Back-Focus 101: The Petzval Lens ft. William Optics RedCat51

In this episode of back-focus 101, I show how to obtain proper back-focus on the Petzval, using the William Optics RedCat51 as an example. This is probably the easiest system to obtain proper back-focus, as you will see in the video.
Are you interested in knowing how filters affect your back-focus? • Back-Focus 101: How Fi...
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[@AstroBlender]

One addendum to the video: You should always try and get your camera sensor as close to the filter as possible. You may have seen that I have a 11mm spacer ring between my camera and filter wheel. The camera should actually be directly threaded into the filter wheel, and the spacer placed on the other side. The way I have it set-up in the video isn't a problem as I'm using a 1" sensor with 36mm filters, but in general you want to get the camera sensor as close to the filters as possible to avoid vignetting. Thanks all and I hope this was helpful!

[@AhmedAlqallaf]

I think I found the answer of my question here

[@alanalain4884]

Thank you for that additional tip... Useful.

[@hobbeeswe7472]

You are the MAN! Your films are so EASY and EDUCATIONAL I often start laughing. THANKS for ALWAYS having the answers to my issues presented PERFECTLY !!! You just also sold me the Redcat, they should send you a sales commission :D

[@richardmadley7455]

Thank you so much for explaining backfocus on the redcat ,great video

[@NorthwesternOntarioAstronomy]

The most informative astrophotography channel on KZbin. Cody never leaves out a single detail in his tutorials.

[@redjr242]

I'm a beginner astrophotographer and have been overwhelmed trying to understand backfocus and worried about getting it wrong. Thank you so much for making such a clear video that explains all of the details, with examples for all the different setups and accessories one could use!

[@Styeeziez]

Thanks for the info but also the brevity. I like the no fluff style. A great lede drew me in and I’m ready to image. 👏

[@janetwillson3502]

This was the best video I have seen on setting up a ZWO camera with a filter drawer on a Redcat 51. THANK YOU!

[@romteed8733]

Hi Cody, thanks for this video, it's very informative especially for an older guy that's just bought a Redcat 51 and is a newbie to astrophotography.

[@stilianoskalatzis7491]

Thanks for yet an other great and informative video Cody, I've learned a lot from watching your posts. Keep up the amazing work.

[@Landice100]

I have been waiting for someone to show me how to attach and use everything you have shown here. Perfect video. Thank you!

[@AstroBlender]

I’m glad you found it useful!

[@floringherman5874]

Thank you so much for the video. It is very useful for me because i just bought a WO Redcat51 and I have the Asi 183MM Pro. I wanted a wide field scope, and I loved this one.

[@Katzenrads]

This was a very helpful video. I now understand what back spacers to buy. Thanks.

[@steveweintraub1113]

Great overview, thanks for this as well as all of your other videos. Very educational. Clear skies!

[@AstroBlender]

Hi Steve, thanks, I’m glad you liked it!

[@marvinwhisman3333]

I really enjoy your videos and learn so much. Thanks for going into so much detail. It really does help. Your whole back focus series is great.

[@AstroBlender]

I’m glad to hear that, thanks!

[@Matt_10203]

These are some of the most helpful videos on astrophotography as not many people discus these topics in such detail. Edit: just bought a redcat 51, excited for it to get here!

[@alimaleki217]

Another great video; thanks Cody!

[@bhattvd1]

Thanks, Cody. I am getting my RC51 today. So I watched this video in preparation for that arrival!

[@AstroBlender]

Awesome, such a great optic!

[@dn5101]

Wow, that's the best explanation so far. I've been on the edge about jumping into astrograph. Starcat or the redcat are what I am looking at as well as an orion quad. Think I'll go the William optic route.

[@blanejnasveschuk6351]

Excellent back focus presentation specific to the Red Cat 51. Quality, ease and versatility of this little scope, such an advantage for imaging larger DSOs.

[@AstroBlender]

I’m glad you enjoyed it. I agree the RC51 is great. I’m excited the new RC71 launches in a few weeks!

[@johnengooyen]

Perfect timing! I literally just put in an order for a RedCat51 before I saw this video!

[@AstroBlender]

Oh awesome, I’m sure you’ll love it!

[@2badger2]

Great review! I just bought it today and your video makes it very clear how to hookup my Nikon DSLR cameras.

