Why I Can't Image the Whole Sun in my Solar Telescope?
MYSTERY OF THE MISSING SUN
Imaging the Sun has been a lot of fun with my Lunt 40mm Solar Telescope. With viewing the Sun with my eyepieces through this solar telescope, the Sun comes into full view. Sunspots, prominences, filaments, and more. But when it comes to imaging the Sun, I am missing a portion of our Star through my ZWO cameras. Where did it go?
My name is Erik and I am an amateur astrophotographer. In today's episode, let's talk about why is the Sun being cut off in my imaging camera, and what you can do to get it back.
WARNING: Before we go into this episode, as a reminder, never point a telescope or viewfinder at the Sun with a proper and undamaged solar filter! The damage to your eye will be instantaneous and likely irreversible.
STARLIGHT
Stars emit light across a range of wavelengths in the electromagnetic spectrum. Our Sun produces electromagnetic waves in the form of gamma rays, x-rays, ultraviolet, visible light, infrared light, microwaves, and radio waves.
Most humans perceive wavelengths in the visible spectrum through the eye, using special cells called photoreceptors, that turns light into electrical signals to the brain. Since the Sun is producing the entire range of light across the visible spectrum, the Sun appears white in the sky, despite it peaking at 500nm at the blue-green wavelength.
Solar telescopes, like the ones that Lunt offers, use specialized filters to allow only a very narrow band of wavelength through - the red Hydrogen Alpha at 656.28nm. It gives us a view in the solar chromosphere, where all the action is at - sunspots, filaments, prominences, flares, and more.
Looking through my Lunt Solar telescope offers some amazing views of our star through my eyepieces. Here is what is looks like through the 32mm. Plossl eyepiece using my Google Pixel 8 mobile phone. The image is challenging to capture with a cell phone, but it is an amazing site.
When I purchased my solar telescope, I purchased everything with the idea of portable and easy of use. I got the reliable Sky-Watcher SolarQuest Alt-Azimuth Solar Mount and the ZWO ASIAIR Mini Astronomy WiFi Smart Device to take images through my astrophotography cameras.
The first camera I used was the one I was using as a guide camera for my telescope - the ZWO 290 mini camera. And although it takes great images, the camera could not get the whole Sun on the sensor. The angular diameter of the Sun is approximately half a degree or 32 arcminutes. The field of view through this astrophotography camera is 0.8° x 0.46°, just short I what I need to capture the entire Sun.
So as a solution, I took two images of the Sun and stitched them together through Adobe Photoshop. Here is an example.
ZWO recently upgraded the 290MM camera to the 220MM. The 220MM has a larger sensor and pixels, higher quantum efficiency and lower readout noise. So I went ahead and purchased this camera for both my guidescope and Lunt Solar Telescope.
On October 14th, 2023, I used my new 220MM camera to image the annual eclipse of the Sun, which was a partial eclipse at my location. Between lots of clouds, my new camera did a pretty good job imaging the Sun that day.
But I noticed that the images were not the same quality as the 290MMcamera. The issue I discovered was that the camera is slightly under sampled for my solar telescope.
So what exactly is sampling?
Sampling is the process of converting an analog signal into discrete values that can be recorded on a memory chip. In digital photography, this is all being done through a photosensitive computer chip and the associated hardware and software.
A properly sampled image is sharp and is properly resolved.
An undersampled image is lacking information, or signal from the source. It generally looks too pixelated. The pixels on the chip cannot resolve the details of the image. Stars become blocky and lose their roundness. Here is an example I created with one of my images to look undersampled. Notice how the details of the Moon are not adequately captured. The edges are jagged into squares and rectangles.
An oversampled image is where the signal (light) is spread over more than one pixel. The pixel size on the imaging chip are small enough that the light or photons are spread over more than one pixel. As a result, it could take more processing time for the camera and possibly result in a soft image.
Having a properly sampled image is an important consideration when purchasing astrophotography cameras. Pairing the right size pixels of your camera with your telescope's focal length for the seeing conditions will result in proper sampling.
