Focusing Moon with DSLR + SCT

[keramos]

Hello Everyone,

Last night/early morning I tried taking my first ever images of the moon. It was amazing in the eyepiece (really amazing) and was even better when I viewed in Live View on my DSLR Camera, crisp view in both camera and eyepiece.

Saved lot of images during the session but later when I saw the pictures on my Laptop, all seem out of focus.

I don't know what I am missing?

I'm using Celestron Nexstar 6 SE (GoTo), with Nikon D3300 DLSR (unmodified) to capture images. I've mounted the camera with and without the diagonal and get the same unfocused Image (I'm using remote shutter release).

I wonder if the GoTo mount is ever so slightly shaking such that it creates a blurry image.

Last question, I could not get the full disc image of the moon, any suggestions as to what I need to do?

Please advice, thanks in advance.

[Owmuchonomy]

What camera settings did you use for the shot?

[JamesF]

It does look a little soft.  It may be that focus isn't perfect, but you might also get problems if your exposure time is too slow.  I think I started at something like 1/1000th @ ISO400.  Eventually I managed to get longer exposures, but only by raising the mirror a few seconds before opening the shutter so the vibration due to the mirror movement didn't shake the camera.

Focus is tricky with an SCT.  I have a Canon DSLR and control it using the PC which allows me to see the image on-screen and adjust the focus until I'm happy (which I do with a motorised focuser).  I'm not sure if that's possible with a Nikon, but by zooming in the camera display you might be able to find a crater or something similar that you can use to gauge the focus.  That can lead to another problem though -- adjusting the focuser wobbles the OTA slightly.  Once you're close you may find that putting a spring type clothes peg on the focuser knob allows you to turn it without causing so much disturbance (which is why I now use the motorised focuser).

Ideally you want a good number of images and to stack them.  This may help there: http://stargazerslounge.com/topic/184192-full-disc-lunar-imaging-with-a-dslr/

The focal length of the C6 is quoted as 1500mm, but that's only then it's focused at one specific position.  As you focus and move the primary mirror the focal length changes.  The focal length is what determines the size of the image.  If you put the DSLR directly on the visual back using a T adaptor you should get the image on the sensor in one go I think because the focal length will be somewhat reduced, but it will still be quite tight.  My experience with my 127 Mak (also 1500mm focal length) suggests to me that if you use the diagonal the image will definitely be too big.  Maks exhibit the same behaviour and the only way I could get the full disc image on my 450D sensor was to have it directly on the visual back. If that doesn't work probably the only option is a focal reducer (or a different scope entirely).

James

[JamesF]

Oh, it could also be that the seeing just wasn't that good when you took the images.  Not much that can be done about that

James

[keramos]

Morning James,

Thank you for the reply, I'll have another go and will keep you suggestions in mind ....I've got lots to learn.

Btw when you mentioned raise the mirrors, did it mean adjusting the focus or movement of the scope itself?

ps : Nice article on Full disc imaging.

[baggywrinkle]

That's a very useful guide James.

I use live view on my Canon and also enlarge that view and try and get a good focus point on the terminator as there is lots of contract and edges.

However you will see a lot of wobbles due to thermals and as James says not much can be done about them. Not having a mororised focuser adds to the problems due to mount shake on whatever mount you are using.

[JamesF]

Btw when you mentioned raise the mirrors, did it mean adjusting the focus or movement of the scope itself?

Sorry, I meant the mirror in the DSLR in that instance.  Again I'm afraid I don't know much about Nikons, but the exposure process in a DSLR first involves lifting the mirror in front of the shutter that redirects the image to the viewfinder, and then the shutter fires.  Whilst hardly huge, the movement of the mirror can cause vibration.

I found that if I tried taking images at 1/200th exposure I was getting ghosting in the image and someone suggested to me that the mirror movement was the cause.  Under USB control the Canon cameras allow the mirror to be lifted independently of the shutter firing, so APT (AstroPhotography Tool, the software I use to control the camera) could lift the mirror, pause to allow the vibrations to stop, then fire the shutter.  Doing so prevented the image ghosting.

James

[keramos]

Hi James,

I think I have seen this ghosting phenomenon when doing some white light solar imaging, Do you use Canon's camera capture control software?

Unfortunately Nikon D3300 is not supported on Nikon's own Camera Capture software, how lame is that? 

[JamesF]

Hi James,

I think I have seen this ghosting phenomenon when doing some white light solar imaging, Do you use Canon's camera capture control software?

Unfortunately Nikon D3300 is not supported on Nikon's own Camera Capture software, how lame is that? 

I don't use Canon's software, no.  Not other than the camera drivers, anyhow.  I use AstroPhotography Tool (most often known as APT), here: http://www.ideiki.com/astro/Default.aspx  There's a similar application called Backyard EOS (BYE), but I didn't get on with it as well as APT when I was looking, so I've not used it for some time.

I seem to recall reading recently that someone is working on something similar for Nikon cameras, but I can't recall what it was called I'm afraid.

James

[Davey-T]

Have a look at this.

Dave

http://www.otelescope.com/index.php?/store/product/4-backyardnikon-10-premium-edition/

[Tommohawk]

Shutter vibration is likely candidate from my experience. With Canon utilities you can lift mirror, and then release shutter to minimise vibration.  I dont have my D3200 any more, but I dont think Nikon allows this. TBH I went over to Canon partly because theres more AStro software for it, and because you can't get Live View on laptop with Nikon - although you can connect to HDMI on TV....  if you happen to have an HDMI TV in your garden. Above link looks promising though, and I certainly got some good pics with the Nikon.

You havent given your ISO/exp settings. Maybe consider upping the ISO, and faster release. To beat vibration I ended up with 1/1000 second and 400ISO - related thread here.

I'm sure you'll crack it!

[JamesF]

To beat vibration I ended up with 1/1000 second and 400ISO - related thread here.

That's exactly where I was at before I started using the mirror lock-up.  I wanted longer exposures so I could reduce the ISO setting and thus reduce the image noise, but I couldn't get any longer without the mirror lock-up.

James

[knobby]

Definitly agree with upping the shutter speed as far as you can, it will help with the vibration caused by mirror slap... also try setting D5 gives a delay after mirror moves before shooting