32 replies to this topic

[BGazing]

I've spent a better portion of the past week reading advice here on CN and perusing book edited by Christophe Pellier, but the ever-shifting market for filters is leaving me somewhat confused.

I have C8 and 678MC and 1600MM. I will have (sporadically, at best) access to friend's C11. 

The main planets are out, so I thought about turning to Venus and Uranus for the foreseeable period, they will be well visible from my balcony.

Visible light situation is clear, 678 and ADC, with limited contrast, if any.

With imaging in IR and UV I have some dilemmas, have never done it before.

 

For Uranus I have to get a IR pass, probably Astronomik 642. I can use both color or mono for this task, which one should be preferred? I ask also because, if I am not mistaken, critical sampling in deep red and IR is not 5x, but rather around 3.5x, so 678 would oversample and 1600 would slightly undersample. Does it matter? Is there any particular advice on focusing in IR in mono or color?

 

As for Venus and UV, looks like quite a few on here recommend NOT using a dedicated 'brutal cut' UV filter for SCT due to correction issues, but rather a #47/Ir-Cut combo? Is that a way to go in absence of Astrodon Johnson U filters. The new transmission curve for Baader Johnson U looks quite different. Any alternatives are reccomended. Can Astronomik L2 be used with #47 (transmission curve looks like it does not really cut at 400), or should one aim at a dedicated IR-cut? I cannot find any transmission curves for #47s that are sold. IDAS 410 has one, but for a price - is it really worth it?

 

I have two more dilemmas re Venus.

First, I see that one should use ADC with mono camera. Why so? I understand for OSC or RGB, but here would be imaging only in UV or UV/blue. Is that because dispersion is at the strongest in short wavelengths? What is more - just how do I do ADC correction in UV...when there are no colors to correct? I am at a bit of a loss there.

Second, critical sampling at 390nm at 3.76 pixel size is around f/23. So I would absolutely have to use barlow in order not to massively undersample in native. Or would leaving out UV-absorbent barlow better than getting close to critical sampling? Venus is small as it is... Are all barlows equal offenders when it comes to UV absorption? I have VIP and PM 2.5...

[BucketDave]

I can answer some of your queries but not all.

Mono cameras and ADCs in UV. I assume that your mono sensor has significant sensitivity in UV. Blue light diffracts/scatters more than red light, UV light even more so. So the far UV may diffract more than the near-UV. Hence your ADC should still be worthwhile.

Although our eyes can't see UV, your camera sensor can. So to focus and set up the ADC properly, you'll just have to iterate. Fiddle. Take a picture. Fiddle some more. I can see that an auto-focusser would be useful.

Edited by BucketDave, 15 February 2023 - 08:25 AM.

[John Boudreau]

While I'm a fan of the Astronomik 642, when imaging relitivly faint Uranus or Neptune generally a longpass filter such as the Baader 610 LP (Baader's visible planetary R filter) gets you noticeably more light than a 200nm bandpass filter like the 642. Contrast will be less vs. a higher cut-on filter such as the 642 or longer, but so will noise, so it's usually a good trade-off. Mono cams are best for this, but the 610 LP has even been used with success using color cams such as the ASI224MC and ASI462MC, so your ASI678 may be worth a try. Use the C11 if you can. Good seeing is a must because of exposure length, you may want to brighten the gamma a bit (for focusing only), and an electric focuser (especially an add-on Crayford type) can really help.

 

I'm one of the folks who isn't a fan of dedicated UV filters for SCTs, largely due to SCT correction issues but also for the corrector's AR coatings cutting much of the UV signal. The Astronomik L2 and W47 combination is certainly worth trying--- it could very well be a fine combination for an SCT. I typically use a permanently mounted 368mm D-K with a dedicated UV (Chroma U-Bessell), but recently bought a Thorlabs 400CWL-40nm bandpass filter for my travel scope--- a C11. It should pass a nice mix of near-UVa and deep blue/violet (I hope). I haven't had a chance to try it out yet, but wasn't planning on imaging Venus for another 2 to 3 weeks. Wratten filter curves are notoriously inconsistent with published values, but I'm sure you'll have a blend of near-UV and deep blue with a typical W47, and the L2 filter should eliminate most if not all of the IR leak. I have tried a stacked UV-deep blue in the past (Schott UG3 with B39) but only with my D-K Cass, and years ago imaged a few times with a Schuler U photometric (UG1 with B39) and the C11. While the Schuler U passed a bit of deep blue, it was largely a UV filter and didn't work very well with the C11. Mono cameras really have a big advantage recording UV--- even more so than their IR advantage, as a color cam's Bayer filters block quite a lot of UV, and the G and R Bayer filters also effectively block deep blue.

