Skyris 618C vs 236C, worth it ?

[MarsG76]

Hello Astroexperts,

Currently I use the Skyris 618C for my planetary imaging... and I'm getting some great results when the seeing is optimal.

I noticed the 236C and even though it has smaller pixels, it captures 9 times the frame resolution... so I'm wondering if anyone knows if upgrading to the 236C would allow me to capture and tease out more detail on the same OTA, namely the 8" SCT. Or if ultimately there would be no (or diminishing) difference??

Thanks,

MG

 

[newbie alert]

I use a 236m on a 8 inch sct, it's a ok camera but always felt it's not an ideal match , maybe the pixels are too small for the focal length..it's great on the moon.. it's mainly used for guiding on my deep sky scope, a bit of planetary (although they're far too low atm) and some solar work..so it gets used as a allrounder 

[MarsG76]

So according to you, would you say that the 618C pixel size (5.6um)  is better suited for a f10 8" SCT? 

[newbie alert]

3 hours ago, MarsG76 said:

So according to you, would you say that the 618C pixel size (5.6um)  is better suited for a f10 8" SCT? 

I'm no pixel expert but seen a few great images done with a zwo 224 with 3.75um pixels 

[JamesF]

Are you using a barlow/extension/anything else in the optical train, or just the C8 at it's native focal length?

James

[MarsG76]

6 hours ago, JamesF said:

Are you using a barlow/extension/anything else in the optical train, or just the C8 at it's native focal length?

James

I use a 2X and 3X barlows and if seeing permits, a 5X powermate.

90% of the time I'll image at around 6000mm focal length.

 

[JamesF]

With your 3x barlow and 6m focal length, I'd suggest the 618C is a fairly good match.  I think you're probably getting as much as you can in terms of resolution at that point.  Moving to a camera with smaller pixels might get you a bigger image, but I suspect you'd not really get any more detail.

What it would allow you to do however would be to reduce the focal length you work at without a loss of resolution, so you could for example drop down to the 2x barlow (or even 1.5x if you could find one) with the 236C.  That may or may not be something you find helpful.

Of course there are other issues such as sensitivity to consider as well, so it's not quite that simple :)

Were I in a similar position and considering an upgrade, I'd probably start from the other end and check out more sensitive cameras in my budget, then look at what sort of focal length/ratio would get the best out of them.  A reasonable rule of thumb for planetary imaging is to start with the focal ratio of the optical system at around five, perhaps six, times the camera pixel size in microns and work from there.

James

[newbie alert]

On 27/06/2019 at 20:59, JamesF said:

 

Were I in a similar position and considering an upgrade, I'd probably start from the other end and check out more sensitive cameras in my budget, then look at what sort of focal length/ratio would get the best out of them.  A reasonable rule of thumb for planetary imaging is to start with the focal ratio of the optical system at around five, perhaps six, times the camera pixel size in microns and work from there.

James

Hi James,  isn't your f ratio going to be at f10 native for a sct? 

So in my own setup at f10 x 2.8um, so does that mean I should be imaging at 2800mm?

[JamesF]

Given a camera with 2.8um pixels, I'd suggest that you should be aiming for a focal ratio of perhaps f/15 to f/18.  Because the focal length of an SCT varies depending on where the image plane is, actually if you have just a barlow and camera in the visual back, the focal length is probably a little under the quoted 2032mm meaning a 2x barlow should be in the right sort of ballpark for the f-ratio of the entire optical train.

With your existing 5.6um pixel size and aiming for f/28 to f/34 then a 3x barlow probably isn't a bad place to start either.

These aren't hard and fast rules though, just a place to start.  And remember that if you need to bump up the barlow multiplier a little then it's often possible to add a spacer between the barlow and camera to achieve that.

James

Edited July 3, 2019 by JamesF

[MarsG76]

10 hours ago, JamesF said:

Given a camera with 2.8um pixels, I'd suggest that you should be aiming for a focal ratio of perhaps f/15 to f/18.  Because the focal length of an SCT varies depending on where the image plane is, actually if you have just a barlow and camera in the visual back, the focal length is probably a little under the quoted 2032mm meaning a 2x barlow should be in the right sort of ballpark for the f-ratio of the entire optical train.

With your existing 5.6um pixel size and aiming for f/28 to f/34 then a 3x barlow probably isn't a bad place to start either.

These aren't hard and fast rules though, just a place to start.  And remember that if you need to bump up the barlow multiplier a little then it's often possible to add a spacer between the barlow and camera to achieve that.

James

Thats the principle I'm ready operating under... I'm just curious whether the 236C would help to capture more detail over the 618C or whether there would be no real world difference.

 

[JamesF]

Just now, MarsG76 said:

Thats the principle I'm ready operating under... I'm just curious whether the 236C would help to capture more detail over the 618C or whether there would be no real world difference.