[@AstroBlender]

Enjoy, it’s a super fun scope!

[@pippinjg]

I love this guy !!!!!!!! you have saved me countless hours .....

[@GETTR2024]

Thank you Thank you Thank you! I needed this tutorial!!

[@tigerchef1969]

Thank God I came across your video

[@SIGP229TPOC]

I've been thinking of purchasing this, but I hadn't seen a good review (out of the many on KZbin). This is, hands down, the best review and you answered all of my questions and gave me insight into the product which I didn't even think about. Thanks very much!

[@AstroBlender]

Oh thanks! It’s not so much a reviews as to obtain proper back-focus, but I’m glad you found value in it!

[@SIGP229TPOC]

@@AstroBlender My mistake and no disrespect intended. Edit: "Hands down, the best Redcat51 proper back-focus tutorial". 😁

[@rudolfbenner4802]

Very helpful. Thanks.

[@brad1958]

Very helpful, thanks!

[@yosmith1]

Such a great OTA. I recently installed the 7 slot EFW along with a Pegasus Rotator backed with a asi2600mm pro....works fantastic...RedCat is just and amazing telescope. Thanks for the demo.

[@AstroBlender]

Agreed! The 2600MM pro is a great choice too for this scope!

[@markmayer9290]

I watched a previous video in the back focus series in which you described the ZWO filter drawer, and based on this ordered one. It arrived to day and will be 'life changing' - just pop filters in and out, and no fiddling with the optical train. Also, when I do need to move the CMOS camera from one OTA to another, the filter drawer comes along. This is probably the most useful information I've picked up from Astro videos. RedCat 51 looks nice as well; would be a good guide scope for a long focal length SCT.

[@AstroBlender]

I’m glad you found it useful! I personally don’t use guide scopes on anything longer than about 1,000mm as off-axis guiding is the way to go, but yeah it could work as a guidescope. Any guiding is definitely better than no guiding on most mounts.

[@Sidecontrol1234]

This was super to watch.

[@AstroBlender]

Glad to hear that.

[@CM-2000]

Hi cody, your video is a great help to me as I am switching from DSLR to the 533MM with wheel. I also use the RC51 and agree with you on all points. I'm already looking forward to the results with this rig. Thank you very much! A fan greets you ;-) Helmet

[@AstroBlender]

Glad it was helpful! The 533MM Pro is a great camera!

[@TheUrbanAstronomer]

Great demo Cody!

[@AstroBlender]

Thank you sir!

[@AntonioPena1]

Happy to see your video on back focusing, I use a RedCat 51 for both Astrophotography and terrestrial with great results, I use a DSLR camera for now and I always always curious if I can use a filter on this lens with DSLR, thanks for the explanation. Great video

[@AstroBlender]

I’m glad you found it useful!

[@AstroQuest1]

Very informative video Cody. The backfocus (or lack of) is another plus for that Redcat51. This is on the top my list when I get another scope - won't be this year since I just got a new scope. Ironically, I discovered the caliper trick recently and have been using them in place of the spacer wrench, it works well. Also there is a grub screw for that ZWO filter drawer to hold in the drawer in the proper place. I mention that because I was imaging recently and is must have come loose during a flip or something so my images had an artifact on part of field edges - fortunately it was a small object so it did not matter. - Cheers Kurt

[@AstroBlender]

Oh that’s a bummer, at least you figure out the slippage before you did more imaging. When you get a RedCat51 I’m sure you’ll enjoy it, it’s a phenomenal optic.

[@catherinegrimes2308]

Very good video, many thanks for doing it. I connect telescopes to cameras using the telescope to Canon EOS adapter then a EOS to Fujifilm X mount adapter, I also have a direct M48 to Fujifilm X mount adapter as well. Or I can use the Canon EOS adapter then a EOS to EOS to ZWO camera adapter. I can also mount a 2X teleconverter between the two EOS adapters.

[@stew8584]

Good demo, I got the Space Cat ( Silver ) with my full spectrum Sony NEX 5T 16MP. Cheers.

[@AstroBlender]

I love the look of the SpaceCat!

[@lupo2824]

Thanks C. Got completely confused with my WO GT81 and the 6AIII Flattener/Reducer and all the competing numbers you can find and consider as to distances. (I've got also the Canon SL2 and the WO 48mm adapter).

