19 replies to this topic

[GabeS]

Hi all, hoping to post some more around here and I’m looking to get some thoughts (and prayers) regarding this silly project I’ve been tinkering on. Yes, I’m afraid you read the title right, I will be attempting astrophotography at f/0.89, although mainly I plan to use this for EAA at live events and meetups. 

 

My set up is based around the use of a 0.64X speed booster made by metabones for a specific pocket cinema camera, I chose this version of this model because it’s optimized for a much thinner sensor stack than what’s in most other micro 4/3 cameras, and I thought it fitting considering there’s only one window/filter above our astro cameras. This optic resides in the flange distance between a DSLR lens mount and mirrorless lens mount, and compresses a full frame image circle down to a Micro-4/3 sensor sized image circle. I also already have the longest focal length full frame f/1.4 DSLR lens, the Sigma 105mm f/1.4 Art. Combining these two give me an approximately 9.5° square field of view when paired with the 533MC sensor for example, at a blistering f/0.89. This is six times faster than a RASA! Now, realistically, it would be more at home in use with my 400mm f/2.8 for EAA and live stacking for an audience (~250mm f/1.7 equivalent exposure), but I will be using it at f/0.89 if only for the heck of it.

 

 

Getting this to work with an astro camera isn’t straightforward, as the adapter protrudes towards the sensor a good amount, and you need to actually remove the plate that holds the window above the sensor in place, but keep the window. That’s how tight the margins are! The window is required to correct for spherical aberrations introduced in its absence. When the window is in place and the speedbooster is extended to its fullest, the correction seems optimal and the image looks sharp. This window changes with the camera used, but at least for the 533 I can confirm a 31mm unmounted filter drops in to cover the gasket and will act as the only place in this optical train with enough physical space to fit any filter. 

 

 

Anyways, the real question I have today is any suggestions for filters I can use, particularly as this EAA will likely be done under heavy light pollution. Obviously a proper narrowband filter would be out of the question due to bandpass shift, and I fear that any old light pollution filter with its bandpass notches would suffer the same fate. I do own, and, thanks to working at an astronomy/telescope shop, have access to lots of filters optimized for high speed imaging. But even those are only optimized for f/2. So what do you all think? Will a regular old cheap UHC style filter be ‘sloppy’ enough in its bandpass to allow full use of the f/0.89 aperture? I know Celestron has light pollution filters with notches optimized for RASAs, but they’re hard to come by. Maybe an IDAS DTD? Love to hear your thoughts, and I’ll keep you all updated on the project!

 

[steveincolo]

Hi all, hoping to post some more around here and I’m looking to get some thoughts (and prayers) regarding this silly project I’ve been tinkering on. Yes, I’m afraid you read the title right, I will be attempting astrophotography at f/0.89, although mainly I plan to use this for EAA at live events and meetups. 

 

 

My set up is based around the use of a 0.64X speed booster made by metabones for a specific pocket cinema camera, I chose this version of this model because it’s optimized for a much thinner sensor stack than what’s in most other micro 4/3 cameras, and I thought it fitting considering there’s only one window/filter above our astro cameras. This optic resides in the flange distance between a DSLR lens mount and mirrorless lens mount, and compresses a full frame image circle down to a Micro-4/3 sensor sized image circle. I also already have the longest focal length full frame f/1.4 DSLR lens, the Sigma 105mm f/1.4 Art. Combining these two give me an approximately 9.5° square field of view when paired with the 533MC sensor for example, at a blistering f/0.89. This is six times faster than a RASA! Now, realistically, it would be more at home in use with my 400mm f/2.8 for EAA and live stacking for an audience (~250mm f/1.7 equivalent exposure), but I will be using it at f/0.89 if only for the heck of it.

 

 

 

Getting this to work with an astro camera isn’t straightforward, as the adapter protrudes towards the sensor a good amount, and you need to actually remove the plate that holds the window above the sensor in place, but keep the window. That’s how tight the margins are! The window is required to correct for spherical aberrations introduced in its absence. When the window is in place and the speedbooster is extended to its fullest, the correction seems optimal and the image looks sharp. This window changes with the camera used, but at least for the 533 I can confirm a 31mm unmounted filter drops in to cover the gasket and will act as the only place in this optical train with enough physical space to fit any filter. 