The "rule of thumb" for the f-ratio is based on matching the camera pixel size to the limit of resolution of the OTA.  Once you've reached that far there's no more detail to be had without moving to a larger aperture telescope.  If you could move to a camera with smaller pixels that was also more sensitive, perhaps you could improve on what you have by being able to capture more frames faster and at a focal ratio that's easier to work with, giving you a greater choice of frames to stack and a higher chance of some of those frames catching moments of very good seeing, but that's probably more difficult to quantify.

I can't find any graphs for the 236C to compare against other cameras, but if that's the way you want to go then I suspect there are better candidates out there for less money these days.  Perhaps the ASI224MC, for instance.

James

[vlaiv]

If we are talking about planetary imaging, things are straight forward, you need to consider following:

1. Critical sampling rate, depends on pixel size and is best considered in terms of F/ratio needed (larger scope - more detail, needs longer FL for given pixel size - max details for given pixel size will always match certain F/ratio, regardless of aperture).

There are couple things here worth noting, critical sampling rate is tied to "display" sampling rate - or to rephrase it - to display all detail there is to be seen under perfect circumstances you need certain sampling rate of the image. In general, recording sampling rate does not need to be that one - it can be higher (less arc seconds per pixel), and in some cases it is preferable to do so - it reduces pixel blur. Notably for OSC imaging you need to consider each color sampling rate rather than pixel size (red and blue sample at twice lower sampling rate than pixel size would suggest - as they are spaced at every second pixel in bayer matrix).

Often quoted figure - x2 or x3 pixels per airy disk is not correct. Actual figure is around x2.4 per airy disk. So proper critical sampling formula is F/ratio = (2.4 *  pixel_size) / (lambda * 1.22)

(where lambda is taken to be around 500nm, but if you want to be "pedantic" about it - go with 400nm for start of capture spectrum, pixel size is in um and wavelength should be also - so use 0.5um or 0.4um instead of 500nm and 400nm).

If you have OSC camera - you should double F/ratio as discussed above.

2. QE of sensor - this one is obvious - go with higher QE sensor

3. Read noise - you want the lowest read noise as you are stacking bunch of frames, each having very little signal (short exposure) and read noise is high enough to be of a concern.

For actual question - Skyris 618C vs Skyris 236C

Both are color? Right? That means double F/ratio.

In first case with pixel size of 5.6um, F/ratio that you should be using is - F/44 - with C8 that means barlow/powermate with x4.4 - so sensible thing for max details is to use x5 one.

In second case, pixel size is 2.8, F/ratio that you should be using is - F/22 - so barlow / powermate should be x2.2 - so sensible option to use is x2 one.

I can't find specs on QE and read noise for those cameras - but one is certain - 236C will have lower read noise given that it is CMOS sensor while 618C is CCD sensor (as far as I can tell).

However, given that Celestron is giving neither read noise nor QE for this sensor, and that I've found following statement on QHY website (this needs to be verified from other sources):

Quote

Compared to the IMX236 sensor, for example, the IMX290 has twice sensitivity for visible light and three or more times the sensitivity in near infrared.   

when comparing two sensors with fairly similar pixel size - 2.8um and 2.9um, I would actually consider skipping Skyris 236C and going for different product - one with high QE (known) and low read noise (again known).

Above statement is actually true and Sony is the source of it in their specs PDF for IMX290:

https://www.sony-semicon.co.jp/products_en/IS/sensor0/img/product/cmos/IMX290_291LQR_Flyer.pdf

If you want that pixel size - look into ASI290 (or camera with the same sensor from another manufacturer) - if you are considering mono version, or like 2.9um pixels, or even better, take a look at ASI385 - that one is color only, with pixel size of 3.75um so you would need to be at F/30, so x3 barlow/powermate matches it well and although QE is not available - according to sensitivity published by Sony after taking into account pixel size difference - it appears to be 8% more sensitive than 290 sensor. 385 sensor has ~0.7e read noise at high gain setting - I think it is currently the lowest read noise in CMOS sensors available for astro imaging (lower even than 224 sensor).

 

[JamesF]

2 minutes ago, vlaiv said:

Often quoted figure - x2 or x3 pixels per airy disk is not correct. Actual figure is around x2.4 per airy disk. So proper critical sampling formula is F/ratio = (2.4 *  pixel_size) / (lambda * 1.22)

How are these numbers (2.4 and 1.22) derived, vlaiv?  The formula you give isn't one I've seen before, so I'm interested to see where it comes from.  I'm happy with the argument that the sampling rate should be doubled for red and blue given that most sensors have alternating rows with red and blue photosites, but I can't see where the other numbers might come from.

