Can you reach the diffraction limit on a 10 meter telescope using lucky imaging and no adaptive optics?

[kingsbishop]

Hello I was wondering can you reach the diffraction limit on any telescope by using the lucky imaging technique?

 

I would think if the seeing is really bad just use a shorter exposure time so say if you had a giant telescope and your trying to image Jupiter and you can do 0.000003 milliseconds exposure that is so short that can you reach the diffraction limit on even say a 10 meter telescope in poor seeing because 0.000003 milliseconds exposure is so short that it would freeze the atmosphere?

[CraigT82]

Post deleted - miss read the question 🙄

Edited November 24, 2022 by CraigT82

[vlaiv]

2 hours ago, kingsbishop said:

Hello I was wondering can you reach the diffraction limit on any telescope by using the lucky imaging technique?

Yes

2 hours ago, kingsbishop said:

I would think if the seeing is really bad just use a shorter exposure time so say if you had a giant telescope and your trying to image Jupiter and you can do 0.000003 milliseconds exposure that is so short that can you reach the diffraction limit on even say a 10 meter telescope in poor seeing because 0.000003 milliseconds exposure is so short that it would freeze the atmosphere?

No

There are number of things that need to be done for successful lucky imaging run.

Freezing the seeing is one of those, and it depends on behavior of atmosphere rather than on size of the optics. Atmosphere is in motion, and it is the speed of this motion that determines how short exposure time you need.

Large aperture will in fact have some advantage over smaller aperture in terms of coherence time in some cases.

If you have 10 meter aperture and 30cm aperture and bad layer that more or less uniformly moves in atmosphere - in time it completely changes "aberration content" over 30cm of aperture it will only change 3% of wavefront over 10m of aperture.

In any case - even when you've frozen the seeing - it is not going to present you with clear image. It is still distorted image on several different levels and those need to be corrected in software.

You need to correct for tilt component with use of alignment points when stacking and you need to stack to average out rest of wavefront errors so you get nice symmetric blur that is close to gaussian in nature so that you can sharpen image nicely.

You also need good SNR to be able to sharpen.

Using shorter exposures will only be counterproductive. Although you have big aperture - if you are imaging at diffraction limit (critical / optimum sampling for given aperture) - you will get same level of signal per pixel in 8" scope as well as 10 meter one, regardless of pixel size.

This is because everything is "matched" - F/ratio will be determined by pixel size so pixel size is no longer a factor, and with fixed F/ratio (and pixel size) - any two scopes will have same "speed" in terms of light gathering.

Using shorter exposure will just give you less SNR per sub and lower total SNR means that you can't sharpen as aggressively as might be needed.

Ultimately, for best results, even with lucky imaging, one needs moments of good seeing. If that does not happen - results won't be as good as possible.

[michael.h.f.wilkinson]

An astronomer I know and I discussed the option of imaging Jupiter with the Gran Telescopio de Canarias (10.4 m, and he had observing time on it), with a very fast multi-band imaging device (HiPERCAM). He had observing time available, and all we asked for was 5 minutes, but the powers that be decided against it. The problem is that even with very short exposure times, freezing the seeing is not straightforward on such a big instrument, according to the technicians of the observatory, as you are requiring a much larger part of the sky to stay put. We were eager to try it in any case, but ultimately didn't get the green light.

[vlaiv]

6 minutes ago, michael.h.f.wilkinson said:

We were eager to try it in any case, but ultimately didn't get the green light.

I hope you get the chance.

I think that results would be very interesting to see.

If you ever get the chance to do it - maybe do couple of runs with different exposure times / focal lengths (with / without a barlow and changing of sensor / barlow element distance). With 10m of aperture, I guess one can afford to under sample somewhat if that will reduce exposure time needed to go sub millisecond - just in case seeing behaves differently on such large apertures (although from what I read on adaptive optics for large telescopes - it behaves pretty much the same).

[michael.h.f.wilkinson]

2 hours ago, vlaiv said:

I hope you get the chance.

I think that results would be very interesting to see.

If you ever get the chance to do it - maybe do couple of runs with different exposure times / focal lengths (with / without a barlow and changing of sensor / barlow element distance). With 10m of aperture, I guess one can afford to under sample somewhat if that will reduce exposure time needed to go sub millisecond - just in case seeing behaves differently on such large apertures (although from what I read on adaptive optics for large telescopes - it behaves pretty much the same).

I don't think a regular Barlow would fit in HiPERCAM

[vlaiv]

17 minutes ago, michael.h.f.wilkinson said:

I don't think a regular Barlow would fit in HiPERCAM

What are you going to use then to capture data?

I'm not convinced that device usually attached will provide necessary capability (very fast readout of small ROI + low read noise?).

[michael.h.f.wilkinson]

11 minutes ago, vlaiv said:

What are you going to use then to capture data?

I'm not convinced that device usually attached will provide necessary capability (very fast readout of small ROI + low read noise?).

It actually is capable of fast imaging, and huge data throughput.

[vlaiv]

36 minutes ago, michael.h.f.wilkinson said:

It actually is capable of fast imaging, and huge data throughput.

Indeed - I just found some specs for HiPERCAM and it indeed allows for ~1000Hz sampling - which is of course much better than regular amateur planetary cameras.

Only issue that I can see is a bit higher read noise than wanted at ~4.5e, but given that sampling rate is 0.081"/px versus critical for up to 400nm and 10.4m aperture is ~0.003967"/px - so about half of that, it might not impact results as much (can be viewed as camera that has 2.25e of read noise at critical sampling rate).

There is only one thing that I can't decipher - and that is relation of readout speed in khz to frame rate. Does it depend on ROI?

It says that above 4.5e is for 263khz readout rate, so I'm guessing that would be 263000 A/D conversions per second, right? With ~200fps (or should we go with 263fps for simplicity) - that is only 1000A/D conversions per frame - and that is 33x33 pixels?

That can't be right?

[michael.h.f.wilkinson]

18 minutes ago, vlaiv said:

Indeed - I just found some specs for HiPERCAM and it indeed allows for ~1000Hz sampling - which is of course much better than regular amateur planetary cameras.

Only issue that I can see is a bit higher read noise than wanted at ~4.5e, but given that sampling rate is 0.081"/px versus critical for up to 400nm and 10.4m aperture is ~0.003967"/px - so about half of that, it might not impact results as much (can be viewed as camera that has 2.25e of read noise at critical sampling rate).

There is only one thing that I can't decipher - and that is relation of readout speed in khz to frame rate. Does it depend on ROI?

It says that above 4.5e is for 263khz readout rate, so I'm guessing that would be 263000 A/D conversions per second, right? With ~200fps (or should we go with 263fps for simplicity) - that is only 1000A/D conversions per frame - and that is 33x33 pixels?

That can't be right?

I can ask the people at the observatory

[vlaiv]

33 minutes ago, michael.h.f.wilkinson said:

I can ask the people at the observatory

I think that it is in fact right. 263Khz is rather slow readout.

https://www.aavso.org/what-download-time-ccd-readout