Ken Elliott

How to use the higher order co-efficients with PCSpectra

From: Kenneth Elliott
Sent: 10 August 2018 14:15
Subject: Re: 3rd order polynomials


Hi Andrew

This is how you use the higher order co-efficients with PCSpectra

Original (linear values entered into PCSpectra calibration file):

70.255, 0.6474

How to enter higher order co-efficients, x2, x3 and x4, into PCSpectra calibration file:

70.255, 0.6474, 0.001, 0.000000013, 0.00000000000121

Alternatively, scientific format can be used for co-efficients e.g.

7.0255E1, 6.474E-1, 1E-3, 1.3E-8, 1.21E-12

Pitfalls of using higher order co-efficients in PCSpectra:

There are two pitfalls

1: You need to have lots more lines if you use higher order fits and any fits outside the wavelength range you have got calibration lines may be wildly wrong as the equation has no constraints, so its fine with the Argon lamp.

2: you need lots of precision in the coefficients to get the accuracy, otherwise if it is multiplied by X**4 it could be wildly out.

How to improve accuracy/usefulness of higher order coefficients with PCSpectra:

Three different ways you could do this:

1. Astrosurf French site which has line idents and fitting programs, but I gave up trying to use it as, although the program runs in English, if there is an error then the error messages are in French.

2. Using STARLINK (UK) software – on Linux and needs a fair level of skill to get going.

3. The HST software both on Linux – on Linux and needs a fair level of skill to get going.


Why different image acquisition software programmes result in need to recalibrate CCDSPEC/QHY6

This follows on from Andrew’s post about needing to recalibrate his CCDSPEC/QHY6 when he changed image acquisition software from Nebulosity 4 to EZCAP:

Re-calibrating CCDSPEC/QHY6 combination for photos taken with EZCAP Image Acquisition software rather than Nebulosity software 4/8/2018

Regarding why recalibration is necessary when image acquisition software is changed:

Hi Andrew

It’s quite simple really.

Although the spectrum appears in exactly the same location on the chip, what comes out depends on how he data is clocked out and that depends on the software. Some chips have non sensitive pixels which are clicked out first and disregarded whilst other programs read out the whole chip which may be bigger than the imaging part.

So the pixel number in both X and Y can be different. Hence, it is essential to do the calibration and observations with the same readout program

Hope this helps.


PS I designed and built a CCD camera Electronics and software 30 years ago