Excellent video, saved me heaps of time researching and measuring all my spacers to achieve proper back focus on my AS294MC.

Exactly what I was looking for! Love your videos!

I got a RC6 + reducer, thanks to this video I figured out how to attach it in the optical train between camera and focuser, thanks a lot

Thanks for this. Just bought my first dedicated Astro cam and the focal reducer for my Nextstar 6 SE.. I needed this information to guide my setup! Just have to resolve a few issues with. R Pi and StellarMate and I will be good to go hopefully this weekend! Clear skies all..

A big thank you for your video which helped me with the backfocus of my C11, a clear and precise video, hello from France

Cool image of the Sombrero... nice informative video 👍

Thanks for the informative video. It was a great help.

Thank you for this excellent video. Back focus and polar alignment are the 2 things that intimidate me about this hobby.

I can't remember if I commented on this video already. I keep coming back when I am ready to image with my C8 Edge HD. It has been very useful for me. Thank you!

yeah nice one, just got a zwo1600 plus filter wheel, currently using on a redcat51but I also have a SCT and focal reducer setup i've yet to use it on, found your vlog very useful and you've gained another sub

Just got a 6" Schmidt Cassegrain optical tube off a Celestron 6se to use with my equatorial go to mount and bought the T-adapter you showed in the video, assembled everything the way you showed and can barely wait for the rain to stop so I can test it on small, bright targets. Still waiting for my HyperStar 6 v4 to come, so I can test it on large, faint targets. Have only used Newtonian reflectors and small refractors up till now. Thanks a lot!

Just what I needed--thank you!

Slymin, awesome topic! very well presented too. Back focus is crucial on SCTs than refractors!

Thank you! I needed this!

Thanks dude that was a big help

When I started this video, I was HOPING you would say 105mm of backfocus haha. I've seen so many different answers on the cloudy nights forum. Glad you're using the same backfocus as I am. Keep up the awesome work!

Great video, absolutely saved heaps of time., thanks. I do wonder with 105mm back focus, is it worth leaving the imaging train assembled when storing the scope or best to disassemble it. Personally, I'd rather leave the imaging train intact, as less risk of dust bunnies getting in and potential of re-using flats. A worry however, long tubing with 1/2 a kilo of camera weight might introduce a slight bend in the train overtime.

I had the same question about three weeks ago and posted a thread on CN. I got all my stuff and got the 294 threaded onto the back of my 9.25, but was surprised at how much vignetting I was getting. Flats definitely helped but I was a little annoyed by having to use them for EAA.

Man this is exactly what i needed. Just fired up my asi462mc last night on a c11 and the stars looked like cotton balls. I have factory 12.5mm of backfocus lol. Off to buy some metal tubes! :)

excellent and to the point.

lol, i just fiddled around with my spacers ‘til it focussed… haha.. nice vid, as usual, well done.

Could you do a simple test with and without back focus from your backyard to a distant terrestrial object I.e. tall building or power line pole. Curious to see with and without scenario.

Why none of your videos mentions about adding 2" filters etc?

@0:25 105mm That's what I needed. That number was harder to find than it should have been. For anyone who just wants to know how far away you need the sensor to be from the focal reducer, you need to have 105mm between the focal reducer and the sensor. Thank you!

Thanks for this info I’m a newbie and I just ordered the ZWOASI183 non cooled. I’ll be using it with a Celestron 9.25 OTA. I have 6/3 focal reducer already and the and the T42 I’m just trying to determine all the other pieces I need to get to 105mm in looking at what comes with the ZWO I’m just not sure what additional items I need. FYI I should receive the camera later this week. Thanks!

Very nice thanks for the info

Could you do an instructional video on Backfocus with a planetary camera? Some of us newbies have just started. I have Celestron C8, a Nexstar 6SE, and a ZWO asi224mc camera. What is the backfocus requirement to achieve? Bonus points: Add a Celestron SCT 6.3 reducer. What's the backfocus requirements now? Same distance for both scopes, but what is it and how to achieve? Your content is very well explained in plain English, as well as well documented. Thank you for taking the time!

Thanks for thios great video. I have a couple of questions please: How did you find out about 105mm being the specifric amount of back focus? Will the 105mm of back focus also apply to Edge HD focal reducers (for 8" SCT) or will they have a different back focus value? Thanks

Thank very useful video!!

