Camera pixel size in relationship with sampling

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Camera pixel size in relationship with sampling

Post by Sol seeker uk »

Hi all, it's that Sampling topic again, following on from barnstorm post on the player one 432..
I decided as I've got a interest in purchasing a Apollo that I'd plug the details into astromany tools cc'd and see what I came up with.. seems like I'd have to purchase the max version with its 9um pixels to keep it within oversampling criteria..
As you see it's with a 2x, 2.5 and a 3x
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Re: Camera pixel size in relationship with sampling

Post by Sol seeker uk »

So as that's with a 1000mm FL scope how do people with extreme focal lengths keep under the oversampling threshold?


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Re: Camera pixel size in relationship with sampling

Post by Dennis »

Sol seeker uk wrote: Sun Jul 24, 2022 11:42 am So as that's with a 1000mm FL scope how do people with extreme focal lengths keep under the oversampling threshold?
How to keep under the oversampling threshold? Why so complicated.. i just use the cam at focal ratios of f/30+ and all is fine (now even pointed out on the player one website).
Even full disk images look ok to me from this cam despite the theoretical heavy undersampling in this case.

Also this is a lucky imaging cam, so it will likely pick up seeing better then your average forecast (if used as intended). So I wouldnt focus too much on the seeing aspect. Give your local seeing a chance to be better then you expect it to be :D .


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Re: Camera pixel size in relationship with sampling

Post by Sol seeker uk »

I've always felt on a planetary imaging aspect you need to be bordering the over sampling line.. lucking imaging and deepsky imaging are totally different
On deepsky I'm happy enough undersampling according to astronomy tools ccd


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Re: Camera pixel size in relationship with sampling

Post by Dennis »

yes.. with deepsky imaging you have a given average seeing that you cant bypass. With short exposure imaging its different.

if you take a closer look to how seeing behaves over time it will be something like in the image below:

you can see that from time to time it changes rapidly. With a lucky imaging cam you can take out the small good moments so your seeing actively increases to the lowest (best) values you see on the graph. Many people seem to get such a cam and image with a low framerate because they dont understand the concept of imaging with max fps, even if it means to use gain.
This gets more important the higher the resolution of your scope and the more you do closeup imaging of fine details.
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Re: Camera pixel size in relationship with sampling

Post by Sol seeker uk »

So according to Wjdrijfhout chart I should be using a x2 Barlow for my 100mm aperture and 1000mm FL setup for Ha.. and if I got the 9um player max camera then maybe just under a x2, Maybe a 1.6 and hope for better conditions than 1 arc sec of seeing..
I'm not going to start reducing aperture since I just bought a 110mm D-ERF


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Re: Camera pixel size in relationship with sampling

Post by Dennis »

Sol seeker uk wrote: Sun Jul 24, 2022 2:39 pm So according to Wjdrijfhout chart I should be using a x2 Barlow for my 100mm aperture and 1000mm FL setup for Ha.. and if I got the 9um player max camera then maybe just under a x2, Maybe a 1.6 and hope for better conditions than 1 arc sec of seeing..
I'm not going to start reducing aperture since I just bought a 110mm D-ERF

What chart is this? Do you have a link?
Its recommended to not use the apollo max below a focal ratio of f30. If the seeing is good enough f/40 is even better to not undersample. The cam is best suited for closeup work of moon, solar, planets.

Regards


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Re: Camera pixel size in relationship with sampling

Post by Sol seeker uk »

Sorry Dennis I thought I'd attached the image
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Re: Camera pixel size in relationship with sampling

Post by Dennis »

Thanks,
i find this a bit counterintuitive, no idea how to read this : D

But it doesnt sound right to be in an optimum at f/20 or even less with this pixelsize. The ratio of "resulting f-number of the setup / pixelsize" should be at 2.8 minimum for h-alpha. This would be F/25 as lower limit.

Also in H-alpha imaging the barlow or focal lenght of the setup is usually first dependent on the needs of the etalon or the needs of the magnification. What kind of etalon do you use?


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Re: Camera pixel size in relationship with sampling

Post by marktownley »

In simple terms, at Ha wavelengths those 9um pixels need f40.

This is regardless of aperture.

If you are at 100mm in Ha, your local seeing needs to be floating just below 1.5" to be capitalising with lucky imaging.


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Re: Camera pixel size in relationship with sampling

Post by Sol seeker uk »

marktownley wrote: Sun Jul 24, 2022 7:25 pm In simple terms, at Ha wavelengths those 9um pixels need f40.

This is regardless of aperture.

If you are at 100mm in Ha, your local seeing needs to be floating just below 1.5" to be capitalising with lucky imaging.
So is there a method or way of measuring this... Or care to recommend a pixel size?


