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Prime Focus

Sun Sep 02, 2018 3:22 pm

Just been experimenting with the new telescope and the camera. This is a shot of an ariel cable junction box on a chimney on a house 200m down the street. The camera is a Sony A300 DSLR and I was never able to get it to focus on the Dobsonium but it seems much more at home on the SCT.

At first the camera was on the A setting and it wouldn't take a picture because there was no lens on the camera. It was ok on the M setting using a remote trigger.

I hope the skies stay clear. Can't wait to point it at Saturn tonight!

Image

Re: Prime Focus

Sun Sep 02, 2018 8:39 pm

Have fun with your imaging.

Re: Prime Focus

Mon Sep 03, 2018 11:00 pm

Dobsonians sometimes have eyepiece holders that are too long to allow standard cameras to focus the image. The image sensor in the camera can't get near enough. Getting a shorter eyepiece holder can sometimes help. Using a webcam or other camera not designed to be used with a lens can often work. I've not heard of any problems with an SCT, so you should be safe.

PS - I think that aerial junction box should be replaced :D

Re: Prime Focus

Tue Sep 04, 2018 9:14 pm

With the camera directly in the end of the telescope I was able to find a focus straight away. But with the barlow lens between the telescope and the camera I wasn't able to focus at all. So it sounds like a similar problem. So I'm probably not going to be able to record a bigger image of Saturn until I get myself a web cam!

Re: Prime Focus

Thu Sep 06, 2018 11:42 am

If you referring to the SCT, then a normal barlow won't move the focussing out of range. Are you getting an out-of-focus image, or no image at all? A barlow will reduce the field of view making it harder to find. It also reduces the amount of light falling on the sensor. You could try focussing on an easy target like the moon first, and then go on the the planet. Try increasing the exposure, and/or the ISO figures.

Re: Prime Focus

Thu Sep 06, 2018 12:43 pm

OK, cheers for that. I probably need to sort the alignment first. I wasn't getting an image at all. I thought my finder red spot was set up accurately but perhaps not!

Re: Prime Focus

Thu Sep 06, 2018 9:24 pm

Unfortunately, prime focus with a DSLR doesn't usually give the best results on planets. I've even tried it with an effective 9 metres focal length! I've used a 2x and 3x Barlow together. Even the ancient (much-maligned) art of afocal projection is often better.

I have a webcam but the driver keeps crashing my PC!

Re: Prime Focus

Fri Sep 07, 2018 12:03 am

I've found you can get a 2nd hand PS3 Eye camera from CeX for 50p.
A £10 1.25" adapter and the instructions here...
http://astrobeano.blogspot.com/2013/03/ ... raphy.html

Re: Prime Focus

Fri Sep 07, 2018 7:20 am

Gfamily2 wrote:I've found you can get a 2nd hand PS3 Eye camera from CeX for 50p.
A £10 1.25" adapter and the instructions here...
http://astrobeano.blogspot.com/2013/03/ ... raphy.html


I used an Xbox cam for a while. One problem with most webcams is that they do not work on all versions of Windows.

Re: Prime Focus

Fri Sep 07, 2018 8:13 pm

Graeme1858 wrote:OK, cheers for that. I probably need to sort the alignment first. I wasn't getting an image at all. I thought my finder red spot was set up accurately but perhaps not!


Using a barlow reduces the field of view quite a bit. Cameras generally tend to reduce the field of view even more, sometimes considerably more A red dot finder may not be accurate enough. To make matters worse, just focussing an SCT can move the image out of view. If you can, focus the telescope on an object easier to find first. (the moon, perhaps). Then increase the ISO to it highest setting, and a 1 second (or more) exposure. Move over to where the red spot, or GOTO says the the planet is, and wander about until you find it. One technique is to use the hand control to gradually 'spiral out' until you see it. Once you have it, reduce the ISO and exposure to give a correctly exposed view.
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