Great video Slymin. I have a celestron f6.3 focal reducer and I am about to buy a ZWO colour camera. I didn't realise the importance of backfocus so this will be very helpful.

HI Slymin - how can we achieve a back focus on a SCT when not using the focal reducer? Unfortunately, I am unable to find the T-adapter as all retailers are out of stock. Thanks!

FYI the information in this video - as well as ZWO's documentation - is not correct. Please see the comment by Chandrasekhar below (posted 2 years ago). The problem lies with the M42-to-M48 adapter ring. As it's only 4mm thick, it sinks ~ 5mm into the 9mm-deep female threads of the 16.5mm long spacer. So with everything screwed together snug, you're at ~100mm back-focus from reducer to sensor, give or take... NOT 105. The easiest solution I found was to replace both the 21mm spacer and the 16.5mm spacer/adapter with a single 37.5mm spacer (M42 on both ends, so no adapter ring required. Blue Fireball makes one, not sure if anyone else does). Another option is to stack another identical adapter ring on top of the first, which when inserted into the 16.5mm pretty much comes up flush (any variance could be made up with shims if desired). Or (ugly hack) you could use some sort of adhesive to permanently seat the adapter ring right at the top of the threads of the 16.5mm spacer, but then they're married for life. Or... a whole bunch of other ways I didn't both exploring since the 37.5mm spacer made things easy! ALL THIS TO SAY: We LOVE your channel! You've been an amazing source of information for us and we greatly appreciate your efforts. I wanted to put the praise before the correction, but I was afraid you wouldn't see it if I did. So thanks for everything - truly - but you really need to update this video ;)

Just what I needed to know . Where would I put my optolong L pro filter . And in you autoguiding scope video you have a dovetail for you C8 . Where did you get it I can’t find the right size . You make very helpful videos thank you . Oh I just bought the 394 mc pro for my C8 and spacecat. Time to learn Nina.

Hi and thanks for all the good advice you give. I have a question. Can I use this setup on Celestron NextGen 6 with ZWO 224?

Excellent video very helpful, one question if I want to introduce 2” filter where can I place it and what accessory I need. Thanks for your help.

Hola, saludos desde chile, gracias por tu video, esta muy bien explicado , pero tengo una duda como sería la configuración de si pones la misma cámara pero con un porta filtro?, ojalá me ayudes

Awesome video. So with DSLR or mirror less, you only need the SCT adapter right? What if I want to add a 2” filter to this?

hi!, nice video!!! only one question please: where does the pollution filter go ???

You saved me from not being able to use my camera when it got here. 93633-A should beat the camera by a couple days.

Thank You. I will attach filter slider means I can remove 21 mm adapter but I don’t have focal reducer so will it work and what about Starizona hyperstar ?

Thank you for your videos! I was wondering - what would you use as the back focus without a Celestron f/6.3 Reducer/Corrector? i.e. So only the C8 SCT (non-Edge) with the ASI294MC Pro camera?

Not sure if anyone has had this issue - I have Celestron CP 925, same focal reducer (Celestron f/6.3), same camera (ASI294MC Pro) and after building the 105 mm back focus was not able to get into focus. Was in a hurry yesterday when trying to do this, will give it another try today.

Is there a chart somewhere, on all the different cameras? I could not find what my canon m-50 back focus is from the sensor to the outer ring of the body mount.

OK, I have a SCT 11" and the f/6.3 focal reducer/corrector. I just stuck a crayford focuser on the tube and no problems.

I tried this same setup on my Mak180 but it seems the adapter ring screws all the way into the tube, (is it supposed to go all the way into the 16,5mm tube?) so I screwed it into an adapter ring and then onto the tube so that it fits snugly on the tube and does not move. Maybe that added another millimeter onto the assembly, but I got the best pictures yet on the moon. First time they have been in crisp focus with my ZWO533, have yet to try my planetary cameras and DSLR´´s.. Great help..

Great info .. how did it work with the larger sensor of the 294MC? Vignetting any issue?

Great video. I have a C8 and want to use an ASI 2600. Would I follow the same back focus and use the same train?

