I set up the scope to see how the filters worked. This is the Celestron 90GT that Santa brought us for Christmas in 2014. It looks a lot like the $189 model from Costco. So far so good. There is a filter on the main scope and one on the smaller spotter scope near the eyepiece.
First light through the new solar filter I put together. This somewhat unimpressive shot of our star was taken on my Samsung Galaxy S7 pressed against the eyepiece of the Celestron 90GT with my DIY filter on the objective end.
I’m still working on a few things here. First, the focus isn’t great because I need some shade out there so I can see what’s going on. Second, I’m still trying to figure out how to use the solar align feature of the NexStar controller, and the Sun keeps drifting out of frame while I mess around with the camera. Third, with this high magnification, the slightest breeze, camera movement, or hand shake blurs the image. I’ll try lowering the tripod so I can sit on a stool or chair for more stability. This is all stuff that can be worked out. No show stoppers.
I’m happy that the 20mm eyepiece makes the Sun’s image just about fill the view. It seems possible to observe surface details with this setup. I have a 4mm I’ll try, but that might be too much zoom for a steady photo. I’m considering a 30mm or 40mm to pull back a little bit more to gain some margin for alignment error and to see more corona during totality.
Testing with the extension tube and T ring on the DSLR remains.
I’m working out how to go from the standard setup (pictured above) to one with a camera attached. I hadn’t seen any specific “do it this way” descriptions for the 90GT. The silver tube is the focus tube. It moves in and out of the telescope to set focus. At the pictured distance, the 20mm eyepiece (also pictured) gives a sharp image to my eye. That image can also be photographed with a smartphone against the eyepiece. The next few photos will show the information that was unknown at the time I bought the extension tube and T ring for the camera.
This camera setup is probably the simplest, and likely best. Slide the diagonal mirror assembly from the back of the scope’s black focus tube end. You’ll be left with a T42 threaded end. Ignore the extension tube that came in the camera adapter kit and just screw the T ring onto the T42 threads. Then you can put the camera on the adapter’s bayonet end. This is the focus tube distance for this configuration.
This shows the camera mounted on the focus tube end. I used the camera’s “live view” to keep the mirror up and show the Sun on the display. I found a 1/50th of a second exposure at 640 ISO gave a pretty good image. The sun fills about half the vertical in the DX frame of my D7100. That’s about ideal. I need more experiments with exposure and ISO on the filtered Sun as well as the moon. The moon will help get the right exposure during totality. Since the 90 degree mirror is not in this setup, it’s kind of hard to see the display on the back of the camera when the scope is pointed up and the back of the camera is facing the ground. At higher latitudes, the angle won’t be as extreme. I might want to use an HDMI cable to run to an external monitor (analogous to the Baja setup). That’s going to need an extension cord or inverter depending on where we are.
Finally, the camera is heavy, and the tracking motors won’t hold it. I’m going to need to figure out how to put a counterweight on the opposite end of the telescope tube.
This is an unaltered shot from the first test of the D7100 on the back of the scope. 640 ISO and 1/50s
There’s still a focus issue due to the fact that I can’t see the screen very well outside in the daylight. I’m going to have to use some kind of hood or monitor. I also forgot to cover the viewfinder hole again. I bet the light leak from that messed up the contrast some. It could also be due to the cloud passing in front and other atmospheric haze.
Finally, my CCD is filthy! All this lens changing and stuff has really put a lot of junk on it. I see specks on the mirror and CCD when I peer inside the camera. Some of the specs could be on the scope lens too. Guh! I’ll work on cleaning that all up.
Here is an alternate configuration that uses the extension tube and the 90 degree diagonal. This makes it easier to see the back of the camera when the objective is pointing up. You start from the original setup and slide the eyepiece from the 90 degree diagonal assembly. Then you add the extension tube and T ring. Then you drop the eyepiece into the extension tube and tighten the screw to hold it in. Finally, attach the camera. Again, I’m showing the focus tube distance for focusing on the Sun. I put a bunch of marks on my tube so that I’d know some good places to start hunting focus. The setup has a hard time holding the heavy camera. The whole setup seems pretty strained and sloppy. I expect I’d need to add more weight to the objective side than with the focus tube end setup.
Here’s the eyepiece nestled in the extension tube. With the diagonal and the 20mm eyepiece, it gives about 3x more zoom at the camera CCD than with the “direct T ring on the focus tube end” setup. The sun’s disk is about 1.5x the vertical size of the DX frame on my D7100.
Here’s what the input side of the setup looks like. I found I had to angle the main filter so it wouldn’t block the finder scope. I kept getting close to lining up the Sun in the finder scope and then losing the Sun in the finder due to the shadow from the big filter. What is going on? Doh! That was a little frustrating.
Side note: When you are out working on this stuff, make sure you put on your sunscreen. No matter what you do, you’ll end up facing straight into the Sun for a long time!
OK, so I got tired of trying to squint into that little finder scope while getting fried by the Sun. I did figure out if you put a piece of white paper on the ground, and look for this image, you’ll be pointed right at the Sun.
So now I’m thinking of making a simpler finder scope with just an empty toilet paper tube that the Sun shines through when it’s lined up.
Wait for it….