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Fast Astrographs - Is the CDK14 faster than the DR350?

Planewave CDK14 Equipment Reflectors Reducer
Rouz Astro
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Rouz Astro avatar
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At first glance, it may seem preposterous to think a "slow" CDK14 can be considered faster than an f/3.0 astrograph - the Planewave DR350.

First of all, lets define what "fast" means to us imagers - Are we merely looking at the focal ratio? In that case, my Samsung phone is the fastest with an f/1.6 lens.

A critical factor to consider as well is image scale - Ideally, we'd (I'd) like to record as much details as we are allowed by the sky. Given this criteria, with "good" median sky conditions at 1.5 to 2.0" of seeing. With 2x to 3x sampling rate, that requires an image scale of somewhere around 0.7" / pixel.

This image scale can be achieved by binning CMOS pixels, I will not consider obsolete CCD technology as they are not on par with modern IMX sensors ( with more improvements in CMOS bound to come).


Luckily, Planewave have designed a focal reducer for the CDK scopes with very good performance. While many may look down at focal reducers, my tests don't show a detectable image quality drop.

The "Hybrid" camera matching allowed with the CDK14 + 0.66x reducer is very beneficial. It allows a generous FOV at a high image scale of 0.46"/pixel which can be further down sampled digitally (hence Hybrid =  Optical + Digital).

Looking at the specs of the DR350, which I believe uses the same Chassis/rear cell/truss components as the CDK14, the light collecting area is considerably smaller. 

How much smaller?  10,080mm2 less. That's is quite a lot considering the "gold standard" Tak FSQ106 has a total objective area of 8600mm2.

Comparing these numbers side by side reveals signal at the Pixel level - Pixel Strength is actually highest on the CDK14 setup.

This means you can achieve critical sampling (or very close to it) and have a higher SNR with the CDK14 compared with the DR350. The main advantage of the DR350 then becomes the larger FOV that is offers (not always an advantage).

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Rouz Astro avatar
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FOV - Another important factor to consider, we'd like our system to have enough FOV to be useful for a relatively large number of targets. With about 1.5 degrees, you can fit the majority of targets in the frame like the large M33 galaxy and Orion with the running man.

Below are some examples with the CDK14 setup  at f/4.7.



Rouz Astro avatar
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Looking at the median FWHM that is 3.85 pixels for this stack of the butterfly nebula, with an image scale of 0.458"/pixel -

That means the median star size across a full frame sensor is   1.76".

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This is what the field looks like in numbers:
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The Achilles heel of the PW focal reducer was the inability to use it with mono camera and OAG.
That issue has been solved by a configuration and designed and tested extensively. The CDK14  at f/4.7 IS capable of running:
  • 0.66x Reducer
  • Rotating Focuser
  • OAG
  • Filter Wheel
  • Mono Camera
Roman Pearah avatar
Roman Pearah avatar
Doing all of the calculations is fun, but it's sufficient just to know that relative speed on an equal resolution basis is always the ratio of the aperture areas, assuming the same QE/transmittance in the system. In this case, it's only 18% faster so I agree that the attraction of the DR really does come down to widefield needs. And when you consider that the resolution is presumed to be the same here, that feels like a powerful argument, since you can always make the field smaller by cropping but never bigger.
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Rouz Astro avatar
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Roman Pearah:
Doing all of the calculations is fun, but it's sufficient just to know that relative speed on an equal resolution basis is always the ratio of the aperture areas, assuming the same QE/transmittance in the system. In this case, it's only 18% faster so I agree that the attraction of the DR really does come down to widefield needs. And when you consider that the resolution is presumed to be the same here, that feels like a powerful argument, since you can always make the field smaller by cropping but never bigger.

*
Yes, it does boil down to aperture but there are also other factors that make aperture more or less "useful" and efficient.

For example, the Edge14 has the same aperture - 14inches - but here, the focal length is just too long which does nothing but restrict the FOV to very small areas.
Once you add the reducer, the FOV is still quite small compared to these telescopes.

You then need to bin almost 3x to get similar critically sampled image scales leaving you with a small 6.8 megapixel final image that isn't very impressive over a less than 1 degree FOV. 

Also, you are now dealing with a corrector plate and the 7 ( SEVEN) element focal reducer. The total system now has 22 surfaces to content with if I'm not mistaken.

