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Sensitvity of Lodestar mono vs Lodestar X2 mono


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Hello everyone,

This is my first post on this forum, although I have been asking EAA questions on CN for a little while. As I've noticed a lot of users of SX Lodestars/Lodestar X2s post on this forum, I thought I would ask the following: Do your experiences with the mono versions of these cameras confirm the comment on the SX website that the ICX829 sensor in the X2 "approximately doubles the Lodestar sensitivity.." versus the original Lodestar ICX429? Has anyone done a direct comparison of the two mono cameras to see how deep each will reach under the same conditions? SX suggests that for guding, the X2 will reach stars about a magnitude fainter, and I'm curious if this is the case for EAA. Looking forward to your comments.

Errol

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Thanks for the link.  Actually,  I saw the  comparison by Lorenzo Comolli sometime  ago, and wondered if anyone else had come to the conclusion that the two cameras had similar sensitivities, or reached a different conclusion. So far I haven't spotted any other test comparisons, and so I am curious if anyone has bothered to compare, perhaps after upgrading to an X2 from the original Lodestar. I should say the reason I am interested in the question is that if both cameras are of similar sensitivity,  I might be comfortable acquiring either one in the used market. If the newer X2 is noticeably more sensitive, then I would probably have a preference for the X2. I'm not worried about the guider port problem that I believe affected some of the original Lodestars as I would use the camera only for EAA.

Errol

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I have a X2 now and had the original Lodestar years ago but never used it for EAA. They are both excellent cameras. But even if they did have similar QEs (which they don't) the X2 will acquire the image faster (i.e. achieve better dynamic range faster) as it has half the read noise of the Lodestar. Which means you can use shorter subs.

Ultimately you can compensate for differences in both QE and read noise with longer sub exposures. But in my view the X2 has the fastest acquisition of any camera I have used (exception being the ASI224 but that is a color camera so not a direct comparison)

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Thanks for reminding me about the differences in read noise, Hiten. I see that the X2 typically has 6e read noise vs 10e for the original Lodestar, so I get your point about faster image acquisition for the X2. On the issue of QE, I see ads showing peak QE of 77% vs 65% @620nm for the X2 vs the original Lodestar, and 45% vs 35% @400nm. The relative QE difference doesn't seem that big, and as you say you can compensate for differences in both read noise and QE by going to  longer exposures. Would it be reasonable to make the following approximation (ignoring all sources of noise except read noise): If an x-second exposure of some object with the X2 is required to reach a certain S/N ratio after subtracting skyglow, then roughly a 2x second exposure would be need with the original Lodestar to get the same S/N ratio ? (The factor of 2 roughly corresponding to the 5/3 ratio in read noise and the improved QE of the X2 relative to the original Lodestar.)

Errol

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Kind of but not quite. Read noise is pretty much the same whether you do a 15s or a 60s exposure. So as long as your exposure is long enough to overcome the read noise and capture the dynamic range you are aiming for you will be ok. Once you overcome the read noise then the exposure time depends on the QE of the sensor (although the two are connected as a higher QE generates signal faster)

But you are right that to capture a certain dynamic range you will generally require longer exposures with a camera that has higher read noise.

As an example with my ASI224 (which has <1e read noise) I can use 5s or even 1s sub exposures and capture almost any dynamic range (even the faintest details) as long as I stack enough subs. The total exposure time in that case is determined by the QE of the sensor. But for higher read noise sensors I will need longer sub exposures and/or significantly longer total exposure.

Edited by Astrojedi
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I think I follow, but perhaps an example might help me a bit. Just to keep it simple (ignoring all noise except read noise): Let's say a 2-second sub is just long enough to capture a faint image detail  with a signal that is just above the 6e  read noise  of the X2. For the original Lodestar,  the signal corresponding to this detail would have to  overcome  10e of read noise and if the signal is just over 6e it would still be buried in the read noise. Assuming both detectors had the same QE, and the signal builds linearly with time, the original Lodestar would need roughly 3-4 sec (= 10/6 * 2 sec) to capture the detail.

So maybe my statement should be that for an image detail right at the threshold of detection, the original Lodestar would require  a sub about twice as long as the X2 for the detail to be detected? For image details that are much brighter (say 3 times or more), both cameras would detect them at the shorter 2 sec hypothetical exposure, as a brighter detail would have a signal above the Lodestar's 10e read noise. So for any given single sub exposure length, the X2 can pull out fainter details than the original Lodestar - corresponding to signals in the 6e to 10e range. (This would seem to be just me restating your comment on dynamic range and camera read noise).

Hope I am not wasting too much time on this, but this is helping me straighten this out in my mind. Hopefully, I am closer to getting the picture right. Thanks.

Errol

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...that the ICX829 sensor in the X2 "approximately doubles the Lodestar sensitivity.." 

The claim for x2 the efficiency has little to do with camera noise, but is taken directly from Sony sales spin :-) The ICX829 has an extended range into infrared, which is useful for night security applications, so the "full-range" sensitivity is indeed x2. But the peak sensitivity is still at most 75% at green wavelengths, compared to the peak sensitivity of the ICX825 of ~55%. Converting to a log scale, there is then little difference between the perceived efficiencies of the two. 

The real advantage for visible light is the higher dynamic range (due to lower noise) which should allow you to see fainter objects. Dynamic Range in Decibels DR= 20log (Fullwell capacity/Readout noise). The interlaced lodestar-X1 image with its higher noise is less good, especially if you have a model with old firmware.

The bottom line is that if you have a lodestar-X1 it's not really worth upgrading to a lodestar-X2, but certainly buy the X2 if you have neither. And go for the Ultrastar if you want to upgrade, eventually adjusting your focal length and binning if you want more signal at the expense of field-of-view.

