Focusing Canon 500d with Skywatcher 130P - Not back focus

[sgroombr]

Hi, Sorry if i havent posted in the right place. I am a newbie.

I have setup my scope, fixed the back focus so there is travel either way. I can get reasonable focus including using a bahtinov mask. The issue I have is when I zoom in on the camera screen - it does not appear to be in focus.  Is this normal?

 

Thanks in advance

[michael8554]

I see elongated faint stars in the first and second images.

You say it's a shot of the screen with another camera ?

You should really be analysing a downloaded image.

The Bahtinov image shows a very slight defocus, but not enough to have caused the elongation.

Unless collimation is really poor, elongation won't be due to focus error.

May be due to a Tracking or Polar Alignment error.

But not on an exposure of less than 5 secs, so not 100% sure what's going on.

Michael

 

[sgroombr]

Thanks. The elongation will be due to no tracking. 15 seconds exposures.

All images are from the same camera. But the one in question is from the live view screen on my computer. I am wondering if the live view function has something wrong with it?

cheers

[ONIKKINEN]

Hello, and welcome to SGL

Looks like the diffraction pattern from your bahtinov mask indicates that the scope was well out of focus. Below is an easy visualization:

The diffraction pattern really does need to be dead center, otherwise the shot will be out of focus. I actually find it much easier to ditch the bahtinov mask and just focus visually looking at star sizes, or better yet, looking at reported HFR values using some software (i use NINA).

But also, untracked images will be smeared no matter the focus. Which scope are you shooting with? The focal length (and pixel size) will dictate how quickly shots become obviously trailed, but with most telescopes this will be less than a second easily.

[sgroombr]

Thank you ONIKKINEN. What are your thoughts on the live view image? It seems a lot more distorted that the other two?

[ONIKKINEN]

38 minutes ago, sgroombr said:

Thank you ONIKKINEN. What are your thoughts on the live view image? It seems a lot more distorted that the other two?

Its hard to say from these images.

The first one shows obvious trailing, so perhaps not the best image to troubleshoot sharpness from. The second is just distorted for some other reason. It could be seeing, bumping the scope, the mount rattling, wind, many reasons really. Could also be out of collimation, but really it is difficult to advice really.

Which scope is it? If you are trying untracked astrophotography you should try to focus on some obvious unmoving target, or better yet, a slowly moving target that has obvious surface features (the Moon!). Recently i took this image:

Of the Moon with an untracked mount. The easy method of taking an image untracked is to try to spend as much time as possible focusing on some very obvious feature, which in this case was the Moon and its very obvious craters, and then hope the focus stays when fiddling with the camera but for deep sky you really want to have a mount that can track the sky. For planetary you could maybe get away with not tracking, but this will be very annoying and tedious to do. I have tried it and its technically possible, but needs more patience than i have available! Below is an attempt of Jupiter with a 90mm refractor untracked (dont know what the circle artifact is to the top, its not real detail):

[sgroombr]

Thanks again. I have a skywatcher 130p reflector on an eq5 and a Canon 500d. I didn’t turn on tracking as I only wanted to show the difference in the images and in particular what live view produced.

[ONIKKINEN]

1 minute ago, sgroombr said:

Thanks again. I have a skywatcher 130p reflector on an eq5 and a Canon 500d. I didn’t turn on tracking as I only wanted to show the difference in the images and in particular what live view produced.

The EQ5 is a plenty capable mount for many types of astrophotography with a 130P, so no worries there for now. Try with the tracking on and with a short exposure (couple seconds at most) and see what you get?

The live view in my 550D is quite temperamental, i think it represents only very short exposures, like 1/4th of a second or so. At this speed the seeing will distort the image randomly to every direction and you could get a funny star shape here and there. 2 seconds and above is where the seeing generally averages out and you get the "true" starshape what your mount can deliver. But with untracked a 2s exposure would be hopelessly distorted also, so bear this in mind.

[sgroombr]

Thanks again. I will now wait for the next clear sky and try that. I guess if the true image from the camera is ok, then the live view isn’t too important. Cheers