 

As BucketDave has mentioned, atmospheric dispersion is strongest in the shorter wavelengths. I use an ADC with my D-K Cass, but the AR coatings on the ADC wedge prisms can be a problem with UV. I use a Pierro-Astro ADC that uses UV friendly coatings and fused silica prisms. The coatings on the popular ZWO ADC were selected to allow quite a bit of near-UV so while not as UV friendly as the Pierro-Astro, it still does a good job. In part, my recent purchase of the Thorlabs 400/40nm filter was to see if it's 40nm bandpass is narrow enough to allow C11 imaging without an ADC with Venus at 30° or higher altitude. It's worth mentioning that the lower a target is, the more UV absorption from our atmosphere. So try to avoid UV imaging below 30° if possible.

 

You mentioned the IDAS 410 filter. It has an IR leak issue according to Christophe Pellier, and he no longer recommends it.

 

Eventually I'll be testing that Thorlabs 400/40nm filter. It's listed here as FBH400-40:  https://www.thorlabs...60&pn=FBH400-40

I've attached a screenshot crop of the curves for the 400-40nm. It has very good off-band blocking optical density of OD5 or better out to 1100nm and at least OD4 out to 1265nm, beyond about 1100nm or so our cameras have virtually no sensitivity.

 

Another interesting choice is the Edmund 400-50nm filter: https://www.edmundop...s-filter/27850/

The curve data PDF is linked on that page under 'Get Product Downloads'.

The Edmund 400-50 has good off-band blocking out to 1200nm, averaging OD4, but it has a few short curve points that dip down into the OD3 to OD2 range. In practice that wouldn't be a problem--- I chose the Thorlabs for it's lower price.

 

Probably the best strictly deep blue/violet filter would be the Edmund 425-50:  https://www.edmundop...s-filter/29361/

The 425-50 won't have the contrast capability of a filter that passes some sub-400nm UV, but it should work well with an SCT and one can always enhance contrast a bit in processing. It's an interesting filter and has nice off-band blocking of OD4 to OD5. It's also mentioned in this Venus filter discussion in 2020 by Christophe Pellier:  https://www.cloudyni...t-blue-filters/

 

Be aware that the Thorlabs and Edmund filters I've mentioned are 25mm diameter and unmounted. You'll need to mount them in a cell.

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Edited by John Boudreau, 16 February 2023 - 09:35 AM.

[Mike_Kar]

Thanks for sharing your knowledge with us John, it's very much appreciated!  I've never imaged Venus before, but I'm putting together my gear selection to give it a go this year.  My choices are largely based on your experience as well as with Christophe Pellier's impressive research on filters. 

 

I too have a C11 but plan on using it more for IR work, with an 8" F12 classical cassegrain dedicated to UV imaging.

 

Looking forward to reading about your findings on the FBH400-40 filter with interest.

 

Mike

[BGazing]

While I'm a fan of the Astronomik 642, when imaging relitivly faint Uranus or Neptune generally a longpass filter such as the Baader 610 LP (Baader's visible planetary R filter) gets you noticeably more light than a 200nm bandpass filter like the 642. Contrast will be less vs. a higher cut-on filter such as the 642 or longer, but so will noise, so it's usually a good trade-off. Mono cams are best for this, but the 610 LP has even been used with success using color cams such as the ASI224MC and ASI462MC, so your ASI678 may be worth a try. Use the C11 if you can. Good seeing is a must because of exposure length, you may want to brighten the gamma a bit (for focusing only), and an electric focuser (especially an add-on Crayford type) can really help.

