Back-Focus 101: Celestron SCT wIth Celestron f/6.3 Reducer/Corrector

[@IgnazioPillitteri]

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

[@dr.joeluciani1482]

Exactly what I was looking for! Love your videos!

[@shawnharvPhotography]

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!

[@AstroBlender]

Celestial Photography Yes, 105mm works great. At f/6.3 and depending on the chip size you could probably get away with +- 5mm. With smaller imaging chips it doesn’t have to be as precise at this focal ratio since you aren’t capturing as much of the edges.

[@shawnharvPhotography]

@@AstroBlender Exactly! I use the Starizona filter tray in replacement of the 11mm ring, so im slightly over 105mm.

[@AstroBlender]

Sounds like a great fit!

[@tomwall8403]

I have an ASI camera and the two spacers. However, I have a standard sct visual back. Do you know if that gives me the same back focus distance? Thanks.

[@jonnyreverb]

@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!

[@universewonders1]

There were alot of reviews online with lots of coma at the edges with the reducer, your sombrero looks nice and flat to the edges. This gives me hope to buy one

[@JodysGotYourCadillac]

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 ;)

[@Nabby13]

You can simply reverse the order of M42-M42 and M42-M48 extenders and use the M42-M48 adapter ring in between them to solve this issue.

[@steppen57]

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!

[@AstroBlender]

No problem. Just remember the C8 EdgeHD backfocus is only 105mm if you’re using the Celestron .7x reducer lens.

[@didiersylvie4216]

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

[@FuRyZee]

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

[@davepastern]

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

[@luckykel2030]

You needn't be intimidated about backfocus. You only need to worry about that when using a focal reducer. And it is straight forward as every focal reducer declares it own backfocus. Just get tubes and put the distance between the reducer and camera. Simple. For polar alignment, I used to be intimidated before until I found easier methods. Polar alignment is no longer as hard as it used to because of technology.

[@davepastern]

@@luckykel2030 still for the newbie, it is very intimidating. Different flange diameters, different spacer widths, having to potentially interconnect differing m sized flanges to each other with step up/down joiners etc...

[@monkeypuzzlefarm]

Get an iPolar for your mount. Takes a minute to polar align.

[@davepastern]

@@monkeypuzzlefarm I already have a Polemaster!

[@bradsnell7676]

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..

[@AnotherAmateurAstronomer]

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!

[@Rurikovihhc.]

на Celestron 6se тоже требуется 105мм удлиннитель?

[@RaysAstrophotography]

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

[@AstroBlender]

Ray's Astrophotography Thabk you sir!!

[@RaysAstrophotography]

@@AstroBlender Cool!

[@LaNa1174]

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

[@mw2077]

You’re a life saver man seriously

[@MrGChuff]

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

[@AstroBlender]

I'm glad you enjoyed it! I envy you, I'd love a SpaceCat51. Thanks!

[@marvhalsey7544]

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!

[@larrycable984]

You should always put the 42-48 adapter ring in place with the notches in the ring to the outside to aid in its removal

[@truckernige]

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

[@AstroBlender]

Haha well that works too!

[@stabbiemchobo]

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.

[@robertn7007]

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.

[@AstroBlender]

Robert N You’re welcome!

[@mattestabrook]

Just what I needed--thank you!

[@stecal12]

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.

[@okeania]

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

[@jcims]

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! :)

[@GaryMCurran]

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?

[@reidchandler6486]

most cameras, as you now probably know but for those who are still new typically are 55mm and offer advice on adapters needed or how to put together the image train and adapters as needed. ZWO does a great job with photos and spacers in diagrams for your camera/s depending on what you use.

[@GaryMCurran]

@@reidchandler6486Thank you.

[@Nabby13]

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.

[@charlesburlison-rush1281]

Yes spot on. The video is wrong with the order in which things need to be connected in and what you have said is correct.

[@TheUrbanAstronomer]

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

[@AstroBlender]

The Urban Astronomer Thanks glad you enjoyed it.

[@bb108]

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!

[@kenn7915]

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.

[@bobbeanbags]

Thank you! I needed this!

[@AstroBlender]

Robert Bean No problem!

[@richardpeterson7201]

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

[@dannyfurdon]

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!

[@fletch591]

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?

[@AstroBlender]

It does, you should compensate for it. I failed to do so in the video.

[@eduardobrandao4307]

@@AstroBlender , I had the same question and almost missed the answer. You have the obligation to correct this fail somehow for future watchers that will increase your income from KZbin...

[@rohitastrofpv7975]

@@eduardobrandao4307 use the 42mm-48mm adapter between 21mm and 16.5mm spacers and then T adapter - it will give you exact 87.5mm

[@rohitastrofpv7975]

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

[@danjensen9425]

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.