Hi! Thanks for the video!! 🙂 At ZWO's website they mention a 55mm back focus. For what purpose this 55mm backfocus applies? A SCT without FR? For SCTs with the 0.63x focal reducer (C8, C11, etc), the back focus changes to 105mm for every camera? Like the 533MCPro or even uncooled cameras like the 585MC? Thanks!

Great vídeo! Maybe a silly question! This back focus number can be used in f/10 mode?

The "magic number" you mentioned was 105mm. IF you/I were to connect an ASI224 camera with the 1.25" eyepiece adapter, WITHOUT the focal reducer, would the standard focus travel eliminate the need for any other extender "add on"? Any suggestions will be welcome. Thanks, Hank P.S. Although the use of the Celestron reducer you use is the "best" way to reduce the FOV, Would an eyepiece screw-in reducer work or do they just not provide the image quality of the Celestron? Thanks again....

Loved to know where these numbers come from. 105mm w\6.3 reducer and 127mm without. Is this listed somewhere ? Do these number work for all F/10 ratios ? except for EdgeHD scopes which have a built in reducer.

I'm curious if you found something official stating the backfocus for the reducer/corrector? I did a lot of searching for that number and came up empty. Sadly, this video was not yet published at that time! I achieved the 105mm (105.5 actually, which is ok because I have 2mm filters inline which should add 0.667mm if I understand correctly) and the images look much better than what I had previously, which was only working out to be in the 80mm range. There is still a little distortion in the corners, so I wanted to ask -- is that your experience as well? I'm on a C8 going into a micro 4/3 sensor. I'm assuming this is the best this setup can achieve, but I would love to know if you or others are experiencing the same. It is really quite good now that I have it set up properly. Thank you for providing this elusive number!

I have a nexstar 5se. I have the 6.3 focal correcter with a .5 focal reducer and a ZWO 120mc camera. What back focus do I need for this image train?

I followed your instructions to have 105 mm back focus on my 5se with 6.3 focal reducer. However when I balance the telescope I have to move forward the whole telescope to balance and now only one bolt holds the dove tail. Do I need a longer dovetail now and what model do I get. It’s a CG5 dovetail and I have AVX mount.

Slymin, I have the same adapters as you, with the 16.5 overlapping the Celestron T adapter, what adapters are you using to compensate for that? I'm just curious what your set up is.. thanks alot, subbed BTW

I have three 1.25 filters moon sky glow, UV-IR and UHC. Do I thread them on top of the 294 or somewhere in the image train. Is the 294 opening able to thread a 1.25 filter. My ZWO is still shipping, just getting ahead of myself. Thanks

Great video Simon - very useful. Question: would you ever consider attaching the ZWO camera after the star diagonal ? I am using a standard mount with a wedge and don't have a lot of clearance left with the focal reducer and 105mm back focus. Thanks, philippe

@AstoBlender Hello Slymin, thank you for your informative instructional videos. My Google-Fu has failed me. How does one calculate the effective back focus distance of a diagonal viewer in mm for an application like the one you are presenting here? Obviously the goal is 105-110mm, but not sure how to measure the diagonal to include into that measurement. I have an Celestron 8SE with NexStar mount and it requires me to use an diagonal for clearance reasons.

Slymin with the back focus 105mm with the focal reducer would that be the same with a zwo as1224mc camera too. 105 mm ? With the reducer use with any camera?

How /where would you add a 2" IR cut filter in this set up? I can bury a 1.25 inch one ( with adapter) in the 17mm extender, but I am concerned it will impact the corners of the image.

Been meaning to ask how you like your C8 with f6.3 for DSO imaging? I have a f6.3 reducer/corrector in my desk and am thinking about buying a celestron C8. I can’t justify in my mind springing for an Edge 8 plus 0.7x reducer given the cost difference and the fact that I already have the f6.3 reducer/corrector. You happy with your DSO imaging with your C8, f6.3 rig? Dr B from Manitoba, Canada 🇨🇦

Friend, I'm a new subscriber to the channel and I like your videos, congratulations on the channel !! I realized that there are videos on the channel with the celestron c8 telescope, I just purchased one and I would like to know if there is a camera for the astronomy market attach to 2-inch eyepieces? I have a 2-inch 31mm tele vue and I would like to attach a camera directly to this eyepiece is that possible?