 

I'm one of the folks who isn't a fan of dedicated UV filters for SCTs, largely due to SCT correction issues but also for the corrector's AR coatings cutting much of the UV signal. The Astronomik L2 and W47 combination is certainly worth trying--- it could very well be a fine combination for an SCT. I typically use a permanently mounted 368mm D-K with a dedicated UV (Chroma U-Bessell), but recently bought a Thorlabs 400CWL-40nm bandpass filter for my travel scope--- a C11. It should pass a nice mix of near-UVa and deep blue/violet (I hope). I haven't had a chance to try it out yet, but wasn't planning on imaging Venus for another 2 to 3 weeks. Wratten filter curves are notoriously inconsistent with published values, but I'm sure you'll have a blend of near-UV and deep blue with a typical W47, and the L2 filter should eliminate most if not all of the IR leak. I have tried a stacked UV-deep blue in the past (Schott UG3 with B39) but only with my D-K Cass, and years ago imaged a few times with a Schuler U photometric (UG1 with B39) and the C11. While the Schuler U passed a bit of deep blue, it was largely a UV filter and didn't work very well with the C11. Mono cameras really have a big advantage recording UV--- even more so than their IR advantage, as a color cam's Bayer filters block quite a lot of UV, and the G and R Bayer filters also effectively block deep blue.

 

As BucketDave has mentioned, atmospheric dispersion is strongest in the shorter wavelengths. I use an ADC with my D-K Cass, but the AR coatings on the ADC wedge prisms can be a problem with UV. I use a Pierro-Astro ADC that uses UV friendly coatings and fused silica prisms. The coatings on the popular ZWO ADC were selected to allow quite a bit of near-UV so while not as UV friendly as the Pierro-Astro, it still does a good job. In part, my recent purchase of the Thorlabs 400/40nm filter was to see if it's 40nm bandpass is narrow enough to allow C11 imaging without an ADC with Venus at 30° or higher altitude. It's worth mentioning that the lower a target is, the more UV absorption from our atmosphere. So try to avoid UV imaging below 30° if possible.

 

You mentioned the IDAS 410 filter. It has an IR leak issue according to Christophe Pellier, and he no longer recommends it.

 

Eventually I'll be testing that Thorlabs 400/40nm filter. It's listed here as FBH400-40:  https://www.thorlabs...60&pn=FBH400-40

I've attached a screenshot crop of the curves for the 400-40nm. It has very good off-band blocking optical density of OD5 or better out to 1100nm and at least OD4 out to 1265nm, beyond about 1100nm or so our cameras have virtually no sensitivity.

 

Another interesting choice is the Edmund 400-50nm filter: https://www.edmundop...s-filter/27850/

The curve data PDF is linked on that page under 'Get Product Downloads'.

The Edmund 400-50 has good off-band blocking out to 1200nm, averaging OD4, but it has a few short curve points that dip down into the OD3 to OD2 range. In practice that wouldn't be a problem--- I chose the Thorlabs for it's lower price.

 

Probably the best strictly deep blue/violet filter would be the Edmund 425-50:  https://www.edmundop...s-filter/29361/

The 425-50 won't have the contrast capability of a filter that passes some sub-400nm UV, but it should work well with an SCT and one can always enhance contrast a bit in processing. It's an interesting filter and has nice off-band blocking of OD4 to OD5. It's also mentioned in this Venus filter discussion in 2020 by Christophe Pellier:  https://www.cloudyni...t-blue-filters/

 

Be aware that the Thorlabs and Edmund filters I've mentioned are 25mm diameter and unmounted. You'll need to mount them in a cell.

Thank you, John, that was incredibly useful. Shame for that Shaymalan ending about the filters being non-mounted (never mounted one, so I would have to google the knowhows). Thorlab solution, in particular, looks interesting.

I will go for 610 and #47 to start with. Astronomik replied that their L2 is at 50 percent at 400nm, a strange claim given the transmission graphs they have published. I have asked about that but no reply yet...Their old UVIR (which, they claim, is the same as present L2) should have cutoff at 380nm.

In the meantime, I've tried some visible light Venus and Uranus. Have to admit it is hard. Uranus is small and requires a lot of exposure, Venus is blindingly bright and hard to focus on...plus it is super low. R6 just creates artefacts on Uranus, on Venus I have learned that S/N ratio favors small quality stacks (like on the Sun), else artefacts, too. Here they are.

