improved contrast at the cost of some brightness, particularly on proms. The double limb is greatly diminished though.

The images below are from 2 brief imaging attempts. Exposure times ~5ms with low gain settings on my non-standard camera (HIKVISION machine vision camera) and ~30 second captures taken at 60-100fps. Aligned in Autostakkert, best 5%, deconvolved and unsharp masked in IMPPG. No flats or flat field correction. All the histograms are linear stretched except the one noted. You can see some vignetting at the edge of the images – they are essentially the maximum field of view for this configuration.

AR2995_2994_2993:

Just a little lift on the histogram for the proms on this one:

Thoughts? I haven’t tried clocking the etalons yet so that is probably next on the list.

Best,
Derek

Post by **marktownley** » Thu Apr 28, 2022 5:24 am

Very promising results Derek. Clocking etalons always finds a best (and worst!) spot for them...

MalVeauX · Post by **MalVeauX** » Thu Apr 28, 2022 12:14 pm

That chamber is awesome! Mind sharing where you had it machined? 40mm front mounted etalons that fit in a chamber like that have lots of potential and are inexpensive relative to their quality for rear-mounted options with a clear 40mm aperture. The air spaced etalon will improve with longer and longer focal-ratio, it doesn't fall off until closer to F100. Mica spaced etalons start to flatten out on gains after F50~F60 in telecentric cone. So if you want to tighten things up, longer focal ratio and use a reducer at the camera for sampling. Other than that it's a dance of tuning. Getting the Quark on band first would be ideal, so you can then real time adjust the Lunt 40 etalon and just watch the limb. If the double limb goes away, you're on band with both. I'm not sure clocking will matter much on this arrangement.

Very best,

Radon86 · Post by **Radon86** » Thu Apr 28, 2022 1:04 pm

Hi Derek,
I also have a Quark, and am thinking of getting a Lunt 40mm scope. Which scope version did you have ? blocking filter size. How is the helical focuser ?
Also how much does the adaptor cost to fit the Quark ?

Thanks
Magnus

Oak · Post by **Oak** » Thu Apr 28, 2022 4:05 pm

Thanks! Down the road, I will probably try to use the enclosure to place the etalon between digerging and converging lenses as opposed to within a telecentric - The T2 threads should make this possible with off-the-shelf parts. That's a more involved project for a differnet time though...

@ Marty: I just tuned the quark to the usual setting and then added the 40mm etalon after it, then looked for the brightest image. Probably the double limb is a more reliable measure of on-band performance, but it was very diffucult to see in general. When I stop the scope down to 80mm (f8.9 vs f7) it definitely gets more contrasty at the expense of some brightness. I'm already using a 0.5x reducer at the camera for better sampling. Really bad newton rings otherwise anyway.

I had the adapter machined at Raf Camera. They have a custom adapter link that I stumbled upon. I roughed something out and over a few emails the finished product emerged. Raf was very responsive and the adapter was made within about 2 days of finalizing the design. Shipping then took >1 month due to all the things that started happening in that part of the world just after my order.

@ Magnus: I have the B1200. It is overkill, but I had planned to mod the scope when I bought it so got a bigger blocking filter for future compatibility. There is a discount on the blockers when you buy with a scope. The helical focuser is ok. There is definitely some sag when I put binoviewers or the quark on the scope, but it still works just fine. It's really nice to double stack with the quark (replace blocker with quark) - still passes a full disk. The adapter/enclosure was 150USD+shipping.

-Derek

torsinadoc · Post by **torsinadoc** » Mon May 02, 2022 10:37 pm

Excellent mod. I am considering something similar for my daystar. I’m not completely happy with my pst daystar double stack. How did it effect your camera exposure and gain? Did you remove the Lunt ERF since it’s behind the quark?

Oak · Post by **Oak** » Tue May 03, 2022 10:05 pm

Thanks. I don't have my prior exposure times handy, but will do a comparison next time I image. The photos above were taken at 5ms exposure with a low gain setting. I did not remove the ERF from the lunt etalon as I wanted to keep the mod quickly reversible. It's plenty bright for visual, but that may be dependent on how bright my quark is. Will update then I have some concrete numbers.

