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DSO DSLR imaging - what else do I need?


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I have a SW130P AZ GoTo and a Canon 40D + T ring adaptor and 1.25" nosepiece. Also have cable remote / timer for Canon.

I understand that the camera body causes problems with the focus. Do I need some form of extension tube. Should I just connect the canon to the 2" fitting on the SW130P?

What are the benefits of either. What filters might I need?

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I'm no expert but reflectors can have a problem where by the focuser doesn't have enough inward travel to achieve focus, so you cant get your camera in close enough.

My Skywatcher 150P has a removable piece on the focuser, if you unscrew that you can then attach the camera further forward. If you still cant focus then a barlow can help by effectively multiplying the focus travel you have to play with.

Say you have 30mm of inward focus but to get your DSLR into focus you need 45mm. A 2x barlow doubles the focal length of your telescope and also effectively gives you 60mm of play on the focuser. At least that's how I understand it.

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There is a bit of an issue with the 130P at prime focus with a DSLR. The focuser is about 1" too long to get the telescope to focus directly onto the chip. Extenders don't work as they are moving the camera in the wrong direction.

The way around this is to insert something in the way that extends the focusing back away from the eyepiece.

Most people use a x2 barlow lens although this also has the dis-advantage of doubling the focal length of the scope to 1300mm and increasing the f-rating meaning imaging is much more difficult.

I'm not sure how accurately the AZ-Goto mount will track but I found the above setup was just too much for my DSLR on deep sky objects.

Other options you might have is a lower powered barlow (x1.25) or possibly a focal reducer. I've also read about about certain coma correctors having the effect of extending the focal point backwards as well.

I'm not sure how sensitive the sensor is in the 40D but I've found that for DSO's, as a minimum, you need to be working with exposures of 15s at ISO1600. Anything below this and the light pollution and noise makes things really difficult and you're into the realm of taking many, many hundreds of photos just to get something viable to stack with.

As a test, you could piggy-back your DSLR to your scope with a 135mm lens to see how long an exposure you can take without getting star-trails. Once you know this, you can work out how your scope will perform.

Hope this helps.

Alan.

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If the mount is an Alt-Azi then you are going to be limited in exposure length by field rotation. This won't matter for planetary or lunar, but will for DSO imaging.

Edited by Zakalwe
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A third vote for getting yourself a copy of Making Every Photon Count (by SGL's very own steppenwolf) before you start buying stuff.

Easy to read, it's full of good advice aimed at the imaging novice, including choosing the right equipment - explaining what kit you'll need and, more importantly, why. Helping you avoid poor choices and costly mistakes. There's also loads of tips 'n' tricks as well as lots of other vital stuff.

Have a trawl through the imaging sections too. They're full of info and some very experienced and talented folk who, I'm sure, will be more than happy to share advice and guidance with you.

HTH :)

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I agree, buy Every Photon. As for the rest of it... as has been said, the 130 is unable to reach focus without a barlow (or modding), a 2x barlow, apart from narrowing the field of view considerably, actually increases the exposure time by a factor of four to get the equivalent exposure. Now, tracking in Alt Az, you can in theory... image at around the 40-60 second mark as long as you don't get too high. You could extend that to about 120s low in the east of west, but the loss rate is very high (at 50% for me) as the drives aren't accurate enough.

One thing to consider trying, if you want to try some imaging with that setup... Remove the scope, mount the camera on a dovetail and use a wide angle lens. That won't make any difference to the duration of each exposure, but being tracked you can take many exposures, the rotation between frames will be removed by stacking, and with a camera lens, you can possibly get to f/4 which is pretty fast.

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Sounds like a good read and a good place to start. Just one more question. The SW 130P allows me to unscrew the focuser to allow you attach 2" EPs which will also allow me to connect the Canon by removing the nosepiece from the T ring - will this get me close enough to focus?

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If you unscrew the inch and a quarter eyepiece adapter off the focusser, you will be able to attach your camera using a Canon T-Ring onto the same thread. Your camera will then be able to fit into the focuser to reach the focal point properly and away you go.

If you want, let me know and I will take a couple of pictures later to show you how I did mine.

If you use a Barlow, your field of view will be extremely small, your image will be extremely faint and you will be causing too many problems for yourself.

Dave

Edited by Dave
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But as stated earlier you have an alt az mount and this will make it impossible to track for any length of time. I would go for a webcam and the moon and planets in your situation.

And also as suggested earlier, read before you spend!!

Olly

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