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[John Boudreau]

Until a couple of years ago we could find some empty 1.25" filter cells on eBay that could take 25mm filter glass, but the source (former owner of Omega Optical) sold the business and the eBay store he sold odds & ends from has dried up somewhat, and the cells are gone--- at least for now. I was lucky to get some cells a few times until about 3 years ago. There are some cells available through other sources--- the Blue Fireball 1.25"cells on Agena Astro is one, but this and most others of these sources have the cells made with wider clear openings that would allow a 25mm filter to fall through--- most popular mounted filters have 26.5 to 28mm glass. One would have to make a couple of thin plastic (or metal) washers that reduce the cells bottom shelf opening and also protect the top of the filter from falling through the screw-in retaining ring. I've done that before years ago, but those eBay cells that had the 24mm clear aperture are certainly missed. The 25mm filters were also best mounted with an O-ring to help center them in those cells as the cells could take up to just over 27mm, and this is also true for the larger clear opening cells that require washers.

 

I agree that Astronomik's claim of only 50% T at 400nm is suspect--- not only from their own filter plots, but I've seen actual filter scans of their old UV-IR cut filter: https://www.astroama.../uvirblock.html

 

Your visible light Venus and Uranus efforts are certainly good ones! You should be able to detect the brightened polar region of Uranus with the 610LP in good seeing. UV/deep blue markings in the atmosphere of Venus can vary quite a bit--- sometimes they're obvious, sometimes not. UV/Violet/deep Blue features are best recorded with the planet at least 30° in altitude, which usually means a very bright sky--- with the Sun only a few degrees below the horizon. In very good transparency I've even detected features with the Sun low in the sky. When approaching greatest elongation from the Sun and in a dark sky, Venus can be so glaringly bright that it's difficult if not impossible to get a good exposure--- I find it much easier in early twilight. These two planets are among the most difficult challenges, and are less tolerant of less than perfect conditions than targets like Jupiter or Mars. Good luck!

[BGazing]

Until a couple of years ago we could find some empty 1.25" filter cells on eBay that could take 25mm filter glass, but the source (former owner of Omega Optical) sold the business and the eBay store he sold odds & ends from has dried up somewhat, and the cells are gone--- at least for now. I was lucky to get some cells a few times until about 3 years ago. There are some cells available through other sources--- the Blue Fireball 1.25"cells on Agena Astro is one, but this and most others of these sources have the cells made with wider clear openings that would allow a 25mm filter to fall through--- most popular mounted filters have 26.5 to 28mm glass. One would have to make a couple of thin plastic (or metal) washers that reduce the cells bottom shelf opening and also protect the top of the filter from falling through the screw-in retaining ring. I've done that before years ago, but those eBay cells that had the 24mm clear aperture are certainly missed. The 25mm filters were also best mounted with an O-ring to help center them in those cells as the cells could take up to just over 27mm, and this is also true for the larger clear opening cells that require washers.

 

I agree that Astronomik's claim of only 50% T at 400nm is suspect--- not only from their own filter plots, but I've seen actual filter scans of their old UV-IR cut filter: https://www.astroama.../uvirblock.html

 

Your visible light Venus and Uranus efforts are certainly good ones! You should be able to detect the brightened polar region of Uranus with the 610LP in good seeing. UV/deep blue markings in the atmosphere of Venus can vary quite a bit--- sometimes they're obvious, sometimes not. UV/Violet/deep Blue features are best recorded with the planet at least 30° in altitude, which usually means a very bright sky--- with the Sun only a few degrees below the horizon. In very good transparency I've even detected features with the Sun low in the sky. When approaching greatest elongation from the Sun and in a dark sky, Venus can be so glaringly bright that it's difficult if not impossible to get a good exposure--- I find it much easier in early twilight. These two planets are among the most difficult challenges, and are less tolerant of less than perfect conditions than targets like Jupiter or Mars. Good luck!

Thank you, John, much appreciated! Fitting those 25mm filters into 1.25 cells looks harder than fitting squares into pegs...or was it vice-versa. That Thorlabs filter you got yourself there looks really really sweet. Not sure how I would mount it, but I will keep an eye on it...

For now, I'll give it a go with #47 and Astronomik L2 and add 610 for Uranus and 742 for Venus. Too bad I do not have the tools to test those filters, especially GSO #47. Lumicon #47 curves just like in Christophe's charts.

Edited by BGazing, 17 February 2023 - 02:31 PM.

[RyzenAMD]

While I'm a fan of the Astronomik 642, when imaging relitivly faint Uranus or Neptune generally a longpass filter such as the Baader 610 LP (Baader's visible planetary R filter) gets you noticeably more light than a 200nm bandpass filter like the 642. Contrast will be less vs. a higher cut-on filter such as the 642 or longer, but so will noise, so it's usually a good trade-off. Mono cams are best for this, but the 610 LP has even been used with success using color cams such as the ASI224MC and ASI462MC, so your ASI678 may be worth a try. Use the C11 if you can. Good seeing is a must because of exposure length, you may want to brighten the gamma a bit (for focusing only), and an electric focuser (especially an add-on Crayford type) can really help.