Oak · Post by **Oak** » Sat Dec 24, 2022 8:41 pm

Hi All,

Just wanted to update this thread with a few things:

I have used this double stack filter system up to 127mm now with good results:

The aluminum etalon adapter is now available commercially from RAF camera:
https://rafcamera.com/adapter-lunt-40mm

I have also now made a 3D-printed version of the adapter with the .stl files available for free here:

https://www.printables.com/model/348580 ... -enclosure

The 3D-printed adapter is basically identical to the aluminum one, but the walls have been beefed up to ~5mm for added strength given the weaker material used. The assembled enclosure has T2 (M42x0.75) threads on both ends. I made two versions of the etalon holder that bolts into the enclosure: one with a female T2 thread (clear aperture 42mm) and one with a male T2 thread (clear aperture 34mm). The Lunt 40mm etalon attaches via M60x1 thread. There are 6 M5x0.8 screws to hold the two parts together, although just using three is probably adequate. The example below was printed in nylon by a commercial service.

Top of etalon holder (male and female T2 versions):

Bottom/inside of etalon holder without etalon:

Bottom/inside of etalon holder with etalon attached via M60x1 thread:

Enclosure with 6 M5x0.8 screws:

Full enclosure assembled with etalon:

3D printed enclosure attached to Quark and size comparison with aluminum version. The etalon enclosure modules screw together so could be used as a double (or triple?) stack.

-Derek

Post by **marktownley** » Sun Dec 25, 2022 8:39 am

Great stuff!

Oak · Post by **Oak** » Sun Dec 25, 2022 1:06 pm

Thanks. I should mention that the 3d printed version is a result of seeing Averton's success with a similar approach.

minhlead · Post by **minhlead** » Wed Jul 05, 2023 6:49 am

Oh this is great!
I find this guy online doing exactly what you did and he got terrific result: excellent double limb supression, extremely even fov.
And he claims that after mounting the Lunt behind the Quark, he no longer tune the Lunt since tuning the Lunt "make no difference". I am baffled. In your experience, is it true?

Oak · Post by **Oak** » Wed Jul 05, 2023 12:57 pm

Tuning/tilting does make a difference to me, but less than with single stack. (You are just trying to maximize brightness and evenness to match the two etalons) . Putting the air spaced Lunt etalon in the telecentric beam behind the quark does blue shift the etalon slightly, which takes the place of some tilt.

AndreaGirones · Post by **AndreaGirones** » Wed Jul 12, 2023 10:23 pm

Hello
Just letting you know I apreciated this thread very much and was able to order the adapter and set up my Lunt 40 etalon with my quark. Still waiting for some nice seeing but here are some of my images so far.

Oak · Post by **Oak** » Thu Jul 13, 2023 1:03 am

Thanks. Glad it's working for you. Also very gratifying to see others trying this approach. That's a nice looking setup and image!

DavidP · Post by **DavidP** » Wed Jul 26, 2023 2:59 am

Very nicely done. Thanks for sharing the details.
Does adding the Lunt change the focus?

Oak · Post by **Oak** » Wed Jul 26, 2023 11:42 am

Thanks. It does change focus, but very little. Less than 1cm. I have had two Lunt 40s behind the quark as a triple stack and there was still minimal focus shift.

vineyard · Post by **vineyard** » Sat Sep 02, 2023 9:40 am

Gosh I'm very late to this mod - thanks for sharing. The etalon needs parallel light doesn't it - how the does the quark generate that (I thought the light that comes out of the quark is a converging beam) or am I missing something? Thank you!

Post by **marktownley** » Sat Sep 02, 2023 10:07 am

vineyard wrote: ↑Sat Sep 02, 2023 9:40 am Gosh I'm very late to this mod - thanks for sharing. The etalon needs parallel light doesn't it - how the does the quark generate that (I thought the light that comes out of the quark is a converging beam) or am I missing something? Thank you!

The light coming out of the back of the Quark is telecentric, so, it works with an air spaced etalon for this purpose.

Oak · Post by **Oak** » Sat Sep 02, 2023 12:06 pm

This quark has the built in telecentric. Also works with Lunt40+TZ4+blocker and no quark better than I thought would. I've been trying just the Lunt 40 mm etalons with a negative meniscus lens and refocusing doublet lately and this also works as it should, but you get a sweet spot.

vineyard · Post by **vineyard** » Sat Sep 02, 2023 2:46 pm

Thanks both. Ah I see - I have the combo quark which doesn't have the built in telecentric. I guess this mod won't work for my kit then - ah well. Cheers!

