SPC900 Focusing Problems

[Skiddins]

I have just been trying out my SPC880 webcam (flashed to 900) in a Celestron SLT 130 but I'm having reall problems getting the focus correct on any object.

I lined up Saturn and the best I got was when the focuser was fully in, but this still wasn't right, almost as if it really needed to go in a bit further.

Am I doing something wrong?

I didn't fit any Barlow's or anything in front of the webcam.

I know the camera is working fine because I have tried it on my desk (with it's lens re-fitted).

Thanks

Skiddins

[jnc71106]

Sounds like not enough inward focus travel

Im not sure but I think if you use a Barlow it might help by making the focus point further out so you may be able to achieve focus properly. Not 100% sure on that. I'm sure someone else will clarify this

Jake

[steelfixer]

Hi there

I had the same problem with mine.

These cameras seem to have a very short focal length.

The good news for me is my scope has a focus tube so I stuck the tube in the lathe and machined 5mm off the end and now I can achieve perfect focus.

Being new to all this I am sorry I cannot advise you on how to cure yours but there are a lot of clever people on here who will know how.

Graham

[Space Cowboy]

You should have no trouble when a barlow is used.

[Parus major]

With mine on my scope the focuser is about 13mm racked out, with a barlow it's more like 40mm so a barlow should do the trick.

[Skiddins]

Thanks all.

Looks like I'll be trying with a Barlow then

Skiddins

[The Sailor]

I solved my problem by just putting a diagonal on, barlow did not work when I tried that.

Jim

[Cornelius Varley]

I solved my problem by just putting a diagonal on, barlow did not work when I tried that.

Jim

Most newtonian reflectors don't have sufficient inwards focuser travel to allow a camera to come in to focus. The solution is to use a barlow lens to move to point of focus further away from the primary mirror. Using a barlow when webcam imaging is also required to improve the image scale. Ideally the barlow should have a power factor that increases the focal length of the telescope to give a focal ratio of about F20 to F32 to get the best out of the camera/telescope combination.

Using a diagonal doesn't work in that situation, it only works on refractors that don't have sufficient outwards travel. The diagonal adds extra length to the light path, With a refractor the usual solution is to use an extension tube approximately equal to the length of the diagonal.

Peter

[The Sailor]

Yes my apologies I should have indicated what I use it on, my refractor it is, the SCT not required, Newt I don't have .

Jim

[Skiddins]

.....Ideally the barlow should have a power factor that increases the focal length of the telescope to give a focal ratio of about F20 to F32 to get the best out of the camera/telescope combination.

Peter

Hmm, err, so how do I calculate what mag barlow would give me those ratio's?

Sorry about the 'idiot' questions but I was also never into photography, so I'm coming to all this a bit late .

Thanks

Skiddins

[Cornelius Varley]

Your telescope is a 130/650 reflector. The focal ratio is 650/130 = F5 (focal ratio is the focal length divided by the aperture). To get to F20 you need to divide the required focal ratio (20) by the existing focal ratio (5). You will require a x4 barlow to get to F20, or a x5 barlow to get to F25.

Peter

[Skiddins]

I tried with my 2x Barlow last night.

This certainly allowed me to get the focus closer ( I could make it go out of focus in either direction) but it wasn't as sharp as I expected.

I increased the video preview to 200% in Sharpcap to try and help

I still took some video with SharpCap (2000 frames) which was setup the same as that demo video on Astronomy Shed.

After running it through Registax it didn't look much different.

It's all on my laptop so I'll try to get the images etc on here so you can see what I mean.

Is there something I should be checking or am I doing something obviously wrong?

What would using a Barlow that gets me to the F20-F30 range give me, larger image, better focus, finer focus adjustment etc?

I am getting there

Thanks

Skiddins

[Cornelius Varley]

By using a barlow lens the focal length of the telescope changes and the field of view of the camera reduces. This means that, for example, you where photographing the moon, you would see a smaller surface area than without a barlow. You will see smaller craters on the surface of moon. In planetary imaging this would mean that the planet would cover more pixels of the camera's sensor than without a barlow, so you would be able to see more detail on the planet.

Boring / science bit coming up...

The optimum focal ratio for imaging usually falls between F20 and F32 which comes from a formula that calculates the number of pixels per arcsecond of telescope resolution, 1 arcsecond being 1/3600 of 1 degree. To get the best resolution for imaging planets and the moon you need at least a grid of 2x2 or 3x3 pixels to image 1 arcsecond of resolution. ie if a crater on the moon had an angular measurement of 1 arcsecond, at an imaging resolution of 1 pixel per arcsecond the crater would not be seen by the camera. At 2 or 3 pixels per arcsecond the crater would be seen by the camera. If you increase the focal length of the telescope further, to get more than 3 pixels per arcsecond of resolution, more pixels are used to record the crater but no more detail is seen than at 2 or 3 pixels.

Basically you get a better image if you image at between F20 anf F32 than at your telescopes focal ratio (F5). You image a smaller area but with more detail.

This is why planetary and lunar imaging is usually done at focal ratios of between F20 and F32.

Peter

[Skiddins]

Here is what I ended up with, I couldn't get the focus much better than this;

[Cornelius Varley]

The image has a ghost above and to the right of Saturn. Is this in all the video frames or just the final processed image?

Peter

[Skiddins]

Only it that processed image.

I tried doing it again just now and the ghost wasn't reproduced.

However, the image and the video look virtually identical.

Skiddins