 

I'm one of the folks who isn't a fan of dedicated UV filters for SCTs, largely due to SCT correction issues but also for the corrector's AR coatings cutting much of the UV signal. The Astronomik L2 and W47 combination is certainly worth trying--- it could very well be a fine combination for an SCT. I typically use a permanently mounted 368mm D-K with a dedicated UV (Chroma U-Bessell), but recently bought a Thorlabs 400CWL-40nm bandpass filter for my travel scope--- a C11. It should pass a nice mix of near-UVa and deep blue/violet (I hope). I haven't had a chance to try it out yet, but wasn't planning on imaging Venus for another 2 to 3 weeks. Wratten filter curves are notoriously inconsistent with published values, but I'm sure you'll have a blend of near-UV and deep blue with a typical W47, and the L2 filter should eliminate most if not all of the IR leak. I have tried a stacked UV-deep blue in the past (Schott UG3 with B39) but only with my D-K Cass, and years ago imaged a few times with a Schuler U photometric (UG1 with B39) and the C11. While the Schuler U passed a bit of deep blue, it was largely a UV filter and didn't work very well with the C11. Mono cameras really have a big advantage recording UV--- even more so than their IR advantage, as a color cam's Bayer filters block quite a lot of UV, and the G and R Bayer filters also effectively block deep blue.

 

As BucketDave has mentioned, atmospheric dispersion is strongest in the shorter wavelengths. I use an ADC with my D-K Cass, but the AR coatings on the ADC wedge prisms can be a problem with UV. I use a Pierro-Astro ADC that uses UV friendly coatings and fused silica prisms. The coatings on the popular ZWO ADC were selected to allow quite a bit of near-UV so while not as UV friendly as the Pierro-Astro, it still does a good job. In part, my recent purchase of the Thorlabs 400/40nm filter was to see if it's 40nm bandpass is narrow enough to allow C11 imaging without an ADC with Venus at 30° or higher altitude. It's worth mentioning that the lower a target is, the more UV absorption from our atmosphere. So try to avoid UV imaging below 30° if possible.

 

You mentioned the IDAS 410 filter. It has an IR leak issue according to Christophe Pellier, and he no longer recommends it.

 

Eventually I'll be testing that Thorlabs 400/40nm filter. It's listed here as FBH400-40:  https://www.thorlabs...60&pn=FBH400-40

I've attached a screenshot crop of the curves for the 400-40nm. It has very good off-band blocking optical density of OD5 or better out to 1100nm and at least OD4 out to 1265nm, beyond about 1100nm or so our cameras have virtually no sensitivity.

 

Another interesting choice is the Edmund 400-50nm filter: https://www.edmundop...s-filter/27850/

The curve data PDF is linked on that page under 'Get Product Downloads'.

The Edmund 400-50 has good off-band blocking out to 1200nm, averaging OD4, but it has a few short curve points that dip down into the OD3 to OD2 range. In practice that wouldn't be a problem--- I chose the Thorlabs for it's lower price.

 

Probably the best strictly deep blue/violet filter would be the Edmund 425-50:  https://www.edmundop...s-filter/29361/

The 425-50 won't have the contrast capability of a filter that passes some sub-400nm UV, but it should work well with an SCT and one can always enhance contrast a bit in processing. It's an interesting filter and has nice off-band blocking of OD4 to OD5. It's also mentioned in this Venus filter discussion in 2020 by Christophe Pellier:  https://www.cloudyni...t-blue-filters/

 

Be aware that the Thorlabs and Edmund filters I've mentioned are 25mm diameter and unmounted. You'll need to mount them in a cell.

Will the Thorlabs 25mm filter mount in a Blue Fireball filter holder from Agena Astro? https://agenaastro.c...-cell-s-1f.html

[RyzenAMD]

Will the Baader SLOAN/SDSS z-s' Mounted Filter - 2" work for a clone of the Wratten #47 filter?

 

I found a 2" Wratten #47 filter at Agena Astro that I am going to try out.

Edited by RyzenAMD, 22 February 2023 - 08:53 AM.