Oak · Post by **Oak** » Sat Sep 02, 2023 8:34 pm

vineyard wrote: ↑Sat Sep 02, 2023 2:46 pm Thanks both. Ah I see - I have the combo quark which doesn't have the built in telecentric. I guess this mod won't work for my kit then - ah well. Cheers!

It will work well with the quark combo. A telecentric amplifier is required before the combo anyway. How else are you getting to F30+?

vineyard · Post by **vineyard** » Tue Sep 05, 2023 11:25 am

Oak wrote: ↑Sat Sep 02, 2023 8:34 pm It will work well with the quark combo. A telecentric amplifier is required before the combo anyway. How else are you getting to F30+?

That's interesting, thanks. I've been using a 2.5x powermate on an f8.6 frac or an f6.5 frac to get to f21 and f16.

Here fwiw is what I get running it at f16:

I do have an SF50 etalon which I've always wanted to try and run at the back of a 4" frankenscope (even just single stack) but I would need to somehow thread a telecentric in front of it, and then the focusing doublet after it and I just can't mount something that long. So I think maybe a LS50+quark may be a more feasible direction for my kit.

But on my to do list is to put the powermate and quark at the back of the SF50 on a 3" scope. I tried it briefly but I have to take the back blocking filter out I think b/c the image came out very very dark.

Cheers,

Vin

Oak · Post by **Oak** » Tue Sep 05, 2023 5:00 pm

A 4x telecentric (baaderTZ4) would give you tighter bandpass with the quark combo. Definitely worth the decrease in overall intensity. Ideally, you want to be around f40 for the mica etalons.

The quark+telecentric should also work on the back of another H-Alpha scope. The quark replaces the blocking filter in this configuration.

vineyard · Post by **vineyard** » Wed Sep 06, 2023 8:33 am

Thanks - I will look into the TZ4 b/c that would be very interesting to see the tighter bandpass. The difference in resolution between the 4" with a quark and the SF50 is palpable. The SF50 has much better contrast though, so anything that can lead to a better combination of the two, resolution + contrast, can only be a good thing. Cheers, tips much appreciated!

EboO · Post by **EboO** » Mon May 20, 2024 2:03 pm

Very interesting mod, i'm near to try it too.
Just a question about etalon, do all etalons from Lunt 40 are equals or is there some (lot ?) of dispersion ?
Last question is there a backfocus to respect ?

And please do you know why rafcamera adapter isn't adapted to Lunt 40 DS module (a little cheaper...) ?

Oak · Post by **Oak** » Mon May 20, 2024 3:16 pm

There is certainly variability in the Lunt etalons, particularly with respect to how much tilt they require to come on band (CWL). Backfocus is not a big issue here because the etalon is in the telecentric output of the quark.

There is a Lunt40 DS (and Lunt 35mm) version of the raf camera adapter. Just search the site for "Lunt". The non DS Lunt 40 is about 5mm shorter than the DS version because of the addition of the thread adapter in the later module.

EboO · Post by **EboO** » Mon May 20, 2024 5:26 pm

Thanks Oak.
So no matter for backfocus with quark and same with a TZ ?
Is there a difference of quality between etalon of Lunt 40 SS and DS ?

Oak · Post by **Oak** » Wed May 22, 2024 12:28 pm

There is a very small change in focus plane and I have had two Lunt 40s in series after the quark (triple stack) with no problem focusing.

I don't think there is a quality difference between the ss and ds etalons. They appear to be the same thing.

KrisJot · Post by **KrisJot** » Thu May 30, 2024 10:08 pm

Hi All,

I'm new here:-).
Lately I bought Lunt 40mm SS, magnificent scope, but inspired by @Oak posts I felt aperture's hunger:-).

I would like to try to combine the etalon from my Lunt 40 one of those scopes: WO80ED F7/TS102ED F7/ Bresser 102mm F10 Achro.
I don't have a quark, I would like to use a telecentric barlow instead to make beam parallel (ES, TV or 2" Siebert Optics OCA).
The optical path would initially look like this.
Refractor -> Baader HAlpha 2" ERF filter -> telecentric barlow -> etalon with enclosure -> blocking filter

Does this make sense?