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Re: Camera pixel size in relationship with sampling

Post by Sol seeker uk »

Dennis wrote: Sun Jul 24, 2022 6:41 pm Thanks,
i find this a bit counterintuitive, no idea how to read this : D

But it doesnt sound right to be in an optimum at f/20 or even less with this pixelsize. The ratio of "resulting f-number of the setup / pixelsize" should be at 2.8 minimum for h-alpha. This would be F/25 as lower limit.

Also in H-alpha imaging the barlow or focal lenght of the setup is usually first dependent on the needs of the etalon or the needs of the magnification. What kind of etalon do you use?
It's a PST mod with a Tal 100rs


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Re: Camera pixel size in relationship with sampling

Post by marktownley »

If you're using a PST mod on the Tal (i've used and got this setup) there are a number of ways of getting optimal pixel size - just depends on the ultimate f-ratio of the system.

Have seen people using a zwo290mm camera straight on the back of it - those small pixels zoom right in on the detail in the sweetspot without the need for a barlow. You could do the same with a zwo183 or zwo178 with ROI but with less frame rate. Fast frame rate is desirable though!

I've used 3,45um pixel cameras with a 1.5x barlow to get the same image scale as the 290mm.

Have also used the IMX174 chipped camera with 2.5x barlow to reach the same scale in the sweetspot.

You could use the 9um camera, but you want to be using a 3 or more likely 4x barlow.

Remember though, the more glass you have in the system the less the light throughput and the longer the exposure time.

Do you have a camera already you can use?

Mark


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Re: Camera pixel size in relationship with sampling

Post by Sol seeker uk »

I have the qhy version of the 290.. so happy days


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Re: Camera pixel size in relationship with sampling

Post by marktownley »

Sol seeker uk wrote: Mon Jul 25, 2022 3:33 pm I have the qhy version of the 290.. so happy days
Just use that 🙂

If the sweet spot is apparent you can always use a Barlow nose piece on the end of the camera to up the magnification slightly.


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Re: Camera pixel size in relationship with sampling

Post by Sol seeker uk »

Thanks again mark


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Re: Camera pixel size in relationship with sampling

Post by Sol seeker uk »

This f ratio/ sampling thing still confuses me
I have a friend who uses a lunt 152, usually with a 2.5 powermate and a 290... He gets fantastic results so I'm sure the system works
So for me the confusion is he's using it at a f ratio of 14.80, and sampling at .27 so he's undersampling and under the preferred f30
If he used a X5 powermate to get this preferred f30 then he'd be sampling at .13... So how can he ever get to the f30 band and the correct sampling? As one conflicts the other


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Re: Camera pixel size in relationship with sampling

Post by Dennis »

Sol seeker uk wrote: Sun Jul 31, 2022 9:03 am This f ratio/ sampling thing still confuses me
I have a friend who uses a lunt 152, usually with a 2.5 powermate and a 290... He gets fantastic results so I'm sure the system works
So for me the confusion is he's using it at a f ratio of 14.80, and sampling at .27 so he's undersampling and under the preferred f30
If he used a X5 powermate to get this preferred f30 then he'd be sampling at .13... So how can he ever get to the f30 band and the correct sampling? As one conflicts the other
A lunt 152 is a different system then a pst mod. A rear mounted etalon like the quark is again another system that has different f-ratio needs.
Maybe read up on the very basics of h-alpha systems to know what f-ratio is ideal for your system. Only after that comes the camera question.


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Re: Camera pixel size in relationship with sampling

Post by marktownley »

Sol seeker uk wrote: Sun Jul 31, 2022 9:03 am This f ratio/ sampling thing still confuses me
I have a friend who uses a lunt 152, usually with a 2.5 powermate and a 290... He gets fantastic results so I'm sure the system works
So for me the confusion is he's using it at a f ratio of 14.80, and sampling at .27 so he's undersampling and under the preferred f30
If he used a X5 powermate to get this preferred f30 then he'd be sampling at .13... So how can he ever get to the f30 band and the correct sampling? As one conflicts the other
Why is he going to image at f30 in Ha with a 290mm - that would definitely be oversampling.

Where are you getting this 'preferred' f30 from? And what is it relating to?


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Re: Camera pixel size in relationship with sampling

Post by Sol seeker uk »

[quote=marktownley

Where are you getting this 'preferred' f30 from? And what is it relating to?
[/quote]

Sorry, so the optimal f30/ F40 refers to the quark system?


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Re: Camera pixel size in relationship with sampling

Post by marktownley »

Sol seeker uk wrote: Sat Aug 06, 2022 6:45 am Sorry, so the optimal f30/ F40 refers to the quark system?
Yeah


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