Astrophotography 101 - focussing with DSLR - how to?

[bradfrankshaw]

Hi all - last night was perfect over Horsham and I sorted my eq3 alignment issue so I thought I'd try a couple of dslr images just to get a feel of the hows and whats. Slewing to Capella and focussing using a wide angle lens, I took my Canon 350d, mounted the T ring and tube, slotted it into the scope to see what I could see. Hmm not a lot. The 350d is older and has no live view but through the viewfinder I could see the mirror with a very out of focus star.

Ok, I thought refocus...but all I got was a large flickering halo. I was missing something I knew but what?? I'd done this same technique 25 yrs ago but unguided on an old reflector and got results....hmm. I checked online and a guide I had but I couldn't figure out the most basic error I'd made? Any ideas or thoughts?

Thanks

Frank

[VigdisVZ]

Is it the 150P or PDS? I think that the focuser on the P might lack movement inwards enough to achieve focus with a DSLR.

[JamesF]

Perhaps the easiest way to do this is with a focusing mask. You'll find many people (myself included) use a Bahtnov mask for focusing, but a simple Y mask can be sufficient. You can buy a Bahtinov mask for your particular scope, but it's quite possible to make one (or a Y mask). It has been known for people to use bits of twig and suchlike when they discover they've left their mask at home.

There are websites that will produce a file to print out an image of a mask that you can reproduce on thick card or something similar, so all that's required is some careful work with a knife.

When placed over the aperture of the telescope whilst pointed at a bright star the mask produces an "X" with a line through it. That line moves one way as you get "outside" focus and the other as you move "inside" focus. At focus it passes straight through the centre of the "X" and you can then slew to your target, first remembering to remove the mask from the end of the scope.

As Carl suggests, there are some scopes that struggle to reach focus with a DSLR. It would be worth checking that you can actually reach focus during the day perhaps.

James

[haitch]

As Carl suggests, there are some scopes that struggle to reach focus with a DSLR. It would be worth checking that you can actually reach focus during the day perhaps.

James

And make sure you perform the check on an object around infinity as closer objects will come in to focus with the tube further out.

[poogle]

... I took my Canon 350d, mounted the T ring and tube...

What is this tube you are using? The 150P will reach focus (unless it's an old model) with a DSLR, I've got one myself.

You should just unscrew the 1.25" eyepiece holder and attach the T-ring directly on to the base plate that fits in the focuser (see

)

/Patrik

[bradfrankshaw]

Hi guys - I've got a 150p (I was the lucky guy whose mate bought me a full eq3 pro 150p set up!) - so if I take the eyepiece holder out, the T ring will go directly into the slot for the holder without the need for the screw in adaptor (on the T-mount)?

frank

[poogle]

Yes, the T-ring will thread directly onto the T-mount. This will give you enough inward travel to reach focus (with like 5-7 mm to spare)

/Patrik

[Neeken]

You probably need to get your dSLR closer, so remove any extension tubes if possible. On my newt. I had to use the collimation screws as well, to get the mirror as far in as possible to achieve focus.

I have a T-ring with a 2" coma corrector attached to it, which fits directly in the focuser without any adapters between them. If you have a 2" tube to put directly on your T-ring, then you should be able to get as close as possible. If I'm correct in assuming that scope has a 2" focuser with a small 1.25" adapter attached to it with screws..

[bradfrankshaw]

The adaptor is similar to this -Antares basic 1.25 nosepiece to T-thread camera adaptor. I'll layout all the components tonight, snap a shot and put up here.

[Purplehayze104]

ooo good video dint realise you could do that, been using the end of a barlow screwed into my t adapter to get focus through my 150p but will give that a go now to see how much difference there is. cheers guys

[Neeken]

Here's a question though, if anyone knows; don't you get more light onto your dslr sensor by using a 2" T-adaptor if your focuser has a 2" slot, than using a 1.25" adaptor? I see many people post 1.25" T-adaptors here, I don't quite understand why. At least not for prime focus on a 2" crayford focuser..

[bradfrankshaw]

Ok I've taken the adaptor assembly out of the scope (using the two thumb screws), taken the 1.25" adaptor out of the assembly and screwed the remaining ring directly onto the T-mount - does that make sense to anyone? The t-mount can now connect directly to the focus assembly. Hopefully this will do the trick as I've been wanting a photo of M31 all my life lol!

frank

[poogle]

Sounds about right I'm sure you'll reach focus now

/Patrik

[poogle]

Sounds about right I'm sure you'll reach focus now

/Patrik