Focal reducer

[stevil]

Hello,

About a week ago I attempted to do some solar imaging using my Celestron Nexstar 127 and a SPC900NC (modded spc800nc) Long Exposure Mode I picked up from eBay.

At first I thought I had been sold a dud webcam however now think I was looking at a closeup of the sun and believe from what I have read on the forum that I should be looking at a focal reducer.

Is this right?

Also, out of interest could someone tell me where in the great scheme of things a focal reducer goes? e.g. next to the webcam or in between the star diagonal and the tube or somewhere else?

Thanks very much

Steve

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[Stargazer33]

Hi Steve,

I bought the Celestron 2" focal reducer for my C8 late last year. It is threaded and so goes onto the image back of the scope. My focuser then screws into the back of the reducer.

HTH

[stevil]

Thanks

Steve

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[lensman57]

Hello,

About a week ago I attempted to do some solar imaging using my Celestron Nexstar 127 and a SPC900NC (modded spc800nc) Long Exposure Mode I picked up from eBay.

At first I thought I had been sold a dud webcam however now think I was looking at a closeup of the sun and believe from what I have read on the forum that I should be looking at a focal reducer.

Is this right?

Also, out of interest could someone tell me where in the great scheme of things a focal reducer goes? e.g. next to the webcam or in between the star diagonal and the tube or somewhere else?

Thanks very much

Steve

Sent from my Nexus 7 using Tapatalk 2

Hi Steve and welcome, first and most importantly, never ever attempt to view or image the sun without taking safety precautions first, and making sure that you have a proper Solar Filter securely fixed on the front of the scope and I mean securely. Secondly a reducer designed for a Celestron such as their f6.3 goes on the back ot he scope and then the diagonal or the camera., a two element 0.5 X reducer such as the ones you can buy from the eBay screws onto the nose piece of the SPC900. I have put the focal length of your scope and the chip size of the SPC900 in the CCDCALC and if I am correct even with a 0.5X reducer the Sun would not completley fit into the chip, the focal length is just too long and you certainly would not need the LX mode of SPC with that scope . I think that you need about 350mm ~ 400 mm focal length max to get the Sun on to the chip.

Regards,

A.G

[JamesF]

I don't think you'll find a focal reducer helps much, to be honest. The image of the Sun is just too big at the native focal length of the 127. You might managed to reduce the size of some sunspots to a manageable size, but I think that's about the best you could hope for.

Do you have a DSLR at all? That would probably be a better choice for solar imaging with this scope than the tiny sensor of the SPC900. (I actually use a Canon 450D and 127 Mak for solar imaging and the Sun almost fills the frame.)

James

[stevil]

Thanks for the welcome A.G.

I have been using an Astrozap filter so my eyes were preserved but thanks for the very valid warning.

I'm sorry for being a bit of a thickie (I am new), does that mean you need 2 reducers? Or should I just try a DSLR instead as JamesF suggests as it looks like I won't be getting anywhere with my current webcam for solar imaging? (I would have to buy one as I only have a bridge camera).

Cheers

Steve

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[ollypenrice]

Only the up market fast frame cameras have big enough chips for the sun other than at very short focal lengths. However, you could use the webcam to capture high res images of interesting active regions (sunspot groups etc.)

Olly

[stevil]

Thanks Olly. That's good to know.

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[JamesF]

I'm sorry for being a bit of a thickie (I am new), does that mean you need 2 reducers? Or should I just try a DSLR instead as JamesF suggests as it looks like I won't be getting anywhere with my current webcam for solar imaging? (I would have to buy one as I only have a bridge camera).

It's probably cheaper to buy a DSLR than two reducers, and to be honest I'm really not sure that would work anyhow.

The problem you have is that the dimensions of the SPC900 sensor are about 3.5mm x 2.5mm. With a telescope of 1500mm focal length such as your 127 the diameter of the image of the Sun, without any space around it, is about 14mm. So to fit the entire Sun on your SPC sensor you'd have to reduce the effective focal length of the scope by a factor of about 7. Hmmm. Isn't the focal length of the Skywatcher 50mm finder about 200mm?

Obviously you don't need to get the entire Sun in a single frame. You can take multiple frames of different areas and create a mosaic for the final image. Even if you halved the effective focal length of the scope however you'd still need perhaps sixteen or twenty-five panels to get a good overlap and a decent image. That's going to be quite tricky and time-consuming to do, especially without an application that will control the mount to create the mosaic panels.

The easy answer then is to use a camera with a much larger sensor. That's really what I meant when I suggested a DSLR I guess. It doesn't have to be a DSLR, though they usually have all the functionality you need to make the process fairly straightforward. A camera with an APS-C size sensor or bigger really helps, but if you can remove the lens and fit a T adaptor that's probably the main requirement. The ability to remotely release the shutter would be good, too. If your bridge camera meets those two criteria then I'd suggest giving it a go. With the T adaptor the camera fits directly to the visual back on the scope. At least, mine does. I don't know if newer models still have the male T-thread on the visual back.

As Olly says, where you may well be able to use the SPC900 is when you want to image individual sunspots. It's a bit tricky to get the hang of at first, but quite possible.

James

[stevil]

Thanks James,

From what you describe it will be quite a task making a mosaic with so many images and the lengthy procedure with my kids running about me will probably end up puting me off. Unfortunately my bridge camera's lens is not removable so looks like a DSLR is the way forward for me.

Thanks

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[JamesF]

From what you describe it will be quite a task making a mosaic with so many images and the lengthy procedure with my kids running about me will probably end up puting me off. Unfortunately my bridge camera's lens is not removable so looks like a DSLR is the way forward for me.

It does prolong the process a fair bit and given the choice it's probably not the ideal place to start.

If you're looking at buying a DSLR then it's probably useful to know that Canon cameras are probably the most widely-supported in terms of astro software though you can use others. If you want to keep the budget under control a bit then you could do a lot worse than a second hand 450D or 1100D.

James