[@psegre]

Watching this 2024. Thanks for the help, but you give two different definitions of where to start the measurement on the focal reducer. At 0:29 is one version, at 4:00 you give a different version. They differ by the width of the reducer threads, about 10mm, a large amount. I’m guessing the 4:00 version is correct? Either way, it’s ridiculous how un user friendly this whole process is, and who knows if the “105mm” is really based on anything.

[@1fastbunny]

Thanks dude that was a big help

[@robertbennett106]

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.

[@GraemeFothergill-dz8di]

Hi, I'm a complete newbie and struggling to understand the science behind this hobby. I've brought a Celestron 9.25 Edge HD and have mounted it on a Skywatcher EQ AZ 6 GT Pro. I have a ZWO ASI 585mc as my primary camera and a ASI 220 as my guide camera fitted into a ZWO OAG. I have a Celestron f6.3 reducer/corrector. I'm reading that you can't use it on the 9.25 edge or is it that you can but it effects the quality of your photography. My budget won't stretch to £440(UK) for the 9.25 Edge reducer.

[@durranaik]

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.

[@Muesli711]

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

[@AstroBlender]

Yes, but I have since switched to off-axis guiding. It’s much better!

[@fermingomezc]

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.

[@cmas-astronomy4715]

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.

[@mostafametwally1]

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.

[@todddunnavant5984]

The proper back focus gives you a flatter image on the sensor. Your stars will be rounder across the entire sensor image.

[@spr1457]

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?

[@fernandomagalhaes4842]

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

[@axford696969]

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

[@AstroBlender]

Yes filters affect back focus. Check out my ‘How Filters Affect Back-Focus’ if you want to see the calculation necessary.

[@andrewkozhanov6800]

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.

[@nightsky8585]

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.

[@gunnarjensen5910]

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..

[@ply17410]

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

[@briankotak403]

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 🇨🇦

[@angelo327able]

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?

[@jamesdougan8789]

g`day slymin good video that 105 mm measurement(for the 6.3 focal reducer) is a hard one to find i had to do a bit of research to find that out the normal back focus for a celestron 6"SCT is 127 mm (without a focal reducer) and a fun fact is that for the edgeHD 8" with the 0.7 focal reducer is also 105 mm or 133.35mm without a focal reducer..... hope these numbers help someone someday cheers james D

[@AstroBlender]

James Dougan Yep, it’s nice the classic and Edge focal reducers are designed with the DSLR standard of 55mm + 50 in mind. It makes it real easy.

[@alalal1979]

Good man James 👍

[@jetpipe28]

How did you know you needed 105mm required????

[@clearskiesastro1028]

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).

[@michaellkovacs]

I'm putting this exact setup to the test tonight. I'll let you know how it goes.

[@KJRitch]

Impossible to get the correct back focus of 105mm if using the Celestron OAG and DSLR. The SCT adapter is 25.3, the OAG body is 29mm. The M42 adapter is 12.5mm. The DSLR T ring and DSLR is 55mm. Total is 121.8 mm back focus. If Celestron included an OAG adapter to replace the 25.3 mm SCT adapter. You can get close to 105mm using a dedicated Astro camera that has 17.5mm internal back focus. Add 15mm spacer gets you to 105.3mm. I haven’t taken any photos yet with my 8” SCT. I guess the stars around the edges of the DSLR will be distorted somewhat but don’t know how much. Maybe cropping will eliminate them from the photo.

[@jodyschultz5870]

OK got it but what if you are NOT using the .63 reducer?

[@VikramHarinath]

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?

[@slzckboy]

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

[@chrislee8886]

Sadly this does not work. If you swap them round the supplied 16.5 42M-48F does not then attach to the T-SC adapter.

[@cellelephant6423]

What is the scope on top? (At the end of the video, there is a scope mounted on top)

[@marcelorsc]

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!

[@henri-julienchartrand3387]

excellent and to the point.

[@robinbarmore549]

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

[@polaco_cosmico]

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

[@terrydufek9635]

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

[@TheMalbroughs]

Would love to know this also...

[@philippefossier7178]

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

[@Felldisulfide]

Hi Philippe, there are many threads on Cloudy Nights on the subject with people falling into two camps: either use a diagonal, or a dovetail extension. Both have their pros and cons. Personally, I use a dovetail extension with a 'counterweight' of sorts. I've been running like this on a wedge with guiding for some time. CS!

[@mohammedgalaxy9131]

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

[@monteroder2708]

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.

[@doco6186]

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

[@mauriziorvt]

Hi docO. I'm using a C8 with an ASI533MC. Eventually the BF distance I'm using is 135mm from the F:6.3 Celestron reducer. This results in a faster F:5.6 ratio (1140mm Focal length) and in shorter exposures. There is a little vignetting in the corners but it's easily corrected by flat field. Stars are pin point and in focus almost to the edge where there is some (very little) coma. The IMX 533 sensor dimensions are: 11.31x11.31 mm (16 mm diag.). My experience is that you can exceed the nominal distance up to 20 mm, if the sensor is not very large as an APS-C, still having a good picture quality. The field of view in this configuration is pretty large, 34'x34', so you can frame the whole Moon or Sun.