I've always quoted the "five to six times pixel size in um" figure because it seemed to agree with my own calculations, but I'm quite prepared to be wrong.  I just need to know why :D

James

[vlaiv]

2 minutes ago, JamesF said:

How are these numbers (2.4 and 1.22) derived, vlaiv?  The formula you give isn't one I've seen before, so I'm interested to see where it comes from.  I'm happy with the argument that the sampling rate should be doubled for red and blue given that most sensors have alternating rows with red and blue photosites, but I can't see where the other numbers might come from.

I've always quoted the "five to six times pixel size in um" figure because it seemed to agree with my own calculations, but I'm quite prepared to be wrong.  I just need to know why

James

1.22 is related to airy disk size - so it comes from first minima in airy pattern

See wiki article on airy disk size https://en.wikipedia.org/wiki/Airy_disk

Second number is "empirical" so to speak. It is measured value from simulations. It has firm mathematical footing - Nyquist sampling theorem. Problem with it is that most people don't quite understand Nyquist sampling theorem - it speaks about band limited signal and sampling rate being twice max frequency component of such signal. That means that you can't "tie" x2 to anything in spatial domain - it needs to be related to max frequency which you can either derive from analytic form of airy disk function by taking Fourier transform of it to get representation in frequency domain - at the time I briefly looked at it and it seemed over my head to do so, or do "simulation" and measure.

Thing is - there are three "concepts" that are related to each other by FT, and those are aperture, airy pattern, MTF. Or Modulation Transfer Function is Fourier transform of Airy pattern and Airy pattern is Fourier transform of aperture.

When you know this, then it is simple matter of generating circular function that represents aperture (0 where light does not pass and 1 where light passes - simple white circle if you will) of certain diameter and doing FFT of that to produce airy pattern of needed size (you set pixel scale to be something like 1" or 0.1" for easy calculation / measurement), and then you take FFT of that airy pattern and observe where MTF hits 0. ImageJ has very nice tool that analyses FFT and gives you info about cursor position in px per cycle - which you can convert to arc seconds depending on your chosen scale for simulation.

I did number of different simulations (different airy disk sizes) and I always get number around 2.4. Sure, rigorous mathematical analysis - doing FT of Airy pattern function and finding where it hits 0 would be certainly more precise, but I think that it will be close enough to 2.4 in value, so 2.4 is good for most purposes in this case.

I've written about this before, let me find that thread for you.

(note I made mistake at first - one that I've made couple of times, that is difference between airy disk radius and diameter, 4.8 pixels per diameter, or 2.4 pixels per radius).

[JamesF]

Thank you for that.  I shall read with interest, but perhaps not quite so close to bedtime

Nyquist looks as though it's the place I went wrong   I was always aware that I was near the limit of my understanding there.

When I did my calculations I worked from the starting point of setting the Rayleigh limit to the camera pixel size, then followed my nose through the maths to see where it would take me.  I think that is effectively the same as using the size of the airy disc.

I did consider the 290MC when I mentioned the 224MC above, but I've read that the sensitivity is not great for the colour version.  I've no idea how true that is.  I agree that the 385MC looks like it could be good too though.

James

[MarsG76]

4 hours ago, vlaiv said:

If we are talking about planetary imaging, things are straight forward, you need to consider following:

1. Critical sampling rate, depends on pixel size and is best considered in terms of F/ratio needed (larger scope - more detail, needs longer FL for given pixel size - max details for given pixel size will always match certain F/ratio, regardless of aperture).

There are couple things here worth noting, critical sampling rate is tied to "display" sampling rate - or to rephrase it - to display all detail there is to be seen under perfect circumstances you need certain sampling rate of the image. In general, recording sampling rate does not need to be that one - it can be higher (less arc seconds per pixel), and in some cases it is preferable to do so - it reduces pixel blur. Notably for OSC imaging you need to consider each color sampling rate rather than pixel size (red and blue sample at twice lower sampling rate than pixel size would suggest - as they are spaced at every second pixel in bayer matrix).

Often quoted figure - x2 or x3 pixels per airy disk is not correct. Actual figure is around x2.4 per airy disk. So proper critical sampling formula is F/ratio = (2.4 *  pixel_size) / (lambda * 1.22)

(where lambda is taken to be around 500nm, but if you want to be "pedantic" about it - go with 400nm for start of capture spectrum, pixel size is in um and wavelength should be also - so use 0.5um or 0.4um instead of 500nm and 400nm).

If you have OSC camera - you should double F/ratio as discussed above.

2. QE of sensor - this one is obvious - go with higher QE sensor

3. Read noise - you want the lowest read noise as you are stacking bunch of frames, each having very little signal (short exposure) and read noise is high enough to be of a concern.

For actual question - Skyris 618C vs Skyris 236C

Both are color? Right? That means double F/ratio.