[@DerHammer.]

Hello, I recently bought the Redcat and am very happy with it. In the beginning I wanted to work with my DSLR and an Off Axis Guider. As far as I know, this is not possible with the Redcat because of the backfocus. But a guiding scope can be mounted without any problems. So that is not a problem. Greetings from Austria

[@alangriffiths1432]

Great video I am saving up for a second set up can you please give much information on what filters you use and what size for the filter draw you on the red cat thanks

[@promod1179]

Thanks for the clear, concise video. Easy to follow... It's always nice to watch a video where there isn't a lot of jibber jabber like so many....

[@AstroBlender]

You’re welcome! :)

[@astroterrace]

My RedCat51 with 30mm adapter to ZWO camera with 12.5mm flange focus (actually there is no flange) allows me to focus almost perfectly. But can't focus at all when I connect Canon EOS M50 Mark II camera with 55mm adapter. With flange focus of 18mm, the back focus needed exceeds required 59.7. So, I need a right adapter to use the camera! Thanks for the video with clear explanation.

[@MrTapoutmma88]

Hey Cody, got any videos on how to do all the wire set up?

[@ozzielfc1568]

Great video Cody. I've just received my Redcat 51 and have the 183MC. Can you tell me what images I can expect from this setup. I'll be shooting from class 2 - 3 skies in just outside of Perth, Australia so probably no need for filters. thanks again

[@ramankrishnamoorthy1308]

Thanks for the video its very clear. The canon you used, is it a Fullframe camera? I recently bought a redcat51 to use with a Nikon 5200. I have a ring and a 2"adaptor for the camera. Yesterday I tried to focus on a distant object but could not get to focus. Not sure where I am going wrong and look forward to some support.

[@AhmedAlqallaf]

thank you this video is very useful for me, but I have a question, I have ZWO294mc with 36mm filters and EFW does it fit with WO RC51?

[@uanesdeiro]

Thank you very much for the video which is very well explained! I need to understand a few things: The shopkeeper who sold me the RedCat51 told me that for use with the Reflex, the T2 ring is enough and I already have the right BackFocus, while other people have told me that in order to have the right BackFocus, I should add an extension. 4.7mm, because there are 44 of the Reflex, 11 of the T2 ring and therefore 4.7 are missing to get to 59.7; what do you think? I use a modified Canon Eos60D SLR and therefore I have an APS-c sensor which is smaller than the FullFrame, but despite this, the image at the edges is not perfect and this is strange, because a correct field is declared up to the FullFrame! Do you think the cause could be that I am missing these 4,7mm? Or would we need to intervene on the Tilt Corrector? Actually I tried and the image improved a bit, but not completely, because I think doing this Tilt adjustment is very difficult and I couldn't find a tutorial that explains well how it's done! The strange thing is that if I photofraph a bright star carrying it around the sensor up to the edges leaving the Bathinov mask inserted, the image is perfectly in focus! Thank you for a possible answer and I hope I was understanding, because I used the google translator.

[@NewtoAstrophotography]

Any chance you can make a video on how to then take photos and get focused with this setup? I try and do live view and I can’t see anything from my screen and when I take photos I don’t see anything. I’m ready to give up

[@markmorel1721]

Thanks. Can I use a filter drawer with the Redcat 51 and a DSLR/Mirrorless camera?

[@michaelclancy7874]

Cody: Thank you for this video. I have a REDCAT 51 with the exact spacers in the video ( no filter wheel) and a 183MC camera. I am using a ZWO ASI AIR Plus. I have a Skywatcher go mount. In preview, I cannot get a clear focus on anything. Last night I was staring at Jupiter. Clearly see it with my binoculars. No such luck with my Redcat. Any thoughts? ( 55mm back focus) Thx Mike

[@Dame1999]

Do you get off-centered halos around bright stars on your Redcat? I've been getting them since I got my Redcat and can not figure out how to get rid of them

[@ScandalBeats]

Hi! I bought the redcat 51 v1.5 and a m48 t ring for canon EOS dslr. After taking the m48 t ring out the dust cap doesn’t fit the redcat 51 anymore… anyone know what in the world is happening?!?!

[@2h4]

how would I hook up a camera with ef-m lens mount to this?

