How to focus a DSO for DSLR imaging?

[mikehab]

Hi,

I'm having problems focussing DSOs when trying to take pictures with my DSLR. I find the DSO and can just about see a faint fuzzy blur through the telescope eyepiece, which I can focus (even using a bahtinov mask on an adjacent star for example), but then obviously when I remove the eyepiece and switch to the DSLR (or simply flip the internal mirror to swith from the eyepiece to the camera when attached at prime focus) the focussing isn't correct for the camera due to the different focal plane etc.

The DSO is too feint to see through the through the camera's viewfinder or LiveView - so can't re-focus. How do you focus on something you can't actually see through the camera's viewfinder or on LiveView??

How to other people manage to re-focus DSOs in the camera when you can't actually see them!??

Any help greatly appreciated...

Thanks

Mike

[Shibby]

The answer is - you don't. You will want to focus on a bright star first, ideally using a Bahtinov mask (you can make you own easily with cardboard), then once focused, slew to your target and away you go!

[Spikey]

As Lewis says the DSO is not where you alter the focus, it's done on a bright star. My usual way to focus is to pick a bright star near the target to align on, I also use this star to focus on with a bahtinov mask over the end of the scope. As I take exposures for focus it's literally only a couple of minutes at the most. Slew to the DSO, frame and expose away Not forgetting to remove the mask off the end of the scope of course

[Shibby]

Yeah, taking exposures is best - I've now started using the tool bahtinovgrabber on the captured image (at 100%) to check that I'm bang on, allowing me to get within 0.1 pixels of focus. A little fiddly but only takes 15 minutes and well worth it.

[mikehab]

Thanks for the replies ... all make sense, however is does of course assume/require that ones GoTo / auto-tracking mount is accurate enough to slew back to the DSO accurately... and I don' think mine is, so I'm having to try to 'find' it manually (hence switching back to the eyepiece). Maybe it's all down to the accuracy of the mount in order to feel 100% confident that I'm actually pointing at something I can't see.

As Spikey says above...

>>Slew to the DSO, frame and expose away....

How do you 'frame' something you can't see? ;-)

I know people say that you can't do DSO imaging with a Nexstar4SE - however, contradictory to that I've also recently seen posts here from people who have successfully imaged M13 with a 4SE (which is what I'm trying to do).... so it's definitely possible :-)

I just need to make sure a) I'm pointing at the right place, and in focus (and c, d, e, f make sure I have subs, darks, stacked etc. etc. etc.)

[Spikey]

As Spikey says above...

>>Slew to the DSO, frame and expose away....

How do you 'frame' something you can't see? ;-)

By taking sub-exposures long enough to show the DSO on a frame

Then tweak the position very carefully and at a very slow slew rate, taking further exposures until you have it centred or framed as you wish.

[pel]

From reading a lot of posts on here - I use Backyard EOS to focus with the mask on a bright star and then try and find the DSO in my finder, if I can't see it in the finder I star hop to the approximate location and take 5-10 second snaps at 1600 ISO to locate the object. When I find the object I frame it and start imaging.

[split_city]

+1 bahtinov mask. All the images I have taken have a bright enough star in the same frame as what I'm imaging to use to focus.

[Gina]

I first of all slew to a bright star, centre it in my finder so that it's in my camera frame, then using either EOS Utilities or APT, in live view, adjust the exposure to show a good clean star. Next put Bahtinov mask on front of scope, readjust exposure if necessary, zoom in and adjust fine focus to give the right image. Then I lock the focus and check it hasn't altered. That's focus taken care of and the mount calibrated for that star. With a star in the general area of the DSO I can usually find it in the frame. Otherwise I use a planet (or moon if available) to check the scope is pointing where CdC says it is.

I do still sometimes have problems finding the faint fuzzy though as I believe others sometimes do.

[Quatermass]

Finding your target after you have focused on a bright star first is the way to go. Get the star focused with your mask first then with that done slew to your target area with the finder scope and start taking 30 second shots until you see it in your images. Using backyard eos makes focusing and framing much simpler. I resisted using a laptop for a year or more but I know prefer the laptop and backyard eos for focus and framing.