[John Boudreau]

Will the Thorlabs 25mm filter mount in a Blue Fireball filter holder from Agena Astro? https://agenaastro.c...-cell-s-1f.html

No, not without modification. Minimum filter glass size is listed at 26.5mm, so a 25mm filter would drop right through it. If you try to modify such a cell, keep in mind that in addition to the cell's base aperture, the threaded retainer is probably too small to retain a 25mm filter. It is possible to make washers out of thin plastic that can be used to narrow the cell's base opening, and also to help the threaded retainer hold the filter in place. The clear aperture of the Thorlabs filter is only 21mm, but that's more than enough for Venus imaging.

 

Will the Baader SLOAN/SDSS z-s' Mounted Filter - 2" work for a clone of the Wratten #47 filter?

 

I found a 2" Wratten #47 filter at Agena Astro that I am going to try out.

No-- the Sloan z-s' is an IR-pass filter, with a bandpass of about 820 to 920nm.

 

As the W47 has an IR leak starting after 700nm, you'd want to use an IR-blocking (cut) filter such as an Astronomik L2 UV-IR blocker. That particular model IR-blocker also still allows UV down to about 380nm to pass through.

[BGazing]

Will the Baader SLOAN/SDSS z-s' Mounted Filter - 2" work for a clone of the Wratten #47 filter?

 

I found a 2" Wratten #47 filter at Agena Astro that I am going to try out.

Which one did you find. Tested Lumicon #47 curves nicely. Could not find tests of any standard China-made #47s.

[BGazing]

No, not without modification. Minimum filter glass size is listed at 26.5mm, so a 25mm filter would drop right through it. If you try to modify such a cell, keep in mind that in addition to the cell's base aperture, the threaded retainer is probably too small to retain a 25mm filter. It is possible to make washers out of thin plastic that can be used to narrow the cell's base opening, and also to help the threaded retainer hold the filter in place. The clear aperture of the Thorlabs filter is only 21mm, but that's more than enough for Venus imaging.

 

No-- the Sloan z-s' is an IR-pass filter, with a bandpass of about 820 to 920nm.

 

As the W47 has an IR leak starting after 700nm, you'd want to use an IR-blocking (cut) filter such as an Astronomik L2 UV-IR blocker. That particular model IR-blocker also still allows UV down to about 380nm to pass through.

Well, I finally found someone who could scan this particular combination - #47 sold by TS and a new L2 UVIR by Astronomik.

Couple of things to note. Unlike other scanned #47 filters, this one has a very weak IR leak, but also goes a bit further into the visible spectrum. Does that still require IR cut? On the other hand, Astronomik L2 really seems to have gone to 400nm proper, just like Astronomik suggested.

 

Here are the links for other #47 filters that have been scanned - Meade and Lumicon have identical curves and strong opening at 700nm.

 

http://www.astrosurf...7-meade4000.gif

 

http://www.carlostap..._wratten_47.pdf

Attached Thumbnails

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Edited by BGazing, 03 March 2023 - 09:02 AM.

[John Boudreau]

Well, I finally found someone who could scan this particular combination - #47 sold by TS and a new L2 UVIR by Astronomik.

Couple of things to note. Unlike other scanned #47 filters, this one has a very weak IR leak, but also goes a bit further into the visible spectrum. Does that still require IR cut? On the other hand, Astronomik L2 really seems to have gone to 400nm proper, just like Astronomik suggested.

 

Here are the links for other #47 filters that have been scanned - Meade and Lumicon have identical curves and strong opening at 700nm.

 

http://www.astrosurf...7-meade4000.gif

 

http://www.carlostap..._wratten_47.pdf

That certainly looks like a nice W47---- it's got the lowest IR leak I've seen by far! However a 10% + leak like that may still be a problem with such a bright target as Venus, especially since it spans from just over 700nm out to beyond 900nm.

 

But regardless it's worth giving it a try both ways---- with and without the L2 UV-IR cut filter. The scan of the new L2 filter is disappointing, as it's clear now that with it's 400nm cut-on, the L2 isn't really the 'old' Astronomik L UV/IR cut. This may very well make the Astronomik L1 filter a better choice with your particular W47, as it would allow some UV and still get most of your W47's IR leak. Another option is to look for an old Astronomik L, but we really don't know when Astronomik changed the cut-on point. 