Kris

Post by **marktownley** » Fri May 31, 2024 5:39 pm

KrisJot wrote: ↑Thu May 30, 2024 10:08 pm Hi All,

I'm new here:-).
Lately I bought Lunt 40mm SS, magnificent scope, but inspired by @Oak posts I felt aperture's hunger:-).

I would like to try to combine the etalon from my Lunt 40 one of those scopes: WO80ED F7/TS102ED F7/ Bresser 102mm F10 Achro.
I don't have a quark, I would like to use a telecentric barlow instead to make beam parallel (ES, TV or 2" Siebert Optics OCA).
The optical path would initially look like this.
Refractor -> Baader HAlpha 2" ERF filter -> telecentric barlow -> etalon with enclosure -> blocking filter

Does this make sense?

Kris

Hi Kris

Welcome to the forum. Start a new thread please.

Mark

Oak · Post by **Oak** » Fri May 31, 2024 6:12 pm

This approach will work, but most would suggest a full aperture ERF for safety. You will want to be at f40+. The baader TZ4 works well. You will be at about 0.9 angstroms fwhm.

OlegLviv · Post by **OlegLviv** » Thu Aug 01, 2024 7:59 am

Have a question about the distance between two Lunt 40 etalons with do it double stack?

Can I not buy large Raf adapters and disassemble the white case of each Lunt 40mm to make an even smaller distance between the etalons?
Won't there be big highlights or double suns when making the shortest possible distance between two etalons?

EdAstle · Post by **EdAstle** » Thu Aug 01, 2024 10:53 am

I have thought exactly the same thing Oleg.
Removing the white nose containing the red ERF shortens them by about 2cm. I could reduce my image train by 4cm.
Your picture is what I wish for every day.

The red ERFs are slightly tilted, so light reflecting off the etalon does not bounce off the rear side of the red ERF and go back down the beam path. At least I think that's why they are tilted.

Increasing the distance between the 2 Lunt40s would help, in my mind.
When close together the reflections are more likely to ping-pong between them.
When further apart (and with small tilt via the tilt wheel) those reflections could miss the other etalon partially or even completely.
Those are my thoughts anyway.

A circular polarizer between them really does help block reflections.
Many "multiple suns" simply disappear when I look down the image train without an eyepiece.
I want your picture but with my CPL between them.

I've thought about taking my 2x RafCamera chambers to a local engineering shop and having them shortened.
If I tried to do it myself with a hacksaw I would not get perfectly flat or level ends.
But I'm not sure if shortening them would make matters worse for the reasons I mentioned above.
I want a shorter light path to be within the recommended working distance for the Baader TZ4S.

I've even thought of putting high refractive index glass between them to increase distance.
Not from a thickness perspective but from a light path perspective.
Off axis reflections would deviate more than in free air. That'd should help move them away from the other etalon.

I have lots of thoughts about this

I fancy trying the "high refractive index" thing out of curiosity, but I'm pretty clueless when it comes to optics - I've no idea what index or what type of glass would be best. This is why I love this hobby - always much more to learn. I'll read up on this tonight so I know what I'm doing!

Regards, Ed.

DavidP · Post by **DavidP** » Thu Aug 01, 2024 3:19 pm

I’m wondering the same about effects of shortening the distance between the two etalons while keeping a polarizer in between.
Ed. Shortening the Raf adapter would be very easy to do yourself. Start with a saw, get it roughly accurate with a grinder, and then finish with a hand file and a caliper. Of course you will need to drill and tap New thumb screw threads.
You go first!

Dennis · Post by **Dennis** » Thu Aug 01, 2024 3:38 pm

DavidP wrote: ↑Thu Aug 01, 2024 3:19 pm I’m wondering the same about effects of shortening the distance between the two etalons while keeping a polarizer in between.
Ed. Shortening the Raf adapter would be very easy to do yourself. Start with a saw, get it roughly accurate with a grinder, and then finish with a hand file and a caliper. Of course you will need to drill and tap New thumb screw threads.
You go first!