 

 

 

Anyways, the real question I have today is any suggestions for filters I can use, particularly as this EAA will likely be done under heavy light pollution. Obviously a proper narrowband filter would be out of the question due to bandpass shift, and I fear that any old light pollution filter with its bandpass notches would suffer the same fate. I do own, and, thanks to working at an astronomy/telescope shop, have access to lots of filters optimized for high speed imaging. But even those are only optimized for f/2. So what do you all think? Will a regular old cheap UHC style filter be ‘sloppy’ enough in its bandpass to allow full use of the f/0.89 aperture? I know Celestron has light pollution filters with notches optimized for RASAs, but they’re hard to come by. Maybe an IDAS DTD? Love to hear your thoughts, and I’ll keep you all updated on the project!

How about the old IDAS NBZ with its 12 nm bandpasses?  Would that be wide enough to accommodate the shift?

[nic35]

This sounds like a job for (filter) superman, Jim Thompson.

 

j

[GabeS]

How about the old IDAS NBZ with its 12 nm bandpasses?  Would that be wide enough to accommodate the shift?

I fear that 12nm is still far too narrow, even if it were 'blue-shifted' to account for the fast f/ratio. some papers I've been finding seem to suggest as much :/

 

This sounds like a job for (filter) superman, Jim Thompson.

 

j

and that sounds like just the type of guy I need, I've just been starting on reading some of his previous work.

[jimthompson]

Hi GabeS,

 

Sorry for the delay in responding.  It took me a while to work out a prediction of filter behavior for optics as fast as you are talking about:  f/0.89.  When I take my measurements of filter spectra, my test rig is able to tilt the filter up to 20deg off axis.  To cover f/0.89 I had to extrapolate my measured data to cover up to 30deg of tilt.  Anyway, the link below is to some graphs for a couple of filters for which I have data, plotted at different optics speeds.  A UHC or CLS filter would work fine, but they have very wide pass bands ... wider than you need and still remain useful at f/0.89.  I think your best bet looks something like an IDAS NB-1 or NB-2.  I have included a plot that compares all the available multi-narrowband filters available so you can see which ones are similar to NB-1/NB-2 and should thus work on your fast optics.  I believe the Altair Quad Band V2 is another possibility.

 

http://karmalimbo.co...ics_Dec2024.pdf

 

Cheers,

 

Jim T.

[steveincolo]

Hi GabeS,

 

Sorry for the delay in responding.  It took me a while to work out a prediction of filter behavior for optics as fast as you are talking about:  f/0.89.  When I take my measurements of filter spectra, my test rig is able to tilt the filter up to 20deg off axis.  To cover f/0.89 I had to extrapolate my measured data to cover up to 30deg of tilt.  Anyway, the link below is to some graphs for a couple of filters for which I have data, plotted at different optics speeds.  A UHC or CLS filter would work fine, but they have very wide pass bands ... wider than you need and still remain useful at f/0.89.  I think your best bet looks something like an IDAS NB-1 or NB-2.  I have included a plot that compares all the available multi-narrowband filters available so you can see which ones are similar to NB-1/NB-2 and should thus work on your fast optics.  I believe the Altair Quad Band V2 is another possibility.

 

http://karmalimbo.co...ics_Dec2024.pdf

 

Cheers,

 

Jim T.

For emission nebulas, I’d be curious how an ultra-fast setup with a wide band pass (such as the NB-2) compares to, say, an f/4 setup with 7nm or less band passes (such as an L-extreme or L-ultimate), in terms of SNR.  If even that calculation can be made!  Perhaps it depends a fair amount on background sky glow.  

