VPhot with Seestar S50

Seestar S50 has already been mentioned several times as compatible with Vphot to perform photometry and in particular to be able to work directly with the FITS in the three bands (TR, TG and TB). However, it would already be appropriate to have the Seestar in the list of telescopes. However, it is not there and I do not know what to enter for certain mandatory values. Could you help me ?

I’m talking about the Gain and the linearity limit because the rest of the parameters, except perhaps the Filter Band Coefficients, are understandable.

For some variable stars, I find it important to be able to use VPhot to verify estimates that I am not certain.

Arnaud:

Actually, it is not appropriate to have a common telescope set up in VPhot for the Seestar S50. Each telescope in VPhot has a specific location (Lat/Long) and this impacts airmass calculation. More importantly, each observer ‘may’ use a different Gain, which BTW is in units of e/ADU NOT decibels

So you should select/measure your chosen gain and your linearity limit. Then you can set up your ‘private’ telescope in VPhot.

Ken

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Thank you very much Ken. I knew very well that it was not appropriate to have a common set up in VPhot for the Seestar S50. I wanted to create my own profile to be able to use VPhot in the rules of the art. In reality, I just had a problem to enter the gain which is common to all observers in the case of the Seestar because locked as a factory setting, unless you use open source software allowing you to intervene on the gain, which is not my case, being satisfied with the proprietary software of ZWO.

I therefore managed to create my own telescope profile and submit to VPhot, the variable star whose magnitude I intended to check because I found it strange when comparing it to other observers. I discovered on this occasion oh how great this online photometry tool is! I did not even have to read the documentation, finding the functions instantly it is so easy. I was thus able to make a comparison of the variable with 18 comparison stars with the consideration of the airmass and deduce an estimate without possible risk of error. And thus validate the accuracy of the value declared in the light curve of this variable (0.2 difference with a fast photometry with ASTAP).

Very friendly and happy.

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I am also hoping to use the Seestar, but need help in setting up the information. How did you find the values to put in for the Seestar?

Hi astron7 !

To create a profile for the Seestar, it’s quite simple!

You need to determine your latitude and longitude exactly in decimals but also your elevation with Google Earth. This allows you to finely enter the criteria of your observation site for the calculation of the airmass which depends on the elevation of the variable star in the sky.

The Seestar S50 constantly operates with a gain of 80% (we find this value in the header of the FITS produced by a Seestar). Using a value of 1.6 e/ADU is very suitable.

The Seestar S50 produces FITS in 16 bits.
The saturation pixels occurs at 65536.
I therefore set the linearity limit to 65000.

The plate scale corresponds to the sampling.
It is calculated from the size pixel of the CMOS sensor and the diameter optic (50mm).
For the Seestar S50, it is 2.39"/pixel.

If you explore the FITS header, you will also see that the day and time data automatically filled in by the Seestar is “Mid-point of exposure”.

There you have it, you have everything to start using Vphot for your photometric measurements with a Seestar S50.

For the CMOS acquisition of a variable star, I use the recommendation of an experienced variabilist who uses the Seestar S50. That is 3 min integration : 18 x 10 sec. This allows for good signal averaging and calibration while avoiding the effects of star flickering.

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Thank you! You explained it clearly and in terms that a beginner can understand. My club has arranged for an online training session in a couple of weeks, so this information will help me prepare for the session!

Arnaud,

The specification for the IMX462 processor states that the bit depth is 12.
ZWO cameras to my knowledge show a count of ~65536 at saturation, whether they have 12 or 16 bit ADC.

Roy

Hi arx !

FITS exists in 16 or 32 bits. The one mainly used in astronomy is 16 bits. It does not exist in 12 bits.

You are absolutely right !

A lot of the ZWO, QHYCCD 12-bit CMOS cameras “modify” the data by shifting it over 4 bits (aka multiply x 16), so bright stars that are actually 4095 count are represented as 65535. This is why you will see gaps in brightness steps. Other cameras like the ones at Diffraction (where in full disclosure I work) and certain other vendors give you the actual unmodified count, so you would see 4095 maximum ADU count.
Saturation happens usually at a lower count level - and is specific to the sensor - and may vary from sensor to sensor. It is common to see saturation around 3600/4095 ADU with some CMOS APS sensors; on 16-bit ones, we often see it around 56,000/65535 ADU.
Don’t confuse the maximum possible ADU value with the point of saturation; worse, some sensors or cameras become non-linear past a certain point, which in the old days could be around 85% of the maximum count. Individual cameras have to be tested for this.