did i get the correct camera for my telescope?

zerodan

Hi! After having a solar telescope for a few years now, I finally decided to get a camera. My goal is to learn the basics of solar photography quickly enough to get a decent photo of the Mercury transit next month. After reading several reviews, I decided to get the ZWO ASI 174MM. But I'm really new to this and I have what is probably a stupid question. I'm wondering if you can help. I have a Lunt LS100THa with the B1800 blocking filter. I'm just learning FireCapture but I captured my first images this weekend. However, even with the ROI set to the max (1936 x 768) I can barely get the entire sun in view. See here for example:

https://i.imgur.com/la2kgkx.png

Did I get the wrong camera? Or do I have the wrong blocking filter? Or am I using FireCapture wrong? Or is this the expected bahavior and everything's fine?

I suspect that I could capture the entire sun plus plenty of extra space around it (in case of large prominences) by simply letting it pass through the field of view while I'm capturing but I don't know if that's the most correct way to approach this.

Thanks in advance for your help.

riklaunim

In general camera sensors are way smaller than average eyepiece field of view. It's common that Sun doesn't fit on one frame. You would have to make mosaics to get a whole disc.

russm

You can figure the field of view of a telescope and camera combination by the following method. You need two things - the focal length of the telescope and the linear dimensions of the camera's sensor. I looked up the focal length of the Lunt LS100THa as 700mm. The ASI174MM camera has a sensor size of 11.3 x 7.1 mm. I'll demonstrate with the specifics for your setup.

You must calculate the angular field of view separately for each sensor dimension. Since the vertical dimension is smallest, that will be limiting as to whether the Sun will fit in. Here's the formula which gives approximate field of view:

Angular Field of View = Sensor Dimension ÷ Focal Length X 57.3°

Note: 57.3° = 360° ÷ (2 pi) = 1 radian

So applying the formula to your case we have:

Angular Field of View (vertical) = 7.1 mm ÷ 700 mm X 57.3°

Angular Field of View (vertical) = 0.581°

According to Wikipedia, the Sun's angular diameter varies from 0.525° to 0.543°, depending on Earth-Sun distance.

So as you have seen, the Sun is a tight fit within your field of view. You didn't get the wrong camera, but you'll just have to be careful to center the Sun's disc. Be watchful if it drifts off-center. You don't have a lot of wiggle room. One positive thing is that you've put nearly the maximum number of pixels under the Sun's image.

zerodan

Russ, thanks for showing me the steps to figure out the correct field of view. You are right - the Sun does fit in the view. I just need to make sure to have accurate tracking on my mount so that it doesn't drift much. I appreciate your help.

Dan