[GabeS]

Hi GabeS,

 

Sorry for the delay in responding.  It took me a while to work out a prediction of filter behavior for optics as fast as you are talking about:  f/0.89.  When I take my measurements of filter spectra, my test rig is able to tilt the filter up to 20deg off axis.  To cover f/0.89 I had to extrapolate my measured data to cover up to 30deg of tilt.  Anyway, the link below is to some graphs for a couple of filters for which I have data, plotted at different optics speeds.  A UHC or CLS filter would work fine, but they have very wide pass bands ... wider than you need and still remain useful at f/0.89.  I think your best bet looks something like an IDAS NB-1 or NB-2.  I have included a plot that compares all the available multi-narrowband filters available so you can see which ones are similar to NB-1/NB-2 and should thus work on your fast optics.  I believe the Altair Quad Band V2 is another possibility.

 

http://karmalimbo.co...ics_Dec2024.pdf

 

Cheers,

 

Jim T.

No worries at all, thanks a ton for that Jim! This might be a silly question, and I doubt I'll communicate it effectively on my first try, but when the transmission is graphed for an f/0.89 optic using a 30-degree angle of incidence, do those transmission values integrate the rest of the light that would be coming in from the central parts of the lens that would be hitting the filter/sensor closer to orthogonal? or are those transmission curves representative only of the light contributed from the outer 'ring' of an f/0.89 optic?

[jimthompson]

No worries at all, thanks a ton for that Jim! This might be a silly question, and I doubt I'll communicate it effectively on my first try, but when the transmission is graphed for an f/0.89 optic using a 30-degree angle of incidence, do those transmission values integrate the rest of the light that would be coming in from the central parts of the lens that would be hitting the filter/sensor closer to orthogonal? or are those transmission curves representative only of the light contributed from the outer 'ring' of an f/0.89 optic?

Hi Gabe,

 

I think I understand your question.  Yes, the plots are weighted averages over the whole area of the objective.  So the outer portion of the objective has filter performance as if were tilted at 30deg off-axis, and the middle of the objective has filter performance with no tilt.  My measurements of filter spectra vs tilt angle are at roughly ~1.7 degree increments, up to a max tilt of 20deg.  After that I used the measured value from 20deg and translated it to estimate filter performance at tilts from 20 to 30deg, in 1 degree increments.  So that is how the objective is divided in my calculation, into rings of 1.7 deg and 1 deg thickness.  Thus the f/0.89 optics calculation is an average of 22 different areas with a different filter spectrum for each area.

 

Cheers,

 

Jim T.

[jimthompson]

For emission nebulas, I’d be curious how an ultra-fast setup with a wide band pass (such as the NB-2) compares to, say, an f/4 setup with 7nm or less band passes (such as an L-extreme or L-ultimate), in terms of SNR.  If even that calculation can be made!  Perhaps it depends a fair amount on background sky glow.  

Let me chew on that question for a few days.  I might be able to calculate something ...

 

Cheers,

 

Jim T.

[GabeS]

For emission nebulas, I’d be curious how an ultra-fast setup with a wide band pass (such as the NB-2) compares to, say, an f/4 setup with 7nm or less band passes (such as an L-extreme or L-ultimate), in terms of SNR.  If even that calculation can be made!  Perhaps it depends a fair amount on background sky glow.  

I bet under perfectly dark skies you'd benefit from the ultra-fast setup for sure, but anything worse than like bortle 3 or 4 and you lose any benefit a faster f/ ratio gives you because of the fatter bandpasses it requires. A simple test would be using the same super-fast optic, and just changing from wider to narrower filters under different ambient light conditions. the narrower bandpass filter will 'reject' any of the light coming off axis from the area of the lens contributing to the faster f/ ratio, and you could treat it as an f/4 optic or whatever the max the filter would allow.

 

Hi Gabe,

 

I think I understand your question.  Yes, the plots are weighted averages over the whole area of the objective.  So the outer portion of the objective has filter performance as if were tilted at 30deg off-axis, and the middle of the objective has filter performance with no tilt.  My measurements of filter spectra vs tilt angle are at roughly ~1.7 degree increments, up to a max tilt of 20deg.  After that I used the measured value from 20deg and translated it to estimate filter performance at tilts from 20 to 30deg, in 1 degree increments.  So that is how the objective is divided in my calculation, into rings of 1.7 deg and 1 deg thickness.  Thus the f/0.89 optics calculation is an average of 22 different areas with a different filter spectrum for each area.

