Backfocus with an ZWO Cooled Camera

Backfocus is the distance where the corrective lens of the flattener produces the optimum image quality onto your camera sensor. My refractors all had a backfocus of 55mm's and my Celestron C8 has a backfocus of 105mm.
In this video I show how I mount my image train to have the correct backfocus.
Een Nederlandse versie van deze video staat hier: • Backfocus met een geko...
#asiairpro #zwo #optolong #ioptron #cem40 #celestron
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[@jr10]

Excellent review. This is the best explanation of backfocus and how to set it up that I have seen yet. Thank you for sharing.

[@mehmetkeremeser9757]

One of the most useful videos I've ever seen on youtube for some time!! 👏

[@Oscaro9928]

Absolutely wonderful clear explanation!! Thank you

[@KopLamp]

You’re welcome

[@tallperson9422]

Exactly why I, as a beginner was looking for. Thanks you so much for this video.

[@KopLamp]

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

[@lukomatico]

Great job Martin :) - keep up the hard work, I'm sure many people will find this useful over time! 👍 Loved the bit of silliness at the end haha 😂

[@KopLamp]

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

[@NikonJax]

Awesome! That was very helpful. Thanks :)

[@KopLamp]

You’re welcome

[@abhijitjuvekar]

Thank you for well-explained video. I have 1600MM-PRO and use GSO 2 inch Coma corrector with 70mm backfocus. So with your method I can now correctly figure out which adapters I need to get that distance correct.

[@KopLamp]

Glad to see my video helped.

[@jordanphillips3611]

Awesome 🙌🙌

[@avt_astro206]

Great tutorial Ones Again Mate!!👍

[@KopLamp]

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...

[@SlimLineHouse]

Excellent video Martin very informative 👍

[@KopLamp]

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

[@SlimLineHouse]

@@KopLamp I'll look forward to it👍

[@michaelm.7389]

Great information......thank you.

[@KopLamp]

You’re welcome!

[@meus2196]

Very informative

[@KopLamp]

Glad it was helpful!

[@streetphotography.globetro7744]

Nice video...Thanks !!!! but is not needed as well to extend the total distance adding half of the thickness of the filter ? 2mm filter 1mm has to be added. ?

[@KopLamp]

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

[@reinaldoaltamura6898]

Thank you Martin for your suggestions. I have a question for you: I need mount my Canon 75-300 EF Lents with ZWO asi183MC camera, what backfocus distance I need to buy?

[@KopLamp]

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)

[@dfinchermd]

Very helpful in demystifying image train. What are the handwritten notations on your TV reducer?

[@KopLamp]

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

[@Dennis-tf2cs]

On my SCT I use the ZWO OAG mounted directly in front of the filter drawer on the ASI294 like you have. How do I get my guide camera to focus? Will I also need 105mm from the focal reducer to the sensor on the ASI120mm camera on the OAG?

[@KopLamp]

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 ;)

[@jds1776]

Thank you for this video ! Very clear ! And if I want to make planetary images using a barlow, how do I insert a barlow in this optic train ? (Simple 1.25’´ Barlow X2 for exemple)

[@KopLamp]

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.

[@jds1776]

@@KopLamp thank you for the reply

[@TheSagi777]

Hey martin thanks for the video it help a lot can u pls guide me frm where u bought 2”sct adapter and nose piece for celestron reducer -TA

[@KopLamp]

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

[@dlindsay1797]

I found your video in my search to understand back focus. Viola! I see, said the blind man. Working with a new WO FLT 91 and ZWO camera and WO is no help whatsoever. Support and documentation is a thing of the past, I guess. Love the telescope, but the support stinks. Thanks!

[@KopLamp]

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 KZbin) and forums like CloudyNights are helpful too.

[@an35tares]

Hi I have a question ! I get a newton last year (200/1000 mm celestron) and a camera (294 mc pro) ! I followed the instrutions of zwo for the back focus (camera, 11 mm, Drawer filter, 16,5 mm and baader coma corrector. However I'm unable to get a focus with this telescope ! I tried with a 2 po adaptor and without... but nothing to do ! I suspect that the focus train is too long ! The only way I got the focus is to remove the 16.5 mm but the stars is like a potatoes same if i have a baader ! Should I change the train focus of the telescope ? Have you another solution ?

[@KopLamp]

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?

[@lorenzodefrancesco4819]

very useful; but I have a ASI ZWO 533MC camera that on tech spec states the backfocus is 17,5 mm instead of 6,5mm is this correct?

[@KopLamp]

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.

[@chazzey7]

Do you adjust/increase your backfocus to take into account the change in reflective index caused by the filter glass?