I have a question. To shoot without the focal reducer, do you still need the T-ring adaptor? Basically just removing the focal reducer, then attaching the T-ring adaptor to the telescope? Seems longer than 55mm focal length since the T-adaptor is 55mm and the 16mm and 21mm are 55mm?

The adapter - your mileage may vary, but the thread direction (stepping up/down, as one way to look at it) needs to be looked at closer (i.e., it's not just any M42 to M48 adapter). Using an ASI224, whose back focus is 12.5, necessitating an additional 5mm if using the 16.5 and 21mm spacers if all just threaded together as is, the adapter is "M48X0.75 Male/M42X1 Female to M42X0.75 Male Telescope Adapter" (for search purposes). And it's actually pretty close if just using the adapter and the two M48-sized spacers because (at least on mine) the adapter doesn't thread on all the way. Maybe it does, I just didn't want to force it based on some product reviews of the rings being stuck together forever if threaded too tightly/or the fit is off of a bit. A common one advertised that has "M42-M48" labeling on the ring is not the correct adapter. This is based on using the same Celestron T-adapter, focal reducer, and 16.5/21mm spacers as shown here. A "Plan B" setup can be achieved using the same narrower spacers (all M42). The SVBONY 5/10/15/20 kit (M42 both sides), plus a 24-35mm extender (it screws in and out, use some tape to keep the barrel from rotating once the width is set) that is set to account for the .5mm difference to get to 55 works. Precision calipers can be used to get the 27.5 on the extender when using 15mm worth of spacers. (12.5 camera, 27.5 extender, 15 in spacer rings, for a total of 55). But, this video definitely got me close to the approved solution for replicating this setup. Thank you.

Hi there Really learn a lot from your videos. How important is it to have the exact back focus? Suppose it is off be up to a few mm, in what ways and how much will the image be degraded? Thanks

Thank you Slymin, nice video and beautiful M104. I have a Question for you... What about the Barlow lens Luminos ? I mean , do you know what the backfocus is when you use the barlow ? Thank you

Do you need to use focus motor to get sharp stars? Or does 105mm fd train make them sharp right away? Thanks

Hello I have celestron nexstar 127slt mak scope. What are the accessories I needed to attached focal reducer and my astrophotography zwo 462mc please 🙏

Question: I have a Nexstar 8SE. I just bought a ZWO Asi224mc. Do I need any spacers to get proper back focus with or without the celestron focal reducer?

I Love this channel. I just got my first telescope which is a Celestetron 5SE. I've been practising taking photos with my DSL on Jupiter and Saturn, and making pretty decent progress. I also picked up an ASI120MC Camera to try. I've seen both 55 mm, and 105 mm referenced for back focusing this camera. Does the focal reducer dictate the use of the 105 mm distance as opposed to the 55 mm spacing? It is still quite confusing for me and any help would be appreciated.

Slymin...do you screw the adapter ring all the way in as far as it will go and then screw in the T adapter? In your videoI couldn;t tell how far in the ring goes.

So, here's a question for you. We know from your video that the 6.3 reducer requires 105mm of back focus. You also said that a DSLR usually requires 55 mm. But, how do you determmine what the back focus requirements for other cameras/telescope/imaging trains requires?

Great video Slymin. Very informative. Great job. What would you do if no reducer in the setup? C 8SE with ZWO ASI183MC. Many thanks and keep the videos coming.

Than you. Can you explain why we need to set the camera at the exact backfocus point when we can just turn the focus knob or move the draw tube until we reach focus. What is the difference between focusing by moving the mirror/ draw tube and having the camera attached at the exact back focus distance.

I have a ZWO ASI290MC camera with a Celestron Evolution 6 and the 6.3 reducer. What am i suppose to use with that to connect it to the scope and get proper back focus? I also have a GSO Crayford focuser i would like to use. Anyone have any ideas how i can set this up? Thanks

Hi there, will this work the same for SVbony sv205 camera? I guess the sensor depth should be pretty much the same.....

great vid thanks. If you reverse the order of the 16.5 and 21 mm spacer you dont need the m42-m48 ring.What guide scope you using at full focal length btw? Look like a beast

He made a mistake. The ZWO M42-M48 adapter ring is designed to connect the ZWO 21 mm M42-M42 extender and the ZWO 16.5 mm M48-M42 extender. If you use the ZWO M42-M48 adapter ring with the Celestron T-adapter for SCT, you will get a back focus that is smaller than 105 mm because the Celestron T-adapter for SCT flange is too narrow to make contact with the M48 flange of the ZWO 16.5 mm M48-M42 extender. So the correct order of connection is: Celestron T-adapter for SCT --- ZWO 21 mm M42-M42 extender --- ZWO M42-M48 adapter ring --- ZWO 16.5 mm M48-M42 extender --- camera.

