Question about imaging using camera lenses

[Lee_P]

I know that when imaging, getting the right focus is critical. If using a telescope, you can use electronic focusers (and perhaps software such a SGP) to regularly check and refocus if necessary. I'm wondering, how does this work if you're using camera lenses? Let's say you're doing an all-night imaging session. Do you need to manually check every hour or so to see if the focus has slipped? Or are camera lenses, often with their lower focal lengths, more forgiving when it comes to getting focus spot-on? 

[vlaiv]

You can add motor focuser to lens as well.

It is usually belt driven device. Not sure if there are commercially available models, but DIY is rather easy to do (just need a stepper motor and maybe look at this https://sourceforge.net/projects/arduinoascomfocuserpro2diy/

Otherwise, yes - check it manually from time to time when temperature changes or maybe capture software can be made to signal if there is change in star FWHM / HFR that will mean that focus is a bit off.

[Lee_P]

Very helpful, thanks for the info!

[david_taurus83]

APT can control a camera lens if it has autofocus.

[Lee_P]

On 27/07/2020 at 20:22, david_taurus83 said:

APT can control a camera lens if it has autofocus.

Thanks for the info, that's interesting. Presumably APT communicates with the lens via a cable connected to the DSLR camera? So this wouldn't work if using a dedicated astrocam?

[wxsatuser]

You can easily focus using 10x liveview and a high ISO.

You should be able to see stars and the correct focus is a small window where the stars are sharpest.

[Lee_P]

2 hours ago, wxsatuser said:

You can easily focus using 10x liveview and a high ISO.

You should be able to see stars and the correct focus is a small window where the stars are sharpest.

Thanks for the info, but what I'm specifically wondering is whether that focus changes over the course of a night's observing. Or, considering camera lenses are often wide-field compared to telescopes, does an image essentially stay in focus all night?

If adjustment is needed, do you need to manually repeat the process you described every hour (for example) to maintain good focus? Or are there automated ways of doing it (such as vlaiv suggested). From your sig I see you use a Samyang lens, so I expect you've got first-hand experience in this!

[wxsatuser]

It could possibly go slightly out of focus, depends on the lens.
Most likely best to check focus once and a while just in case.

When I have used my Canon lenses on a dslr never had to refocus as far as I can remember.
May be I was lucky, but my lenses were the really expensive red rings, your mileage may vary.

I have never had my Samyang need refocusing even over several hours, it seems rock solid
with a belt drive, btw I use mine on an Atik CCD and use their software to focus.

[david_taurus83]

3 hours ago, Lee_P said:

Thanks for the info, that's interesting. Presumably APT communicates with the lens via a cable connected to the DSLR camera? So this wouldn't work if using a dedicated astrocam?

Yes a simple mini USB cable connected to camera and laptop enable APT to control it. This is my experience with Canon camera and Canon lenses. The Samyang range are manual so wouldn't apply.

What we your plans or intentions and maybe we can give specific advice.

[Lee_P]

Thanks, all good info!

 

2 hours ago, david_taurus83 said:

What we your plans or intentions and maybe we can give specific advice.

I've previously had an imaging rig -- telescope, mono CMOS, filters, etc. I sold all of that due to a lack of time for the hobby, and am tentatively pondering a return sometime in the future. I reckon I'd go down the cooled CMOS route, and a key question will be: telescope or camera lenses? I'm a photographer and have some nice Nikon lenses. But 'scopes like the WO 61mm seem like camera lenses designed for astrophotography, so may be a better choice. My old rig had an EAF, and I liked how that took care of focussing, and kept everything in focus all night. So this thread is just to figure out if such a thing is possible with camera lenses

[Sp@ce_d]

Yeah it is possible.. I guess it depends if you are wanting to automate the imaging process or just checking each time you change filter.. temp fluctuations etc.. bahtinov mask would do you for manual perhaps?

There's some interesting stuff going on here...  Just for Canon lenses so far. https://astromechanics.org/

https://www.firstlightoptics.com/adapters/astromechanics-ascom-aps-c-m42-to-canon-lens-controller-mark-ii.html

and the Samyang 135 F2 thread is a great read

 

If you've already got some nice lenses why not try them first anyway. You could rig up a motor & belt drive fairly cheaply.. less than investing in a scope.. to see if it suits you.

https://www.motionco.co.uk/pulleys-belts-c-25.html

There's a rather expensive pre cooked one for the Redcat here..(bearing in mind you also need the controller) but you get the idea.. https://www.firstlightoptics.com/focuser-controllers/starlight-instruments-redcat-electronic-focusing-system-si-rcef.html 

 

[Lee_P]

Thanks for the info -- those lens controllers in particular are the kinds of things I was imagining, and I had no idea they actually existed! Shame they're currently just for Canon.

On 29/07/2020 at 20:09, Sp@ce_d said:

If you've already got some nice lenses why not try them first anyway. You could rig up a motor & belt drive fairly cheaply.. less than investing in a scope.. to see if it suits you.

That's a fair idea, the reason I haven't just tried it yet is that I don't have a suitable camera to use. My inner-city skies rule out my regular camera. What I'll do is let these ideas swim around my head for a while, and then start a new thread about a whole imaging set-up that might fit the bill. 

Thanks everyone for the replies, you're all awesome

[nfotis]

I have noticed that Backyard EOS (which has also a version that also works with Nikon equipment) offers accurate focusing via software in three different steps (obviously, this doesn't apply on manual focus lenses like the Samsung 135mm)

Only the software and a USB cable are needed for your laptop.

 

You can download a trial version and see if it's what you want.

 

Cheers,

N.F.

 

Edited August 7, 2020 by nfotis