 

Cheers,

 

Jim T.

awesome, I was wondering if you added up rings of a given 'thickness', or if you were doing the maths and doing some fancy integral.  

 

 

FWIW, here's IDAS's reply to me with their estimates of the DTD filter at f/0.89 as well: I liked that it gave me some bandpass in the near-IR where there's not much light pollution, but where galaxies are still bright.

[jimthompson]

Happy New Year everyone!  So, over the break I had some time to calculate a prediction of how image SNR changes with optics speed, filter FWHM, and sky darkness.  My calculation assumes an ideal sensor, so Read Noise and Dark Current are zero.  Making that assumption reduces the calculation of SNR to the following:

 

SNR = Target Signal / sqrt(Target Signal + Background Signal)

 

I can get predicted Target Signal and Background Signal values from my filter performance calculator, so I just had to vary my inputs and turn the crank to generate the attached table.  I have done the calculation for one object type, H-alpha dominated emission nebulae.  The calculation also assumes that the same exposure time is used regardless of the filter selected, optics used, or sky darkness.  Interestingly it appears that very roughly one can get the same SNR under dark skies with f/4 optics as they can under light polluted skies with f/0.89 optics.  Note that f/0.89 optics give you just over 20x the light gathering that f/4 provides, so quite a large advantage.  So much of an advantage that you do indeed get a better SNR (about 3x) using a wider filter with fast optics than you do with a narrower filter and slow optics.  I would be very interested to see if this prediction is correct based on Gabe's observations.

 

Cheers,

 

Jim T.

Attached Thumbnails

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Edited by jimthompson, 08 January 2025 - 05:35 PM.

[GabeS]

Happy New Year everyone!  So, over the break I had some time to calculate a prediction of how image SNR changes with optics speed, filter FWHM, and sky darkness.  My calculation assumes an ideal sensor, so Read Noise and Dark Current are zero.  Making that assumption reduces the calculation of SNR to the following:

 

SNR = Target Signal / sqrt(Target Signal + Background Signal)

 

I can get predicted Target Signal and Background Signal values from my filter performance calculator, so I just had to vary my inputs and turn the crank to generate the attached table.  I have done the calculation for one object type, H-alpha dominated emission nebulae.  The calculation also assumes that the same exposure time is used regardless of the filter selected, optics used, or sky darkness.  Interestingly it appears that very roughly one can get the same SNR under dark skies with f/4 optics as they can under light polluted skies with f/0.89 optics.  Note that f/0.89 optics give you just over 20x the light gathering that f/4 provides, so quite a large advantage.  So much of an advantage that you do indeed get a better SNR (about 3x) using a wider filter with fast optics than you do with a narrower filter and slow optics.  I would be very interested to see if this prediction is correct based on Gabe's observations.

 

Cheers,

 

Jim T.

Happy new year's Jim, thanks for this! That was also not my expectation, I would've guessed the hyper-fast optics would lose their advantage once any amount of light pollution is involved, and I certainly never would've guessed the f/0.89 setup to still hold a 3x SNR advantage over f/4 optics under a Bortle 9 sky!

My first light with the setup was not ideal, there's a large amount of purple fringing from the Sigma 105mm f/1.4, and the stars fall apart in the outer 5% or so, right in the corners of the image. But I'm not too worried, I plan on using a filter like the IDAS GNB, or SVBony's new equivalent; something with ~10nm bandpasses around hydrogen and oxygen, and then a window in the NIR for galaxies. for the aberrations that would still be present with that type of filter, like spherical aberration, I'm happy to report the stars are acceptable at f/0.89. Tonight I'll be trying this setup at my shop's meetup for some live stacking using my 400mm f/2.8, with either an Antlia ALP-T 5nm Highspeed dualband, or an Antlia 3nm dualband in the lens' filter drawer. With the speedbooster it'll bring it down to 256mm f/1.79, not groundbreakingly fast but good enough for some nice EAA, especially since the filter can be placed before the speedbooster and tighter bandpasses can be used. Will report back with my findings!