 

Your C8 corrector's AR coatings may well act as a good relatively 'early' UV cut-on itself, probably slopping around the 390 to 380nm mark to allow some upper end UV in addition to the violet/deep blue.

 

A typical W47 filter with the strong IR leak has been used by some for adjusting ADCs on a star (with a camera)--- it would pass deep blue and some IR, causing a low altitude star to appear as two separate disks. Adjusting the ADC then draws the deep blue and IR disks together when properly adjusted. I've done this myself--- it works well, but it wouldn't work as well with your W47! 

[BGazing]

That certainly looks like a nice W47---- it's got the lowest IR leak I've seen by far! However a 10% + leak like that may still be a problem with such a bright target as Venus, especially since it spans from just over 700nm out to beyond 900nm.

But regardless it's worth giving it a try both ways---- with and without the L2 UV-IR cut filter. The scan of the new L2 filter is disappointing, as it's clear now that with it's 400nm cut-on, the L2 isn't really the 'old' Astronomik L UV/IR cut. This may very well make the Astronomik L1 filter a better choice with your particular W47, as it would allow some UV and still get most of your W47's IR leak. Another option is to look for an old Astronomik L, but we really don't know when Astronomik changed the cut-on point.

Your C8 corrector's AR coatings may well act as a good relatively 'early' UV cut-on itself, probably slopping around the 390 to 380nm mark to allow some upper end UV in addition to the violet/deep blue.

A typical W47 filter with the strong IR leak has been used by some for adjusting ADCs on a star (with a camera)--- it would pass deep blue and some IR, causing a low altitude star to appear as two separate disks. Adjusting the ADC then draws the deep blue and IR disks together when properly adjusted. I've done this myself--- it works well, but it wouldn't work as well with your W47!

Thank you for the explanations! I was surprised when the machine started plotting 700 plus part. For a budget #47 it certainly looks decent, but I will pair it with a dedicated IR only cut, makes more sense than L1.
I'll get a hold of a new Lumicon #47 in about two weeks and will try to scan it for comparison. If earlier specimens give any hint, it will have a tighter curve and higher tramsmission...and a bigger IR leak.
So C8 corrector smothers it at 380, just about killing any signal on a dedicated Venus filter. Would a triplet lens have the same effect?

Sent from my SM-S9010 using Tapatalk

[John Boudreau]

Thank you for the explanations! I was surprised when the machine started plotting 700 plus part. For a budget #47 it certainly looks decent, but I will pair it with a dedicated IR only cut, makes more sense than L1.
I'll get a hold of a new Lumicon #47 in about two weeks and will try to scan it for comparison. If earlier specimens give any hint, it will have a tighter curve and higher tramsmission...and a bigger IR leak.
So C8 corrector smothers it at 380, just about killing any signal on a dedicated Venus filter. Would a triplet lens have the same effect?

 

I just spent some time on the Astronomik website. Until reading your note here I was unaware that they still offered the old IR-blocking filter!!! Thought that was superseded by the three new types of L filters. Thanks--- always nice to learn something! 

 

So yes, I agree that would a fine filter for your W47. Attached is the T% curve for a Starbright coated C8 corrector. In practice due to the combination of corrector, filter stack, and ASI174MM camera AR-coated protective window the total UV bandpass T% would be somewhat less than in this corrector chart. Be aware that this chart's base is @ 50% T.

 

 

 

 

 

 

Also, take into account that within your W47's BP, the actual light signal from Venus is stronger in the violet/deep blue than in the UV. Attached is a Venus spectrum from Christophe Pellier that illustrates this nicely.

 

 

 

 

 

Edit: I forgot to address your question of UV T% through the AR coatings of a triplet refractor. Yes, the AR coatings would mute the UV --- how much I have no idea, but if it's an airspaced triplet that means 6 coated surfaces. But refactors--- even most APOs, don't have very good UV correction. The exception is the Takahashi TOA scopes. I have seen some nice UV Venus results from a TOA150, but I have no idea how long the exposures were.

 

 

 

Edited by John Boudreau, 03 March 2023 - 07:34 PM.

[BGazing]

Thank you, John, the fact that, despite that transmission graph you posted, people still get great results with SCTs and dedicated Venus filters is amazing.  The way I understand it, W47 straddling both sides of 400 helps get a bit more of contrast and a bit more of signal.

This is going to be fun, for sure (or really frustrating, can't tell ). We probably won't have clear skies for the next two weeks, by then Venus is going to be much closer and higher.

