Effective focal length and best sampling factor

My Helios collegue, Walter, asked me about the effective focal length of my set up when imaging Saturn (august 22, 2023).

So here it is:

I uploaded my picture of  Saturn in WinJupos. Using the measurement - adjustment tab it's possible to obtain the diameter of the planetary disk in pixels (Di). The apparent diameter of the planet in arc seconds  (Dp) is provided by ephemerides tab. 

Sampling S = Dp/Di = 19/128.8 = 0.1475 arc seconds per pixel

The image sampling is equal to :

S = 206 P / F  (P = pixelsize of the camera and F = effective focal length in mm)

So F = 206 P / S = 206 3.75 / 0.1475 = 5237 or F/D = 5237/200 = 26.18 

Conclusion :

My set up with barlow x2, ADC and ASI224MC (3.75 micron) on TAL200K f/8.5 has an effective focal length of f/26.18. This means that my magnification is 26.18/8.5 = 3.05

Based on this f/26.18 and a Dawes resolution power (RP=1.02 wavelenght/diameter telescope) of 0.5838 it possible to measure the sampling factor k (k = RP / S).

k = RP / S = 0.5838 / 0.1475 = 3.95. 

Conclusion :

The Nyquist-Shannon theorem requires a k>2 but in practice a sampling factor of  3 to 3.5 is recommended according to Christian Viladrich - Solar Astronomy page 300 - Planetary Astronomy page 80). Increasing the sampling factor k beyond this would not bring a significant gain. Wirh my set up of 3.95 I will investigate how to reduce this to a value of 3 to 3.5. 

For information: the formule on page 81 of Planetary Astronomy should read F =206 Tp Di/Dp and not using factor 260.

Saturn reworked picture adjusting white balans

My previous reworked pictures of Saturn had some issues with colour setting. Again I reworked my pictures using AstroSurface U4 for stacking and sharpening and CS4 for adjusting white balans (RGB adjustment).

The result is more bright and the image has more colours. 

Saturn reworked

My picture of August 22 is reworked using Astrosurface U4. Below the picture I added all related information.

Software used: SharpCap 4, Astrosurface U4 and CS4

Saturn 5 days before opposition

Capturing Saturn 5 days befores opposition.

Setting: TAL200K f/8.5, ASI224MC, ADC, Barlow x2

Filters: IR/UV blocker

Software: SharpCap 4, Autostakkert3!, CS4, DeNoise AI

Seeing: Good 

Transparency : Good

Sun un H-alpha and AS3! setting

SolarMaxIII 70/400 f/5.7 DS BF15 with ASI290MM and reducer x0,5
Lights: 82fps 3000 frames (via SSD Samsung T7), flats 

Software: SharpCap 4, AS3! (10%), CS4

Setting in AS3!

Some nasty circles around the sundisk appeared when min brightness was set on "0". After adjusting to 15 the circles dissapeared.

Actual Frames per Second (FPS)

SSD Samsung T7 (2Tb)

I'm using a regular Hard Disk of my PC to capture my astro images. For the Sun and Planets my camera's typicaly goes up and beyond 80fps. But after capturing for a few seconds, the frames per second drop. I always thought the rate would keep up in the back but  I found out that this was not realy true. 

This morning I did a test when capturing the Sun with ASI290MM:

When using an external SSD (Samsung SSD T7) the actual frame rate = 82,1097 fps

When using my existing Hard Disk on my PC, I start with 82 fps but my actual rate drops to 27.0601fps.

Conclusion: be aware that capturing time is depending on your set up eg. type of hard disk, connection (eg. USB2, 3), ....

Sun in H-Alpha

Some images of the Sun in H-alpha from this morning.

Imaging was done with an external SSD resulting in an avg frames per second of 81.

Setting: 

SolarMaxIII 70/400 f/6 DS BF15 ASI290MM x0,5 reducer

Software : SharpCap 4, AstroSurface U4, CS4