Why do you need the focal reducer? Just to get a wider angle for imaging?

what happes if i use a diagonal? Wont the diagonal consume focus length as well?

Great video and easy to understand the back focus concept. Well done! I've a question about the SCT primary mirror. As you know, when you're focusing the image this mirror is moved up & down which implies a modification of the f-ratio. My question is, does it have any impact in the back focus distance where the primary mirror is located or on the contrary it is advised to pull it down as much as possible?

Wow! Thanks! You just saved me a lot of frustration. If you do not use the reducer, would you still calculate 105mm of back focus? Is there a mathematical formula to calculate the back-focus? Keep up the great content!

what abourt using DSLR + reducer 0.63 + AOG celestron ?? can i make 105 mm back focus with this set up ?

Curious if you've ever done a plate solve with the 0.63x on your non-Edge SCT? I have a C9.25 and when I plate solve with my backfocus set to 105mm I get a focal length of ~ 1320mm (160mm off the theoretical 1480mm which I would get if I was getting the 0.63x redcution). So I'm getting 0.56x reduction which I've read others seeing on CN forums. Can you explain why this is? My backfocus is off by a max of 1mm if at all, I'musing 50mm T-adapter, 16.5mm extension, ZWO filter drawer (21mm), and my asi2600mc (17.5).

How would I incorporate an Optolong l pro filter into this setup?

Hi, in this setup, where do i place the 21mm zwo oag ?

friend I have a celestron nexstar 8se and I would like to put that adapter and an extra telescope what is that type of adapter called

Is it the same lenght for a Meade sct lx90 10” telescope? 🇩🇰

what if you're using the ZWO filter wheel?

Hello, I have a black Celestron cpc1100 tLX GPS, can I install a Reducer Lens .7x - cpc 1100 hD

Hello Simon: You don't say which telescope you're using in this video. The OTA tube is BLACK in color but I've seen you using an OTA Carbon Fiber EdgeHD and the standard C8" OTA in your video's.. .... So which witch is which?

Hello, Do you need this extension if you are not using the focal reducer? I just ordered the Zwo ASI294 for my Celestron 8 in evolution and am curious. Thank you

Coming off the reducer are the fittings M48?

Use the 42mm-48mm adapter between 21mm and 16.5mm spacers and then T adapter - it will give you exact 87.5mm

Hi, exellent video. @4:23, are you using your piggybacked refractor as a guide scope?

Have had my QHY163C working fine with 1.25" diagonal and camera, why useca diagonal you ask? As its an alt alz mount the camera sticking out, you on can get the 63 degrees before the mount hits the mount. Have even used the 2" diagonal and focuses also and tried both options in the day

Great video, thank you. Quick question, please? When you use the 42mm-48mm adapter it goes some distance into the adapter (by at least 5mm!) so how does this not compromise the whole 105mm measurement?

Hi, this is going to sound absolutely stupid I guess. I am into EAO and use a Celestron C6 SCT with an Atik Infinity camera. So far I have simply inserted my Atik onto my C6 using a Baader Click Lock and just focussed. Just like I do when I go visual and use eyepieces. Now I learn about this 'backfocus' and wonder what have I missed all this time. I found out meanwhile that the backfocus of my Atik infinity is 13mm. If I use extensions to get to this 105mm (because I also use a Celestron focal reducer 0,63) will I see a difference? What's the purpose of the focusknob then? I'm confused. Sorry.

Hey Slymin, I finally got my Celestron Evo 8. Bought a Rebel T6 to get started in astrophotography. In order to connect my camera to my telescope are the only things that I need are T adapter and a T ring?

How is the back focus affected if you add in a filter, would you add a few 1mm spacers? Great video though!