In first case with pixel size of 5.6um, F/ratio that you should be using is - F/44 - with C8 that means barlow/powermate with x4.4 - so sensible thing for max details is to use x5 one.

In second case, pixel size is 2.8, F/ratio that you should be using is - F/22 - so barlow / powermate should be x2.2 - so sensible option to use is x2 one.

I can't find specs on QE and read noise for those cameras - but one is certain - 236C will have lower read noise given that it is CMOS sensor while 618C is CCD sensor (as far as I can tell).

However, given that Celestron is giving neither read noise nor QE for this sensor, and that I've found following statement on QHY website (this needs to be verified from other sources):

when comparing two sensors with fairly similar pixel size - 2.8um and 2.9um, I would actually consider skipping Skyris 236C and going for different product - one with high QE (known) and low read noise (again known).

Above statement is actually true and Sony is the source of it in their specs PDF for IMX290:

https://www.sony-semicon.co.jp/products_en/IS/sensor0/img/product/cmos/IMX290_291LQR_Flyer.pdf

If you want that pixel size - look into ASI290 (or camera with the same sensor from another manufacturer) - if you are considering mono version, or like 2.9um pixels, or even better, take a look at ASI385 - that one is color only, with pixel size of 3.75um so you would need to be at F/30, so x3 barlow/powermate matches it well and although QE is not available - according to sensitivity published by Sony after taking into account pixel size difference - it appears to be 8% more sensitive than 290 sensor. 385 sensor has ~0.7e read noise at high gain setting - I think it is currently the lowest read noise in CMOS sensors available for astro imaging (lower even than 224 sensor).

 

Thank you for this... this is great help... and you have explained a lot to me..

I wasn't necessarily looking at upgrading my planetary imaging camera unless there would be a chance of considerable amount of improvement.. and judging by the formula and info you have written, I doubt that there would be much improvement in getting a smaller pixel or larger chip camera for planetary when planets already fit into the frame and I'm matching the pixel size to the scopes detail limiting factors (plus seeing)...

I see that the only way to substantially improve the capture of planetary detail is to increase my aperture.

 

[vlaiv]

6 hours ago, MarsG76 said:

Thank you for this... this is great help... and you have explained a lot to me..

I wasn't necessarily looking at upgrading my planetary imaging camera unless there would be a chance of considerable amount of improvement.. and judging by the formula and info you have written, I doubt that there would be much improvement in getting a smaller pixel or larger chip camera for planetary when planets already fit into the frame and I'm matching the pixel size to the scopes detail limiting factors (plus seeing)...

I see that the only way to substantially improve the capture of planetary detail is to increase my aperture.

 

Judging by your signature, you already have the scope suitable - 14" Goto Dob, why don't you give it a try at planetary imaging, you might be surprised by results

[MarsG76]

4 hours ago, vlaiv said:

Judging by your signature, you already have the scope suitable - 14" Goto Dob, why don't you give it a try at planetary imaging, you might be surprised by results

Believe it or not, I never got any results with the dob that were better or even matched the SCT!!?!! I suspect that it's focus related and  if I added a electric focused to it than I'll hit focus and that situation might change.

[vlaiv]

15 minutes ago, MarsG76 said:

Believe it or not, I never got any results with the dob that were better or even matched the SCT!!?!! I suspect that it's focus related and  if I added a electric focused to it than I'll hit focus and that situation might change.

Could be focus issues, but also collimation and cool down issues as well. I think it's worth trying though. In any case, you can use 14" Dob to produce same resolution image as with C8, but using shorter exposure time and getting higher SNR (less frames needed to be stacked - you can be picky about quality of frames and include only the best) - just match focal lengths and you'll get the same scale.

Best tip for focusing is to do it on something that is point like - if you are shooting Jupiter for example - concentrate on one of its moons - and try to get the smallest "footprint" of it in frames - much easier than trying to focus on planet features when seeing is not perfect.

[MarsG76]

10 hours ago, vlaiv said:

Could be focus issues, but also collimation and cool down issues as well. I think it's worth trying though. In any case, you can use 14" Dob to produce same resolution image as with C8, but using shorter exposure time and getting higher SNR (less frames needed to be stacked - you can be picky about quality of frames and include only the best) - just match focal lengths and you'll get the same scale.

Best tip for focusing is to do it on something that is point like - if you are shooting Jupiter for example - concentrate on one of its moons - and try to get the smallest "footprint" of it in frames - much easier than trying to focus on planet features when seeing is not perfect.

I found that focusing on the moons is not the answer... when features are in focus than the moons are slightly out, and vice versa... I find that increasing the gamma or contrast until the darkest features are obvious and focusing on them is more accurate... much easier with a electronic focuser.