Edited by GabeS, 08 January 2025 - 06:10 PM.

[steveincolo]

Happy New Year everyone!  So, over the break I had some time to calculate a prediction of how image SNR changes with optics speed, filter FWHM, and sky darkness.  My calculation assumes an ideal sensor, so Read Noise and Dark Current are zero.  Making that assumption reduces the calculation of SNR to the following:

 

SNR = Target Signal / sqrt(Target Signal + Background Signal)

 

I can get predicted Target Signal and Background Signal values from my filter performance calculator, so I just had to vary my inputs and turn the crank to generate the attached table.  I have done the calculation for one object type, H-alpha dominated emission nebulae.  The calculation also assumes that the same exposure time is used regardless of the filter selected, optics used, or sky darkness.  Interestingly it appears that very roughly one can get the same SNR under dark skies with f/4 optics as they can under light polluted skies with f/0.89 optics.  Note that f/0.89 optics give you just over 20x the light gathering that f/4 provides, so quite a large advantage.  So much of an advantage that you do indeed get a better SNR (about 3x) using a wider filter with fast optics than you do with a narrower filter and slow optics.  I would be very interested to see if this prediction is correct based on Gabe's observations.

 

Cheers,

 

Jim T.

Very interesting!

[jimthompson]

... But I'm not too worried, I plan on using a filter like the IDAS GNB, or SVBony's new equivalent; something with ~10nm bandpasses around hydrogen and oxygen, and then a window in the NIR for galaxies. ...

Funny you mention the new SV240 filter as I just did my first testing with it this past Sunday night.  I originally ordered it from SVBony back in early November, but due to the Canada Post strike I didn't receive it until this past Friday.  I don't have the test report written yet, but you can see the test images I captured at the Flickr album linked below.  The SV240 is pretty much identical to the IDAS GNB.  They both provided an improvement in the view of galaxies from my Bortle 9+ location, but not as good as the Antlia Quadband ALP, which is in my opinion the current champ for all-purpose filter.  Note however that these filters don't have O-III/Ha pass bands as narrow as 10nm, they are more like 20 to 30nm wide.  Their performance on emission nebulae is inferior to an NB-2 or NBZ.

 

https://flic.kr/s/aHBqjBXq6X

 

Cheers,

 

Jim T.

Edited by jimthompson, 09 January 2025 - 12:05 PM.

[MarMax]

Funny you mention the new SV240 filter as I just did my first testing with it this past Sunday night.  I originally ordered it from SVBony back in early November, but due to the Canada Post strike I didn't receive it until this past Friday.  I don't have the test report written yet, but you can see the test images I captured at the Flickr album linked below.  The SV240 is pretty much identical to the IDAS GNB.  They both provided an improvement in the view of galaxies from my Bortle 9+ location, but not as good as the Antlia Quadband ALP, which is in my opinion the current champ for all-purpose filter.  Note however that these filters don't have O-III/Ha pass bands as narrow as 10nm, they are more like 20 to 30nm wide.  Their performance on emission nebulae is inferior to an NB-2 or NBZ.

 

https://flic.kr/s/aHBqjBXq6X

 

Cheers,

 

Jim T.

Jim, I've noticed in the link above that the graph for the Antlia Quadband ALP is different for NIR as compared to the Antlia site. Agena Astro also shows the same graph as the Antlia site. Can you explain this?

 

I've re-acquired my Antlia Quadband ALP filter and hope to give it a second chance with the RASA8 and 2600MC with the AR protect window.

[GabeS]

Funny you mention the new SV240 filter as I just did my first testing with it this past Sunday night.  I originally ordered it from SVBony back in early November, but due to the Canada Post strike I didn't receive it until this past Friday.  I don't have the test report written yet, but you can see the test images I captured at the Flickr album linked below.  The SV240 is pretty much identical to the IDAS GNB.  They both provided an improvement in the view of galaxies from my Bortle 9+ location, but not as good as the Antlia Quadband ALP, which is in my opinion the current champ for all-purpose filter.  Note however that these filters don't have O-III/Ha pass bands as narrow as 10nm, they are more like 20 to 30nm wide.  Their performance on emission nebulae is inferior to an NB-2 or NBZ.