Then you have the mechanical and tube issues as well.
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Roman Pearah avatar
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Right, but that's why I made the qualifying statements about QE/transmittance/FOV.
Dark Matters Astrophotography avatar
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Focal ratio and pixel size are the two ways you can influence speed. You also need to factor in the central obstruction. The CO is much larger on the DR350, which your calculations seem to show impacts the effective etendue enough that the reduced CDK14 is faster. The difference in transmission caused by the glass in the reducer on the CDK 14 should also be considered though. Not sure that's in the sheet or not.
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Roman Pearah avatar
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Bill,

Yep, but that f-ratio and pixel size all get absorbed into the equal pixel scale assumption here, making it a matter of aperture and transmittance. Like you, I expect that the original calculation, which already gives the CDK14 a merely modest speed advantage, is being generous to the CDK14's transmittance. I wouldn't be surprised if that turns things around.
Dark Matters Astrophotography avatar
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Roman Pearah:
Bill,

Yep, but that f-ratio and pixel size all get absorbed into the equal pixel scale assumption here, making it a matter of aperture and transmittance. Like you, I expect that the original calculation, which already gives the CDK14 a merely modest speed advantage, is being generous to the CDK14's transmittance. I wouldn't be surprised if that turns things around.



Yeah, I'm aware of that. I'm not so sure the benefit of the 1.62 binning in post would be the same as putting larger native pixels on the system to begin with. But for a quick on paper analysis this is an interesting exercise.
Roman Pearah avatar
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Yeah, might be better to shoot with a larger pixel and Drizzle up the sampling to match given that it's not an integer.
Rouz Astro avatar
Rouz Astro avatar
Focal ratio and pixel size are the two ways you can influence speed. You also need to factor in the central obstruction. The CO is much larger on the DR350, which your calculations seem to show impacts the effective etendue enough that the reduced CDK14 is faster. The difference in transmission caused by the glass in the reducer on the CDK 14 should also be considered though. Not sure that's in the sheet or not.

*That's right Bill, the secondary is larger and the primary is slightly smaller. The difference is more than a Tak FSQ106 worth of objective.

The CDK and reducer have 2 + 2 = 4 glass elements. The DR350 has 3 elements so there is 1 more glass element in the CDK14 setup. That might equate to 2% light loss maybe?
Rouz Astro avatar
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Roman Pearah:
Bill,

Yep, but that f-ratio and pixel size all get absorbed into the equal pixel scale assumption here, making it a matter of aperture and transmittance. Like you, I expect that the original calculation, which already gives the CDK14 a merely modest speed advantage, is being generous to the CDK14's transmittance. I wouldn't be surprised if that turns things around.



Yeah, I'm aware of that. I'm not so sure the benefit of the 1.62 binning in post would be the same as putting larger native pixels on the system to begin with. But for a quick on paper analysis this is an interesting exercise.

*I would love to get these Sony IMX specs on a camera with 5 or 6 microns pixels but they don't exist (yet). 

The KAF sensors are no longer really available, the 4040 large pixels come with a lot of dark current so the option I was left with was resampling ~60%.
Rouz Astro avatar
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I approximated the following for losses:

Mirrors 96% reflectivety 
Each glass element 2% loss.
CDK14 + reducer = 4 glass elements
DR350 = 3 glass elements

Dark Matters Astrophotography avatar
Dark Matters Astrophotography avatar
Rouz Astro:
Focal ratio and pixel size are the two ways you can influence speed. You also need to factor in the central obstruction. The CO is much larger on the DR350, which your calculations seem to show impacts the effective etendue enough that the reduced CDK14 is faster. The difference in transmission caused by the glass in the reducer on the CDK 14 should also be considered though. Not sure that's in the sheet or not.

*That's right Bill, the secondary is larger and the primary is slightly smaller. The difference is more than a Tak FSQ106 worth of objective.

The CDK and reducer have 2 + 2 = 4 glass elements. The DR350 has 3 elements so there is 1 more glass element in the CDK14 setup. That might equate to 2% light loss maybe?



The CDK has the corrector lens plus the reducer. Is that the 4 you are counting?
Rouz Astro avatar
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The advantage of the CDk14 setup is that the native resolution is actually 0.46 "/pixel.
I have had very good nights in the Summer (no that often) where seeing goes below 2.0" and then you can really stretch the CDK14's legs at higher resolution.