[@eaojnr]

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

[@sabikiss]

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.

[@figzastro-photography]

@Sabi Kiss did you ever get the answer to your question?

[@sabikiss]

@@figzastro-photography nah .. still waiting lol

[@CaptainCaveman60]

i 'think' it's 127 mm. Celestron says 5 inches

[@arshadwm]

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 ?

[@HollomanUFOLanding]

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

[@pabloazurdia]

Great video, question; Do you need to add a field flattener? or does the f/6.3 reducer acts as one?

[@AstroBlender]

The Celestron f6.3 is a reducer and flattener. The EdgeHD models it’s just a reducer as there is already a field Flattener in the baffle tube of the scope.

[@pabloazurdia]

@@AstroBlender I appreciate the feedback and response! thank you

[@gwzapo]

Great vid but forgive me, please....I'm confused with the back focus.....so if my 294 camera manual states the back focus is at 6.5mm. Is this something different? I have this same camera and two SCT telescopes. I just want to get this sorted out. Thank you for your vids and you're great at explaining things!

[@AstroBlender]

The 294MC Pro has 6.5mm of back-focus, but the black ring on the front adds an extra 11mm, so it's at 17.5mm total. If you unthread the black ring, then you'll be at 6.5mm.

[@gwzapo]

@@AstroBlender ok....thank uou so much for your quick response. I should have realized that.

[@AstroBlender]

No problem!

[@gunnarjensen5910]

Does this method work with a 2x Barlow lens ?

[@Star_boy369]

What if for some reason your back focus comes up to 150 to 160mm, how will that affect the image quality and will you still be able to get good focus?

[@gunnarjensen5910]

Does fitting a starizona onto the corrector plate provide decent pictures compared with an Edge or a Rasa setup ?

[@AstroBlender]

Yes, if you put on a Hyperstar lens there is virtually no difference between the Edge and normal SCT. I’d say the RASA is still a bit better but they’re close.

[@luciennone6757]

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

[@denodan]

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

[@timpearce2281]

I have the same problem and I have yet to find a solution.

[@rcaballeron]

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?

[@AstroBlender]

Yes the primary does move, but the focal reducer sets the back-focus requirement when it is installed. The shift in focal length isn't too large, so at f/6.3 and 105mm of back-focus it doesn't need to be be absolutely perfect, there definitely can be some give. For best focusing results, I would say to make your last adjustment against gravity, not with it.

[@rcaballeron]

@@AstroBlender Thanks a lot for your feedback. Would it be different is we don't have in place the FR and therefore we have a f/10?

[@jimpoop]

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

[@KJRitch]

I notice ZWO cooled cameras come with Mia selection of spacers while their planetary don’t have all those spacer just a 1.25” nose piece. The sensor is still 6.5 mm. Is suppose the 105mm is still required so you have to purchase a selection of spacers? I looked up back focus on Celestron but only found 5” for 8” SCT. 105mm is less than 5”. Nothing came up about the focal reducer back focus specifications including in the reducer user guide. Frustrating.

[@AstroBlender]

Most people don’t use a focal reducer when planetary imaging, so they just focus their planetary camera and are good to go.

[@ash20500]

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

[@TheAndyoesq]

but with that much length and the camera on the back, how do you keep the camera from hitting the mount when the telescope of using the goto function for anything straight up

[@LaNa1174]

in my cem26, I added a minipier for separate the optics and tripod legs

[@davidfayolle4432]

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

[@AstroBlender]

David Fayolle Not sure on that. It shouldn’t matter as much though since you’re increasing the focal length and narrowing the field of view. The edges won’t matter as much.

[@anata5127]

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

[@KJRitch]

I’m not doubting your results but when I look through my C8, 6.3 focal reducer manuals and the manual for your camera there is no mention of the back focus except the 6.5mm distance on the camera. Did you find out about online in some forum?

[@AstroBlender]

It’s published on Celestron website.

[@johnnyredfireastroimaging5266]

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?

[@steelerfan4386]

Will this work with the asi224mc?

[@garyantesr]

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.

[@gaminguser4968]

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

[@patrickhoran2675]

Awesome

[@jasonmathias5343]

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?

[@bigbob5603]

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.

[@stephanebourque490]

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!

[@lancewilliams8732]

I want this info as well

[@CaptainCaveman60]

i think it's 127mm

[@jr10]

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.

[@luciennone6757]

In the setup wich you show in the movie, where do i place the zwo oag (21mm) ?

[@explorelearnshare8222]

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!