 

https://flic.kr/s/aHBqjBXq6X

 

Cheers,

 

Jim T.

This is very helpful, thanks again Jim. We have a spare CPC1100 and a Hyperstar 11v4 on the way, I will soon have it up and running and, when used in conjunction with this Metabones speedbooster and a 294MC pro, will make for a mighty fine f/1.19 EAA scope for outreach and live events. The Antlia quad band would be perfect for all setups, if not for that pesky bandpass in the blue/purple region for colour balance, as I know it will transmit the purple fringing with the 105mm f/1.4. But it might be the perfect filter for the C11 Hyperstar setup! I’m surprised to see the quad band and GNB/SV240 filters hold their own on emission nebulae when compared to the other filters with tighter band passes, despite the light pollution.

 

I haven’t taken correct flats for it yet, but I used the speedbooster on my 400mm f/2.8 for some first light EAA at our last public meetup last week, and I’m happy to report the speedbooster did not negatively affect the stars, at least from what we could see in the initial live stack. Will share the image once I can take those pesky flats.

 

EDIT: here's the 21 minute stack from a bortle 9+ industrial zone, with some likely improperly taken flats applied and a histogram stretch, but nothing else. Despite the industrial light gradients, and poor flats, I think we can still learn some things. The stars look more than usable for EAA, out past the illuminated field. An illuminated field that realistically should be more like a 533 sized sensor, anyways. We were seeing the horsehead in less than a minute, which was impressive considering it was hard to even pick out Orion's Belt visually. I think this speedbooster will be excellent when used with a 533MC Pro on an 11" Hyperstar! As for the f/0.89 imaging that started this thread, that optical setup on a 533MC Pro gives an approximately 9 degree square view, so it's neat as a party trick or under extra dark skies where large molecular clouds can be revealed, but for outreach and live stacking for an audience I think the subject isolation of a larger telescope is necessary.

Edited by GabeS, 12 January 2025 - 11:07 PM.

[jimthompson]

Jim, I've noticed in the link above that the graph for the Antlia Quadband ALP is different for NIR as compared to the Antlia site. Agena Astro also shows the same graph as the Antlia site. Can you explain this?

 

I've re-acquired my Antlia Quadband ALP filter and hope to give it a second chance with the RASA8 and 2600MC with the AR protect window.

I just read on another thread that Antlia has changed the filter to have a narrower pass band in the IR because users were complaining about poor focus on some refractors.  That is unfortunate as I think the narrower IR pass band will hurt performance.  I will have to do some calculations on this ...

 

Cheers,

 

Jim T.

[MarMax]

I just read on another thread that Antlia has changed the filter to have a narrower pass band in the IR because users were complaining about poor focus on some refractors.  That is unfortunate as I think the narrower IR pass band will hurt performance.  I will have to do some calculations on this ...

 

Cheers,

 

Jim T.

OK, so it is safe to assume that the pass band of the filter I bought matches the graph that came with the filter? I think mine is an early one with the wider pass band (I hope so). You would think Antlia includes the correct graph with each filter.

[jimthompson]

OK, so it is safe to assume that the pass band of the filter I bought matches the graph that came with the filter? I think mine is an early one with the wider pass band (I hope so). You would think Antlia includes the correct graph with each filter.

That would be my assumption as well, that the graph that is packaged with the filter is for that particular filter batch.  You probably bought a filter from existing stock at that particular vendor.

 

Cheers,

 

Jim T.

[GabeS]

a new optical setup to experiment with! Now I’m curious to see which version of the antlia I have on our shelves right now…

 

EDIT: We got a shipment of four recently, checking the graphs that came with each of them they seem to be the narrower IR bandpass versions. 

Edited by GabeS, 14 January 2025 - 06:33 PM.