I wonder why it is 20mm longer then needed in the first place. I would agree with Ed about doing it without the proper tools and experience: almost not to avoid to be left with unwanted tilt. Unless you can mill it off which again might be difficult with the holes etc.

Oak · Post by **Oak** » Thu Aug 01, 2024 3:53 pm

The enclosure is longer to accommodate the white housing (preferably minus EEF) so it can be used with the compression tuning mod. This should limit or eliminate banding caused by etalons with too high a CWL. If your etalons don't require much tilt/don't need compression to come on band, then the white housing is not necessary and cutting out a few cm would be feasible. I need compression on both of my etalons.

I'm sure that Raf Camera would make a smaller version of the piece that needs shortening for a very reasonable price. Worth an email if you are interested.

-Derek

Oak · Post by **Oak** » Thu Aug 01, 2024 4:01 pm

Here is what I mean by compression:

viewtopic.php?p=388259#p388259

OlegLviv · Post by **OlegLviv** » Thu Aug 01, 2024 4:50 pm

What I want to say that 40mm stock lunt f10 when we are do not use telecentric sysem works double stack very good, but if you do it telecentric system on your big telescopes 100-127-150-235mm the unevenness of the strip and also the glares and ghosts are beginning to appear! This is the main problem with the double stack of these etalons on telecentric system!

Dennis · Post by **Dennis** » Thu Aug 01, 2024 5:31 pm

Oak wrote: ↑Thu Aug 01, 2024 3:53 pm The enclosure is longer to accommodate the white housing (preferably minus EEF) so it can be used with the compression tuning mod. This should limit or eliminate banding caused by etalons with too high a CWL. If your etalons don't require much tilt/don't need compression to come on band, then the white housing is not necessary and cutting out a few cm would be feasible. I need compression on both of my etalons.

I'm sure that Raf Camera would make a smaller version of the piece that needs shortening for a very reasonable price. Worth an email if you are interested.

-Derek

You are absolutely right Derek, its actually not longer then needed but fits perfectly with the housing of the Lunt etalon.

DavidP · Post by **DavidP** » Fri Aug 02, 2024 3:01 am

OlegLviv wrote: ↑Thu Aug 01, 2024 4:50 pm What I want to say that 40mm stock lunt f10 when we are do not use telecentric sysem works double stack very good, but if you do it telecentric system on your big telescopes 100-127-150-235mm the unevenness of the strip and also the glares and ghosts are beginning to appear! This is the main problem with the double stack of these etalons on telecentric system!

Yes. Glare and ghosts are the issue. A circular polarizer between the two helped greatly on my set up.
I’m just wondering if bringing the two etalons closer together, though with a polarizer in between, will worsen the reflection problem

OlegLviv · Post by **OlegLviv** » Fri Aug 02, 2024 4:20 am

Which brand circular polarizer do you use????
Friends, what brands do you use polarizer?

Oak · Post by **Oak** » Fri Aug 02, 2024 12:19 pm

Hoya 52mm HRT Circular PL Polarizer UV Multi-Coated Glass Filter. There is a void under the etalon in the Raf adapter that fits this filter.

B+w would be the more popular choice though.

viewtopic.php?p=431765#p431765

DavidP · Post by **DavidP** » Fri Aug 02, 2024 4:02 pm

B + W F-Pro Kaesemann HT Circular Polarizer Filter 49mm

OlegLviv · Post by **OlegLviv** » Fri Aug 02, 2024 4:03 pm

Oak wrote: ↑Fri Aug 02, 2024 12:19 pm Hoya 52mm HRT Circular PL Polarizer UV Multi-Coated Glass Filter. There is a void under the etalon in the Raf adapter that fits this filter.

B+w would be the more popular choice though.

viewtopic.php?p=431765#p431765

Sorry Hoya 52mm HRT Circular PL Polarizer UV Multi-Coated Glass Filter looks good not expensive for me but will be very good have m48 filter for more practical use, but how rotate it if it is closed in the Raf adapter?

You say about B+w would be the more popular choice though but why?

What do you say about Baader 2" Double Polarization Filter with Rotating Filter Cell or Lunt - 2" Polarizing Filter for White Light Solar Wedge?

