Live View Focusing
Our astrophotography question this month comes from Marlin Sandlin. Like many of us, Marlin is starting out in astrophotography by learning to shoot the biggest and brightest object in out night sky. This is a great way to start, since as a strarting point, it is fairly easy to get very acceptable pictures, even with a smartphone camera. To get really good high resolution lunar images take though requires different techniques, and perhaps different equipment. Here is Marlin’s question:
The adjustment on the Nikon viewfinder will be difficult to get right to allow you to focus the telescope by eye. The camera and lens system is designed to do the critical focus and the viewfinder adjustment is only so you can see well enough to frame your daylight photo. Most cameras have these adjustments and use clickstops. Because of this you will not be able to set the viewfinder focus in between the stops, and the viewfinder image is not big enough to assess critical focus.
Your best bet is to use the camera’s live view function. It will display what the camera sees on the LCD screen, and should allow you to zoom in on the image.
Move the telescope to a bright star and center it. Using live view, zoom all the way in on the star. Focus the telescope until the star is the smallest and brightest by going one direction just past focus, then back again. Now that the telescope is focused, move back to the moon. Set your ISO low about 400, and set the shutter speed so the image is properly exposed or better yet about 2/3 stops underexposed. This should work better for you.
Before you jump in and spend money on new equipment, I would encourage you to get the most out of your current set up that you can. The experience you gain will transfer directly to a new equipment set up. Jumping up to hi-res lunar and planetary imaging requires a big aperture OTA which also means a bigger mount that can easily carry the weight of the OTA and track accurately.
Here are a few tips to tune up the equipment you have, and to up the quality of your resulting images.
1. Make sure your OTA is well collimated. If it is not, no matter how hard you try, your focus will never be as sharp as it should be. Here is a good resource to help with your C5 https://starizona.com/tutorial/collimating-a-schmidt-cassegrain/
2. Add a Bhatinov Mask in front of the OTA to improve your focus using a focus star. It will make this process even more accurate and repeatable.
3. Slightly underexpose your image. As you already have discovered, you want to use a lower ISO setting and slightly under expose so you don't max out the expousure level of the very bright areas. This obliterates any detail in the bright areas. Look at the histogram of any image you take and make sure that there is not a spike on the right side of it. You can adjust the image brightness later in post processing, but if there are any over exposed areas, the detail there cannot be recovered.
4. Consider trying "lucky imaging" by shooting video rather than single frames. The atmosphere is not static and this changes the focus of your target.This instability is what causes stars to tinkle and is generically called seeing.
By shooting video at a high frame rate, you will catch a few frames that are perfectly focused out of a short video clip. You will need to acquire some software to help you automatically grade the focus of each individual video frame and align and stack the best ones to cut down the shot noise. Fortunately there are two freeware programs that work really well. Both also have tools to help you sharpen and adjust your image to make it the best it can possibly be. They are:
a) Registax https://www.astronomie.be/registax/
b) Autostakkert https://www.autostakkert.com/
Marlin - I hope this helps. Keep on imaging!