Here, only minimal resampling might be enough. 

Here, seeing was better and the image could be processed at a higher scale of 0.5"/pixel:

https://www.astrobin.com/h97ni5/C/
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The CDK has the corrector lens plus the reducer. Is that the 4 you are counting?

*The CDK has 2 elements in the corrector assembly.

 As far as I know, the focal reducer has 2 elements as well (it could be a cemented pair and another lens) but that should act as 2 elements . The system ends up with 4 lens elements.
Rouz Astro avatar
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Roman Pearah:
Yeah, might be better to shoot with a larger pixel and Drizzle up the sampling to match given that it's not an integer.

*There really aren't many choices for large efficient pixels as of now.

Using the KAF at native with no reducer (it wont support the KAF16803). The Signal drops dramatically and the FOV is smaller:


Pixel Signal is what we are looking at.


Dark Matters Astrophotography avatar
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Rouz Astro:
Roman Pearah:
Yeah, might be better to shoot with a larger pixel and Drizzle up the sampling to match given that it's not an integer.

*There really aren't many choices for large efficient pixels as of now.

Using the KAF at native with no reducer (it wont support the KAF16803). The Signal drops dramatically and the FOV is smaller:


Pixel Signal is what we are looking at.





The documentation for the CDK14 reducer shows it will support the 16803. Am I missing something?
Skorpi79 avatar
Skorpi79 avatar
Hi Rouz,

A very interesting topic.
 
I also own a CDK 14 + Reducer.
I use a FLI ML KAF 16200 Mono Camera with 6µm.

But I found the point interesting when you mentioned the 6µm.
Well it is a CCD not a CMOS.

My last shot was M42 with this system.
75 x 60sec HA Astrodon 5nm.

I get with my camera + reducer to 0,72"/pixel.
I was also surprised that the CDK 14 + the reducer has an enormously gite imaging performance.
Due to the 6µm pixels, the system collects a lot of light.
Thus, I did not need a long exposure at all for this image and also for other objects.
Even though I have a CCD camera with a not so high QE.

Best regards
Matthias



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Rouz Astro avatar
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Rouz Astro:
Roman Pearah:
Yeah, might be better to shoot with a larger pixel and Drizzle up the sampling to match given that it's not an integer.

*There really aren't many choices for large efficient pixels as of now.

Using the KAF at native with no reducer (it wont support the KAF16803). The Signal drops dramatically and the FOV is smaller:


Pixel Signal is what we are looking at.


The documentation for the CDK14 reducer shows it will support the 16803. Am I missing something?


Bill,

Pixel peeping the corners at full res shows the star of deformed stars.
The vignetting is substantial at around 45% of the IMX455.

Furthermore, the short side of the IMX455 is where the OAG prism goes. The square sensor wont allow that light to be used for the guide cam.

Rouz Astro avatar
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I think that wishful thinking that the 0.66x reducer can work properly with the 52mm KAF sensor.

Besides offering a relatively les res. image scale of 1.1 "/pixel there is another major constraint.

the KAF16803 has a diagonal of 52mm - to get that with 0.66x of reduction means you need a native image circle of   ~ 52/0.66 = 79mm. That's larger than what the native CK14 can support and the CDK14 has one of the largest image circles rivaling the CDK24. The rest are mostly good to 50mm.

More importantly, the output baffle of the PW reducer is exactly 50.0mm. So there is no way it can support a 52mm sensor 44mm from that baffle has the light cone will only get smaller at the sensor.

I would confidently say the FF sensor is the largest practical size.
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Rouz Astro avatar
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I think that wishful thinking that the 0.66x reducer can work properly with the 52mm KAF sensor.

Besides offering a relatively les res. image scale of 1.1 "/pixel there is another major constraint.

the KAF16803 has a diagonal of 52mm - to get that with 0.66x of reduction means you need a native image circle of   ~ 52/0.66 = 79mm. That's larger than what the native CK14 can support and the CDK14 has one of the largest image circles rivaling the CDK24. The rest are mostly good to 50mm.

More importantly, the output baffle of the PW reducer is exactly 50.0mm. So there is no way it can support a 52mm sensor 44mm from that baffle has the light cone will only get smaller at the sensor.

I would confidently say the FF sensor is the largest practical size.
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