Backfocus with an ZWO Cooled Camera 

Thanks Luke. Let’s hope these interactions also help this videos exposure, because now I am making these for a rather small audience 😇

You’re welcome

Glad it was helpful! Let me know if you have video suggestions. And feel free to tag me on socials in your resulting images. 👍

You’re welcome

Thank you! Deliberating what to film next. If only it got clear... I want to test out the new scope and feature it during a head to head shoot on a target like M13 in both native as reduced setup. Will probably take a long time to complete though...

Glad to see my video helped.

You’re welcome!

You’re welcome! A new video is coming (hopefully next Wednesday)

@@KopLamp I'll look forward to it👍

Glad it was helpful!

I have read it indeed. But I think because I am using a cropped sensor I get away with it.

Glad my video helped. The information is out there, but it can get complicated real fast. I found the role that physical telescope shops play in this invaluable. Online resources like social media (including RU-vid) and forums like CloudyNights are helpful too.

That was an attempt to have an idea of the camera rotation. I had the 90 degrees points marked.

It is not necessary but depending on the object you are shooting it can help bring out details which would otherwise stay invisible.

I am no expert in lenses with ZWO cameras. I do know that the lenses are made for the DSLR, which requires a backfocus of 55mm. Keep in mind that the sensor is recessed in the camera body. Normally the ZWO camera should come with adapter rings to reach that 55mm (which is pretty standard)

You wouldn't be using a flattener/reducer then. I don't own a Barlow, but my guess is that it will come in place of the reducer. If I understand the optical implications good enough, a barlow will mess with the magnification of the optical train, but not with the shape of the field. Hence a back focus distance is not applicable. YMMV.

@@KopLamp thank you for the reply

The sensor is 6.5mm inside the main body of the camera. There is an 11mm ring attached to the front of the camera. That makes the total back focus to take into account for the camera 17.5mm. Most optical systems are designed to have the perfect focus at 55mm from the last lens element. So usually you can mount an included 16.5mm and a 21mm extender ring to that to get to the 55mm. The zwoastro website has some of examples mentioned in the specs for your camera.

I would advise to use astronomy.tools website to determine the image circle of your camera in relation to your optics and deduce from that if you can get away with 1.25”. I have no experience with this specific optics. I do everything on 2”

@@KopLamp: Thank you very much for the kind response.

I have no experience with an OAG. But from what I've seen, the OAG basically splits the light from the telescope towards the two camera's. Both lightbeams need to reach that correct backfocus. So in less words: yes ;)

You’re welcome

It is not dependent on the camera, but on the optical system. The lenses determine where the focal point is and where you should put your camera sensor for best performance. Some camera's such as DSLR's have their sensor deep inside their body. That makes that telescope manufacturers make their focal point be some distance outside of the backend of the telescope. Usually this is 55mm backfocus. Focal Reducers as shown in the video have their own backfocus. The one shown has a backfocus of 105mm (as found on the manufacturers website). Some telescopes, such as the Celestron EdgeHD can have a different backfocus when used without a reducer. See for instance page 13 in this document: s3.amazonaws.com/celestron-site-support-files/support_files/edgehd_whitepaper_final.pdf

People say it is needed. I never noticed a difference although I must say that I either shoot with the L-eXtreme or with the L-Pro. They might have a similar effect…

by using filter, I also need to plus 1mm (55mm to 56mm) for back focus.

Finding the correct back focus is an iterative process.

In a perfect setup, one would not need a guidescope indeed. But, there are always inconsistencies in the tracking. Due to mechanical imperfections or just simply by a not perfect polar alignment. Active guiding will keep the scope pointed at the exact position throughout your imaging session. By the way, leveling your mount is not necessarily the cause of tracking errors. In tracking, only the polar alignment is of meaning.

@@KopLamp Thank you so much for your time and for sharing your knowledge. I appreciate it very much.

I'm afraid I don't have experience with Newts and coma correctors. Perhaps this website's example images of how to mount things gives you a few hints how to achieve focus?

The nose piece I had already and came with a T2 ring. The 2" SCT adapter is this one: www.teleskop-express.de/shop/product_info.php/info/p240

I have no clue about barlows, but I would guess the same. I am in the process of scripting a new version of this, in which I dive in to the world of fitting an OAG in the mix (as well as doing things on EdgeHD)

I’ve done some reading and it appears that a Barlow does not call for an exact distance in the same way as a flattener would. A flattener under- or overcompensates when not used with a proper backfocus. For a Barlow the distance only effects the zoom. So you’ll best determine the maximum your scope can provide and stick with that.

I have received your image. Thanks for reminding me. I have 100+ gigabytes of data to stack, so I am a bit busy. Also I want my newest video to be finished... so much things to do. In case of not using a flattener it depends on the used telescope. In most cases there isn't a specific backfocus to reach. Then only important factor then is achieving correct focus. It is the flattener/reducer that induces the need for a proper backfocus (at least to my understanding)

@@KopLamp I wish someone in there videos would start with (If you do not have a field flattener you do not need to do this )..... I think that would have saved me 3 years of trying.. LOL

Mostly L-Pro and L-eXtreme

As far as I know, this telescope does not have specific needs for a certain backfocus other than reaching focus. There is no corrective lens such as is the case with an EdgeHD. Therefor reaching focus is your only task.

@@KopLamp thank you. I will let you know how it goes!

I never did this, but that is because I am either using an Optolong L-Pro or the Optolong L-extreme. Secondly my scopes are fairly slow and my cameras are not full frame. That gives me some leeway

I use the ASIAIR and it doesn't do a gradual warming of the sensor. So basically I just let it warm itself on the ambient temperature.

@@KopLamp Thank you...

Can you achieve focus without the flattener? Also I think with a Newtonian a flattener designed for a refractor telescope will not work. For Newtonians you’ll need a coma corrector instead of a flattener.

@@KopLamp I guess it is like this....thks for answering

@@KopLamp and yes, I get focus without the flattener, I just want sonething to avoid weird stars in the corners so that I don't have to crop the photo....

You’re welcome?

The filter only needs to be put in the drawer once. It is advised to have separate drawers for each filter you own. Only then you get the benefit of changing filters without having to deconstruct the imagetrain every time. Yes, screwing in the filter is a bit cumbersome. I suggest doing that indoor and at a slow pace.

Depends on the telescope. If it is an EdgeHD you might want to look at my most recent video, which details that. In case of a non-EdgeHD C8 it doesn't matter that much I think. Your field edges will not be pinpoint stars anyway.

@@KopLamp a c8 as this is a c8 video :)

@@CDigata Fair point. Well, the EdgeHD and C8 name are used interchangeably. Even by Celestron themselves. See this page for some advise on Back Focus distance with a C8. www.celestron.com/blogs/knowledgebase/what-are-the-optimum-back-focus-distances-for-celestron-scts So, 5 inches or 12,7 centimeters...

Thanks

Correct. I was using the classic C8 at the time of shooting that video. My EdgeHD came later and has a dedicated reducer for it (0.7x)

back focus spacing to be exact. But most just refer to this as back focus.

@@KopLamp Actually that's a perfect description Back focus spacing