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The real advantage for visible light is the higher dynamic range (due to lower noise) which should allow you to see fainter objects. Dynamic Range in Decibels DR= 20log (Fullwell capacity/Readout noise). The interlaced lodestar-X1 image with its higher noise is less good, especially if you have a model with old firmware.

The bottom line is that if you have a lodestar-X1 it's not really worth upgrading to a lodestar-X2, but certainly buy the X2 if you have neither. And go for the Ultrastar if you want to upgrade, eventually adjusting your focal length and binning if you want more signal at the expense of field-of-view.

Thank you very much for the advice. Also, interested to hear your comment on the old firmware of the original Lodestar. I had not followed this carefully.

Errol

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  • 2 weeks later...
On January 10, 2016 at 03:00, Hibou said:

The claim for x2 the efficiency has little to do with camera noise, but is taken directly from Sony sales spin :-) The ICX829 has an extended range into infrared, which is useful for night security applications, so the "full-range" sensitivity is indeed x2. But the peak sensitivity is still at most 75% at green wavelengths, compared to the peak sensitivity of the ICX825 of ~55%. Converting to a log scale, there is then little difference between the perceived efficiencies of the two. 

The real advantage for visible light is the higher dynamic range (due to lower noise) which should allow you to see fainter objects. Dynamic Range in Decibels DR= 20log (Fullwell capacity/Readout noise). The interlaced lodestar-X1 image with its higher noise is less good, especially if you have a model with old firmware.

The bottom line is that if you have a lodestar-X1 it's not really worth upgrading to a lodestar-X2, but certainly buy the X2 if you have neither. And go for the Ultrastar if you want to upgrade, eventually adjusting your focal length and binning if you want more signal at the expense of field-of-view.

I completely disagree. It depends what your requirements are.

Sensitivity is a combination of a number of factors.

The LX2 has nearly half the read noise which in theory itself significantly increases the 'sensitivity' all else being equal.

Here is the highly simplified equation for SNR to emphasize relationship between SNR and read noise.

SNR = S / sqrt(S+B+D+Read Noise^2 x n)

Let's assume background and dark current are zero... We get 

SNR = S / sqrt (S + Read Noise^2 x n)

Note S will also increase with n (number of sub exposures) but more slowly for the lodestar due to the lower QE

Hence the lower read noise allows you capture a certain dynamic range much faster even with the same S. In theory to achieve the same dynamic range with the Lodestar as the X2 you will require much longer sub exposure time to overcome the read noise. This holds for visual and non-visual wavelengths.

The lower QE of the Lodestar across the spectrum further reduces 'S' which also slows down the image acquisition (for 2 reasons - takes longer to capture enough S to overcome read noise and second takes longer to achieve an image of certain 'brightness'. 

In summary with the Lodestar with sufficient exposure time you can achieve the same results as the X2 but dynamic range achieved (i.e. faint detail captured) for the same sub exposure time will be lower (due to lower S)

Not sure how firmware comes into play here. Can you explain more.

In my view X2 is a worthwhile upgrade if you value speed in EAA.

Edited by Astrojedi
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4 hours ago, Astrojedi said:

I completely disagree...

Hence the lower read noise allows you capture a certain dynamic range much faster even with the same S

Hmm. I'm not quite sure what you "completely disagree" with. I wrote "The real advantage for visible light is the higher dynamic range (due to lower noise)" with which you appear to agree "even with the same S".  I repeat, if you already have the Lodestar-X1 and want to upgrade, buy the Ultrastar and skip the Lodestar-X2. Is that what you don't agree with ? If so, you're wrong :-)

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5 hours ago, Astrojedi said:

Not sure how firmware comes into play here. Can you explain more.

The first Lodestars were especially noisy and some had what appeared to be hot columns. A firmware upgrade at the beginning of 2014 largely cured that, but Terry sent it to me directly and I can't see it on the SX site. I think the problem resulted from the necessary compromise between readout speed and noise. If you bought a lodestar from a distributor in the US in 2014 (who might have had older stock), then I begin to understand why we don't agree... completely :-)

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That was a readout timing problem in the firmware. I also had to apply a patch to my Ultrastar. This is more akin to fixing a bug and not relevant to this discussion.

You cannot just change the fundamental characteristics (QE, readout noise) of the camera by a firmware update. The former is determined by the sensor design and the latter by the camera circuit design and architecture choices i.e. if a camera has readout noise in the range of 10-15e depending on read mode you cannot just change the range to say 6-10e by a firmware update.

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The Lodestar X2 is fantastic with the innovation foresight on axis guider which uses near infra red emitted from most stars to guide. This overcomes much of the scintillations of the stars so you get much better guiding according to the blurb from the OEM. I should get my C11 back shortly so I will be able to test this theory. At normal wavelengths i.e. normal guiding the X2 is superbly sensitive-I have never struggled to find a guide star- but I have no comparison experience with earlier models.

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If you haven't already done suggest you check the Sony site for these sensors and see what they say.  From my long experience with both cameras the doubling of sensitivity is reasonable but think it may include better sensitivity to near-IR which is fine for me with my reflector [SCT] but may be inhibited when used with IR block filters on refractors.  Just a thought.

Nyte

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On Friday, January 29, 2016 at 06:50, nytecam said:

If you haven't already done suggest you check the Sony site for these sensors and see what they say.  From my long experience with both cameras the doubling of sensitivity is reasonable but think it may include better sensitivity to near-IR which is fine for me with my reflector [SCT] but may be inhibited when used with IR block filters on refractors.  Just a thought.

Nyte

Thanks for the direct comparison, Nyte.

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