8 April 2024

Cacalotan - Mexico

Local circumstances

Location : North +23° 04' 03.87", West 105° 50' 31.24"

Duration of totality : 4mn 14s

Conditions of observation

Equipement :

ZWO FF 80/600 refractor (continuous check of focus up to about 5 min before totality).
Nikon Z7II, 14-bit RAW mode (non destructive compression), electronic shutter mode. Pixel size = 4.35 micron. Image scale = 1.5 arsec/pixel

Exposure times for totality : 1s to 1/250 s (9 EV bracketing at 1 EV interval, 9 frames taken in burst mode at 3 s interval) 

Mount : iOptron HAE29EC.

Seeing conditions : cirrus of variable thickness. 

Estimation of the resolution :
Star FWHM on RAW images = 2.8 pixels (= 4.2 arsec).

Processing of the image of totality

1) Pre-processing
- flat/offset correction,
- hot pixels correction (home-made Julia code).

2) Registration of the image on solar corona
- registration of images on solar corona using zdeta Psc (mag 5.2) position (measured to 0.1 pixel) and correction of differential motion Sun/stars (based on ephemerids at local observation site).

3) HDR processing
- home made Julia software,
- calculation done in floating 64-bit, output fit file saved in 32-bit integer,
- image weight function = sinus function (except for 1 s and 1/250 s exposures),
- sensor response function is assumed to be linear.

4) Display of HDR image

The basic idea is :

     image_visu = HDR_image / [gaussian (HDR_image, sigma) + constant]

- a large value sigma increases the visibility of long streamers and large scale structures away from the solar center (and decreases apparent noise in the resulting image),
- a small value of sigma is more appropriate to increase the constrast of small scale structures near the solar limb (and also reduces Gibb effet near the solar limb),
- the constant increases/lowers the effect of the gaussian mask.

The color images displayed here are the result of using a variable value of sigma (from 10 pixels to 30 pixels) as the distance r from the solar limb increases.

Mid-exposure = 18:11:31 UT
9 x 9 exposures from 1/250 s to 1s (at 1 EV interval), total exposure =  18 s

Version 1

Link to full-resolution image
Stars are visible to  mag 11.4

Version 2

Link to full-resolution image

Motions in the solar corona

Blink of two HDR stacks taken on 18:08:37 UT and 18:11:45 UT (i.e. about 3 min apart)

Click on image for zoom 50% animation (7 Mo)

Click here for zoom 100% animation (35 Mo)

Notes :
- 1h and 5 h directions : dark voids going downward,,
- 11h, 9 h and 5 h directions : expanding streamers,
- 9 h direction : expanding bubble.

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