[YossiZ]

Some very useful information in here, thank you for that!

I am going to try imaging Venus in UV with my Mak 150mm (I know, I know...) and ASI178MM. Two reasonably priced filter options are on the way and may work:
1) Baader Bessel Photometric U
2) Lumicon #47 + Astronomik IR Cut

[RyzenAMD]

I got my Wratten #47 and Optolong UV/IR filters the other day. I will try to image Venus next clear evening with my Evolution 6.

[BGazing]

I got my Wratten #47 and Optolong UV/IR filters the other day. I will try to image Venus next clear evening with my Evolution 6.

Looking forward to seeing your result. I got IR-cut only and IR742...but am away from home, so the first test will have to wait.

[RyzenAMD]

I have the IR 642 ProPlanet, which I might try as well.

[Jcoogler]

I received an ir cut filter yesterday and have the w47, just waiting on clear skies! Can’t wait to see everyone’s photos

[BGazing]

Well, Venus is difficult. It does not matter that seeing is really, really bad at the moment. UV/violet was unusable, and the best I could do with IR was a subtle hint of something...cannot push it because it becomes noisy, really starved of quality data. Wind...

Attached Thumbnails

• 

Edited by BGazing, 18 March 2023 - 02:24 PM.

[John Boudreau]

Well, Venus is difficult. It does not matter that seeing is really, really bad at the moment. UV/violet was unusable, and the best I could do with IR was a subtle hint of something...cannot push it because it becomes noisy, really starved of quality data. Wind...

You've captured some IR features! Nice job! Attached is your shot after a high pass filter and curve tweak in PhotoShop (hope you don't mind).

 

And at virtually the same time as your shot, Luigi Morrone was shooting from Italy with a C14 and a Sloan photomertric 820 to 920nm BP filter to get this fine high-rez result: http://alpo-j.sakura...3/v230318a1.jpg

 

You can detect the bright '>" chevron features in both images!

 

I thought the 610LP filter was worth a try although it's usually looked at as a Uranus/Neptune filter, but actually a near-IR  LP filter that cuts on at ~700 or 800nm or so will result in better contrast. There will be a brightness penalty vs. the 610 though.

Attached Thumbnails

• 

[KMH]

The UV Venus picture at the bottom won't win any awards - seeing was quite poor and there were some capture issues.  But, I wanted to acknowledge use of the Thor Labs 400/40nm filter that was mentioned by John in post #3.  The other capture information is:
Celestron Edge11

2x PowerMate

ASI462MM with 4.0 ms exposures and gain 202 (33%)

8000/74000 frames stacked

Firecapture/Autostakkert/AstraImage/Photoshop

I have multiple video captures that show the same pattern, so I think they are real clouds rather than processing artifacts.  Please excuse the edge rind.

 

Because of the seeing, this image is certainly not representative of this filter's ability to capture detail.  But, I note that I was able to decrease exposure time by a factor of ~15x relative to a Baader U Venus filter (which is basically worthless for this setup).  So I think the 400/40nm filter has tremendous potential for use with SCTs and barlows.  Hopefully I will get some better seeing soon to try again, and I imagine John will be testing his.

 

BTW Thor Labs also sells "lens tubes" for mounting 1" optics and filters.  There is a 0.3" long version that does well as a filter cell for 1" filters.

https://www.thorlabs...ctgroup_id=3307

The catch is that the threads are SM1, which is 1.035"x40 (1.25" eyepiece threads are M28.5x0.6).  There are two solutions:

 

Thor also sells C-mount to SM1 adaptors.
https://www.thorlabs...tgroup_id=12627

Here the cell+adaptor can be threaded into a video camera.  Some shimming (like metallic tape) is necessary when mounting to the scope, because the SM1 diameter is 1.20".

 

Alternatively, an SM1 to M28.5x0.6 adaptor can be used.  I found one on ebay (from RafCamera - they are now out of stock), or someplace like Precise Parts can make one.

 

Hope this helps!

Kevin

 

PS - I have long experience purchasing from Thor Labs, and they are a good company.  Plus, you get "Lab Snacks"!!!

 

Edited by KMH, 07 April 2023 - 04:08 PM.

[KMH]

Just for the completeness, here is an IR image from the same day, taken using a Baader 685nm long-pass filter.  Maybe some cloud features???

 

Kevin