[@KopLamp]

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…

[@leftoff3rd]

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

[@KopLamp]

Finding the correct back focus is an iterative process.

[@jeffreyvictor3678]

May I ask where you find the data for the necessary back focus for each camera?

[@KopLamp]

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

[@DeereX748]

Does your filter glass not change the optical back focus, even though the mechanical back focus is correct? Isn't the rule to add 1/3 the thickness of the filter glass to the mechanical back focus? It may be just a millimeter or tqo, but will make a difference.

[@KopLamp]

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

[@RaviHegde1975]

Very informative video. I need a suggestion from you. For the f/9.8 (102/1000mm) refractor, which is the suitable 0.5x focal reducer (FR) for astrophotography using ASI294MC Pro camera? 1.25 or 2 inch FR?

[@KopLamp]

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”

[@RaviHegde1975]

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

[@allenbaylus3378]

slightly off topic here but which filters do you use on your Color camera on this setup?

[@KopLamp]

Mostly L-Pro and L-eXtreme

[@massimomelodia3670]

Thanks for watching ?? Noooooo....Thanks to you for explanation !!!!! Great !!!! 👍👍👍👍

[@KopLamp]

You’re welcome

[@brendanforward7322]

Awesome video, question do you need a filter for a ZWO colour camera?

[@KopLamp]

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

[@NMagturo]

What would the backfocus be if I didn't use a flatner? I am attaching a ZWO ASI385MC to a Celestron CPC 8 SCT without a FR/Corrector? Thanks for your video.

[@KopLamp]

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.

[@NMagturo]

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

[@michaelm.7389]

Hey Martin, good morning. Hoping you can clarify this for me. I have the ASIAIR Plus and the Explore Scientific iEXOS100 equitorial go to mount.....with an astrotech AT102ED refractor.....why would I need a guide scope ? I thought lining everything up was the purpose of the ASIAIR Plus ?? I have gotten pretty good stacked images so far but last night it would not keep the target centered ?? Thinking I might have not checked the scope for being level before I started. Thanks for your time.......Mike

[@KopLamp]

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.

[@michaelm.7389]

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

[@Steve_The_Ignorant_Astronomer]

Martin Back focus is my bugaboo too. 2 questions , I dont use a flattener so where do I measure from, have to same camera as you , the 294 ... Second question , have you been able to get to the picture I sent you for stellerium ? Thanks for the vids

[@KopLamp]

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)

[@Steve_The_Ignorant_Astronomer]

@@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

[@jjaimesmx]

Excellent Video!!! so I'm assuming that same rule would apply to the Barlow Lense right?

[@KopLamp]

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)

[@KopLamp]

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.

[@travelanddrones]

Hello, great video. But my problem is that I cannot get the right focus with my Celestron 150/750 Newtonian tube + ZWO 294 Color camera + William Optics Z73A flattener...I guess it is because the flattener is W.Optics and not celestron...not sure though, I'm just learning these things in astrophotography , I hope you can help me with the right set up. Regards from Chile

[@KopLamp]

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.

[@travelanddrones]

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

[@travelanddrones]

@@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....

[@shutterstock2015]

Greetings from Kaliningrad!!! She came to me 533mm pro ... I want to ask you how long it takes to defrost it after cooling? After all, if you cut it down sharply, it will flow and can flood the sensor or short it out. Raise a degree per minute or 5 degrees per minute or something else ????How is it right? Lower the temperature, too, after all, gradually, so as not to shock it?

[@KopLamp]

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.

[@shutterstock2015]

@@KopLamp Thank you...

[@okeania]

Excellent video but the problem is changing even attaching filter to Zwo Filter drawer terrible pain in the ass. I cant put my finger and rotate filter to fit to drawer. Whether Im terrible at it or its worst invention. My L pro and L extreme got finger prints because of Zwo Filter Drawer. Im using nose adapter instead.

[@KopLamp]

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.

[@CDigata]

whats the optimal BF on a C8 without fl/reducer.. 127mm?

[@KopLamp]

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.

[@CDigata]

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

[@KopLamp]

@@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...

[@Hughejazshole]

Thank you?

[@KopLamp]

You’re welcome?

[@robertsipes7391]

Ditto to jr10's comment!

[@KopLamp]

Thanks

[@mauricezundel2333]

L'objectif réducteur/correcteur #94175 n'est pas COMPATIBLE avec les systèmes optiques CELESTRON EDGEHD.

[@KopLamp]

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)

[@Mistr_A]

backspace not backfocus

[@KopLamp]

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

[@Mistr_A]

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