[@geopolska]

Hi, please shine a light of knowledge upon my head. I am having an issue with focusing my Cat along with DSLR. I literally can't see diffraction spikes, even when I hit magnifying button to zoom and get more help with focusing spikes are soo small it makes the process very difficult. It's just cat, T-ring and DSLR, no spacers. I've done some astrophotography in the past but with ED 80Pro and never experienced such a problem.

[@davidletz9123]

Hi asrto-photo newbie here, just ordered a Red Cat 51, so this vid helps a lot! I know you have another video on how filters affect back focus, but you did not mention anything about it in this video. Is it critical to adjust back focus on this telescope? Thanks.

[@AstroBlender]

Glad I could help! The Petzval just needs to be close in terms of back-focus spacing. Once you’re in focus, you know the back-focus is correct, so adding a filter on a Petzval lens won’t really change things. Just re-focus after adding the filter and you’ll be good to go!

[@NewtoAstrophotography]

What’s the numbers on the back of the red cat towards the camera? Does turning it do something?

[@thomasphennigan]

Hey Cody . Late to this party but thought I’d ask anyway. Is it possible to achieve back focus on the RC 51 when using a DSLR and filter drawer ? Great channel !!!

[@Ian-sk7by]

You actually don’t need to backfocus since the RC51 has a petzval-like design.

[@michaelclancy7874]

My Redcat 51 has the exact same spacer set up for the 183MC camera. The only focus I can get is with the adjustment turned so the front lens is almost all the way in. Is this normal? Seems like I have no more adjustment. Would more back focus, say another 11 mm help? Thx, Mike

[@AzCrawler00]

My RedCat51, the latest revision purchased earlier this year, would not achieve focus at the specified backspacing of 59.7mm. ~55mm worked perfectly. James Lamb made the same observation from his first hand experience. This is not the first WO recommended spacing that has been incorrect in practical application from my sample set of five WO 'scopes.

[@AstroBlender]

Thank you for the note! At least the good news is you can get to the proper back-focus just by focusing. In your case I’m guessing you just removed a spacer and then focused?

[@AzCrawler00]

@@AstroBlender Yes, you got it.. Just rearranged some adapters to achieve ~55mm of back spacing to the sensor plane.

[@FamiliaColl]

Right now I got a DSLR but I wanna add a filter drawer can I do that, redcat-filter drawer-dslr or is gonna be too long?

[@alanalain4884]

Thank you for this very informative video. I have a question though. When one uses this/telescope/lens with a DSLR for birding, will one get an upside down image view in camera when looking through the viewfinder or in llive view ? Thank you for any insight.

[@AstroBlender]

I use mine in live view all the time while birding. It is really designed as a "camera lens" and provides a correct-view image.

[@alanalain4884]

@@AstroBlender Thank you for your reply, greatly appreciated...

[@henryriverasiervo]

Hello astroBlender I have a issue with my red cat 51, william optic adapter and my canon RP does not wanna take the shoot but when I used my canon T7i everything work just fine no problem at all; any help? May be I'm missing something in the canon RP setting?

[@thetruthabouteverything5399]

Would have been helpful seeing you attach the camera to the redcat instead of fitting the filter, fitting the m48 then next we see the camera on the scope. Did you have to screw the whole camera on? That's a bit awkward.

[@skye7690]

Why are you trying to get 55mm Backfocus on a Petzval design that doesn't require it?

[@camrun2011]

I use rc51 with nikon d5600. Its still out of focus.

[@ddiver7908]

why does it look like there is black electrical tape on your zwo efw? 🤔

[@AstroBlender]

There is. I have it covering the holes so dust doesn’t get in.

[@ddiver7908]

@@AstroBlender 😦 holes? might you pls be able to elaborate? why are there holes? a defective unit? is there any light leak? thank you!

[@AstroBlender]

ZWO drills and taps holes on both sides of the filter wheel so you can use their off-axis guider and bolt on cameras. I don’t use either of those (for now), so I put electric tape over the holes to prevent dust and light from leaking in.

[@whatmattersmost6725]

First!

[@void_presence]

should really title these videos to reflect where in the series each episode is

[@dummag4126]

Please, stop to use DSLR for astrophotografy with little scope.....pixel are to big!! not cooled!!! ugly image with noise and square stars!