This subject is deep, and we could easily get lost in math formulas and charts. To make things easier, check out your cameras pixel size. Then use one of the available online calculators to see if you are going to get a good sampling size with your telescope.
Astronomy.tools has a calculator to help with this. Select CCD Suitability calculator and use the dropdown menus to match your telescope with your camera or the one you are planning on purchasing. It will tell you if your pixel size is adequate.
178MM Camera
So I decided I wanted a dedicated monochrome camera for my solar imaging. After checking astronomy.tools and the online reviews, I purchased the ZWO ASI 178MM monochrome camera. I did an unboxing video this Summer on this camera. This 6.4 megapixel image sensor has a large array of pixels at 3096 by 2080. This astrophotography camera a field of view at 1.06 degrees by 0.72 degrees, more than adequate for our Star that has an angular diameter of 0.5 degrees.
The pixels are 2.4um (microns) in size, perfect for the Sun and Moon. High quantum efficiency, fast frame rate, and back-illuminated.
With my camera loaded upon a sunny day in my Lunt Solar telescope, I began imaging the Sun by using the lucky imaging technique. Lucky imaging is where you use the video feature of your astrophotography camera to take a high number of short exposures so it minimizes the effect of Earths atmosphere. In post process stacking, only a small percentage of those exposures are stacking, proving a sharp, crisp image.
As I used my ZWO ASIAIR Mini Astronomy WiFi Smart Device through an old iPad, something was wrong. A portion of the Sun was clipped! No matter what I did, I could not get the entire Sun in my images.
HARDWARE LIMITATIONS
You may find that your solar images are being clipped as well. So let's review what could be the issue.
1. Is it the blocking filter? My Lunt 40mm Solar Telescope came with the B600 blocking filter. The filter is 6mm in diameter. Lunt recommends this blocking filter with telescopes with up to 400mm in focal length. They do recommend a larger blocking filter as some astrophotography cameras need it for back focus. Check out Lunt's website for more information.
2. Is it the astrophotography camera? By checking the specifications of my cameras by using astronomy.tools, my ZWO 290MM camera is too small to image the Sun, but my 220 and 178 are larger enough.
3. Is it a hardware issue? I have been using the ZWO ASIAIR Mini Astronomy WiFi Smart Device to take videos. The ZWO website says that their devices support VIDEO MODE, but I could not find any more details. Thanks to subscriber Bart Sadali and ZWO customer service, the entire line of ZWO ASIAIR Astronomy Smart devices are limited to 1080p mode. That means that the smart devices cannot use any pixels outside an area of 1920 x 1080. This means only 1/3rd of my camera chip is available.
So if you have the ZWO ASAIR Mini, 32GB Plus, 256GB Plus devices, we are limited to 1080p due to hardware limitations of the device. This is unfortunate and I hope that ZWO is able to update this device in the future.
DRAG OUT THE LAPTOP
If you want to take advantage of the full features of the astrophotography camera with a sensor larger than 1080p mode in video mode, you will need to use some other hardware and imaging software like ASIStudio, SharpCap, FireCapture, or N.I.N.A.
With my laptop, I finally was able to get a full image of the Sun with my ZWO 178MM camera and enjoy the details. I hope this episode as been helpful. If you haven't subscribed and your interested in learning more, be sure to smash the subscribe button and give me the thumbs up.
Also, if your like me, there are no telescopes stores my area, not even in my state. I purchase all my astronomy gear and solar viewing gear from High Point Scientific. They have a huge inventory of astrophotography cameras, solar telescopes, solar eclipse viewing glasses, and more. By using the High Point Scientific link below in the description, you can help this channel. Thank you for your support.
High Point Scientific
https://www.highpointscientific.com/?rfsn=7755489.66ed8e
I'll leave you some words from the famous philosopher Plato.. "Astronomy compels the soul to look upwards and leads us from this world to another". Take care.
Until next time, clear skies!
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