Help - Full Sun Disk

[Jonathan Rees]

Help! 

With the eclipse approaching,  I am trying to get a full sun disk with my Celestron NexImage Solar System Imager through my Celestron NexStar 5se telescope.  I am a teacher and can't take off what will be the first day of school.  However, I can live stream the eclipse for the school. The problem is, my camera only gives me a view of (roughly) 25-ish % of the sun. 

I have a focal reducer (.5), though it is a cheap one. I tried using it with the camera, but I couldn't tell a difference. Do I need an eyepiece extension? Whatever I do, I need to do as cheap as possible--makes for a happier marriage.  Incidentally, I am very much considering upgrading to the Meade Instruments 07545 LX f/6.3 Focal Reducer and Field Flattener, but I'm not sure if it will help if I'm not using it correctly (I'm not even sure a focal reducer will help at all to make the sun smaller).

Technical stuff you might need to know:
Camera: 1280x720 resolution, 3.0 micron square pixel size, sensor size is 3.86mm x 2.18 mm

Telescope: Schmidt-Cassegrain, 125 mm aperture, apparent field of view 1 inch, focal length is 1250 mm

I appreciate any help you can offer! I'm fairly tech savy, but pretty much every place I've visited speaks in a technical vernacular that's above my experience. 

[ronin]

Sun is half a degree, so with 1250mm focal length that is D = Tan(0.5) * 1250 = 11mm image of the Sun = bit too big for the sensor size of 3.86mm x 2.18 mm.

With a 0.5 reducer that simply means 625mm focal length so = 5.5mm diameter image, again too big for the sensor.

You need a bigger sensor or smaller focal length scope. All values are a bit approxomate, just easier then try to talk down to 4dp. Which is pointless.

For 2.18mm the focal length needs to be less then about 250mm, actually 250mm focal length gives a sun diameter = 2.18mm so need lesser still.

Focal length like this are rare/difficult so really a bigger sensor.

25% of the sun covers 2.7mm so I guess you are getting about that in the long edge and about 20% of the sun on the short edge. 20% (0.1 deg) in the scope is 2.18mm. So what you are seeing is about right.

 

[Jonathan Rees]

How big of a sensor? I was wondering if there was exponential growth as sensors go up. I'm looking at a camera that is 5.7mm x 4.28mm. Perhaps you can provide me the equation i need.

[Peter Drew]

I would have thought that a DSLR with a telephoto lens would be a better bet, common enough to borrow one?  

[ronin]

The equation is fairly simple geometry. The image size formed is the Tangent of the anlge that the sun subtends , 0.5 degree, multiplied by the focal length of the scope, 1250mm unreduced in this case.

So image size D = Tan(0.5)*1250 = (0.0087)*1250 = 10.908 = 11mm as close as it matters.

Since the 0.5 reducer halves the focal length to 625 that results in 5.5mm diameter image.

Technically it is a close approximation but the difference will be vanishingly small. Should be 2*Tan(Sun angle/2), only say that as someone may come along and point it out.

Part of the problem will be how much of the sun and the surround do you want to capture to display as a live feed. I would suggest double the sun so that is 1 degree. So to get 1 degree that is tan(1) * 1250 = 22mm size sensor or with the 0.5 reducer 11mm sensor. Both are quite large.

Problem is that webcam sensors are small, to keep the cost low, and the scope you have has a long focal length.

I am not aware of the chip sizes used on the various easily available webcams. Even then with the scope focal length I still think you will have a bit of a problem.

[Davey-T]

Can you borrow a different scope camera combo ? you only need something like  a SW ST80  and a bigger sensor camera, icx 274 sort of thing which will fit in the Sun with some sky around it which is what you need.

Unfortunately your small sensor plus long focal length are both conspiring against you.

Any local astro clubs in your area to lend you something suitable ?

As Peter said a DSLR and telephoto lens would be better suited you can display the live view feed on a PC screen

Don't forget you'll need a solar filter on the front of whatever you use.

Dave

[Davey-T]

If you click on the FLO logo at the top of the screen and select, help and support then astronomy tools then FOV calculator you can stick in various combos to see how much Sun fits in.

Here's a 5SE plus  DSLR with and without a 6.3 focal reducer.

HTH

Dave

[Jonathan Rees]

Thanks, everyone!

One last question. I bought a Canon Rebel T6 DSLR. I have also purchased the required telescope mount.

Will I still need a focal reducer? The Rebel's sensor size seems to be 22.3 x 14.9 mm. 

[Jonathan Rees]

9 minutes ago, Jonathan Rees said:

Thanks, everyone!

One last question. I bought a Canon Rebel T6 DSLR. I have also purchased the required telescope mount.

Will I still need a focal reducer? The Rebel's sensor size seems to be 22.3 x 14.9 mm. 

I may have answered my own question based on Bright Giant's equation. The short answer is (I believe): it wouldn't hurt, but it might be close. 

[Jonathan Rees]

22 hours ago, Davey-T said:

If you click on the FLO logo at the top of the screen and select, help and support then astronomy tools then FOV calculator you can stick in various combos to see how much Sun fits in.

Here's a 5SE plus  DSLR with and without a 6.3 focal reducer.

HTH

Dave

Very cool! Thanks for the info.