EdAstle · Post by **EdAstle** » Fri Aug 02, 2024 11:40 pm

"KSM HTC-POL MRC nano" 49mm
https://schneiderkreuznach.com/en/photo ... /polarizer

Edmund Optics look handy:

"Circular Polarizers (CP42HE and CP42HER)"
(HE = left, HER = right - won't matter to us which one)
https://www.edmundoptics.co.uk/f/glass- ... 2he/39839/

2 variants; uncoated (42% transmission) and coated (45%) for an extra £10.

EdAstle · Post by **EdAstle** » Fri Aug 02, 2024 11:42 pm

Oak wrote: ↑Thu Aug 01, 2024 4:01 pm Here is what I mean by compression:

Thank you for this - completely missed this one - added to favourites for later.

EdAstle · Post by **EdAstle** » Sat Aug 03, 2024 12:14 am

OlegLviv wrote: ↑Fri Aug 02, 2024 4:03 pm What do you say about Baader 2" Double Polarization Filter with Rotating Filter Cell or Lunt - 2" Polarizing Filter for White Light Solar Wedge?

Hi Oleg - I replied to your PM but don't see my reply in my "sent". So sending similar from here...

1. you can not use a linear polarizer to block reflections
2. a double polarizer is just 2 linear polarizers stacked on top of each other

It has to be a circular polarizer.
Not a round one, one that only allows circularly polarized light through.

https://www.edmundoptics.co.uk/knowledg ... -advanced/

Edmund Optics sells things:
1. Linear polarizer
2. Waveplates, specifically 1/4 waveplates

When you combine those two things you get a "circular polarizer".
I'm no expert. I learnt that last night 2am with a bottle of Nemiroff for company

The key thing is - when circular light reflects off a surface it goes from clockwise to anticlockwise (or visa versa).
Because the circular polarizer is "programmed" for one direction of spin it wont allow the opposite type through.
It will block reflections coming back through it that are the wrong direction.

This is why it has to be a CPL; not an ND filter, not a basic linear, but a circular polarizer which is actually made from a linear polarizer + 1/4 waveplate film bonded together.

Cheers, Ed.

OlegLviv · Post by **OlegLviv** » Sat Aug 03, 2024 6:13 am

EdAstle wrote: ↑Sat Aug 03, 2024 12:14 am
OlegLviv wrote: ↑Fri Aug 02, 2024 4:03 pm What do you say about Baader 2" Double Polarization Filter with Rotating Filter Cell or Lunt - 2" Polarizing Filter for White Light Solar Wedge?
Hi Oleg - I replied to your PM but don't see my reply in my "sent". So sending similar from here...

1. you can not use a linear polarizer to block reflections
2. a double polarizer is just 2 linear polarizers stacked on top of each other

It has to be a circular polarizer.
Not a round one, one that only allows circularly polarized light through.

https://www.edmundoptics.co.uk/knowledg ... -advanced/

Edmund Optics sells things:
1. Linear polarizer
2. Waveplates, specifically 1/4 waveplates

When you combine those two things you get a "circular polarizer".
I'm no expert. I learnt that last night 2am with a bottle of Nemiroff for company

The key thing is - when circular light reflects off a surface it goes from clockwise to anticlockwise (or visa versa).
Because the circular polarizer is "programmed" for one direction of spin it wont allow the opposite type through.
It will block reflections coming back through it that are the wrong direction.

This is why it has to be a CPL; not an ND filter, not a basic linear, but a circular polarizer which is actually made from a linear polarizer + 1/4 waveplate film bonded together.

Cheers, Ed.

What do you say about this two exzamples ?
Need to be KSM Käsemann too?

EdAstle · Post by **EdAstle** » Sat Aug 03, 2024 10:53 am

I have the one on the left. Either would work.
I paid extra for the "high transmission" label

More T% = less gain / faster exposures.

I prefer adjusting gain down over exposure down.
Then exposure up over gain up.

Less gain = greater the dynamic range the camera can produce.

Look at Gain Value vs Full Well and Dynamic Range:
http://www.astrosurf.com/viladrich/astr ... meras.html

As always it's a balancing act.

I stubbornly stick to "16-bit" through the entire process to maintain the maximum dynamic range I could capture.
Only when I convert to jpg/png for upload to here/astrobin do I convert to 8-bit.

That's not pointless IMHO. I have far more latitude for processing styles/techniques with 16-bit.
Opinions vary