ZWO OAG Introduction

Overview of the ZWO Off-Axis Guider parts, assembly, and its integration with my Explore Scientific ED102 refractor and some thoughts on its use.

[@centenojoel]

Thanks James, your videos are amazing. Being an engineer I thoroughly enjoy the visuals and explanations. This specific video comes in handy as I'm installing an OAG on my ED102

[@Aero19612]

Thanks, Joel! Good luck setting up your OAG.

[@sreeshab4093]

extraordinary video. thanks so much James. first off it requires tremendous patience just to learn, leave along making a video and sharing this way. secondly, it goes to show how magnanimous you have been to share this across...the video is made so professional, every single detail. great work, and thanks again.

[@Aero19612]

Thanks Sreesha! Glad you're getting something useful out of the videos.

[@forerunner75]

Thanks for this very detailed video! That helped me while installing my first OAG 😊

[@Aero19612]

Great! I’m glad it could help some. Thanks for watching!

[@bernarddelangue844]

Hey James, thanks for your analytical way of presenting things, this is very helpfull.

[@Aero19612]

Thanks, Bernard! And thanks for watching!

[@dennisthemenace191]

Great video and a great explanation! You sir are an educator! Thank you!

[@Aero19612]

Haha. Thanks, Dennis! Welcome to the world of off-axis guiding.

[@celestromel]

Great video indeed. It would have saved my buddy Bruce and I a heck of a lot of time had I found it earlier. Thanks heaps.

[@Aero19612]

Thanks for watching Melvyn! It's take me forever to adopt the OAG over the guide scope. Good luck with yours!

[@mattwier4824]

Great overview! ZWO should link to that video from their online instruction manual... their own documentation left me with a lot of unanswered questions.

[@Aero19612]

I know what you mean. I had a lot of questions too--that's one of the reasons I make these video. It would be nice if manufacturers would provide clear enough information so that the buyer could determine what other parts might be needed to make it all fit. Oh well. Thanks for watching, Matt!

[@dhill730]

Great video! Very helpful. Thanks!

[@Aero19612]

Thanks for watching, Dennis!

[@The_old_geezer_geek]

Very good video, thanks for the very detailed information.

[@Aero19612]

Thanks! And thanks for watching.

[@whatmattersmost6725]

GREAT JOB James!

[@Aero19612]

Thanks Christopher! I hope it was helpful. Clear skies!

[@tempusfugit6820]

Thanks for the great video. Will you post another one on your experience when imaging with this system?

[@Aero19612]

Absolutely. I need to try out the Celestron OAG with my C9.25 SCT and this OAG with the ED102. I'm sure I'll have "lessons learned" from these trials--I always do...

[@rishigarrod7227]

Thanks very much for this explanation. I am curious about the CCD Height and CCD Width you filled in in Stellarium. Surely the Height is the smaller measurement of the chip and the width the longer?

[@Aero19612]

Hi Rishi, The "Height" dimension should be the shorter of: (a) the prism length or (b) the dimension of the camera sensor side parallel to the scope axis The "Width" dimension should be the shorter of: (a) the prism width or (b) the dimension of the camera sensor tangent to the OAG (perpendicular to the scope axis) I hope that helps. Thanks for watching!

[@olly7248]

Really good video, thanks 👍🏻

[@Aero19612]

Thanks for watching, Olly!

[@blueastrophotography6131]

First of all thanks for all the videos regarding the oag, they are very very useful. I also have an 9,25 “ sct and im planning to buy a zwo oag, but I have one question: i use the focal reducer f6,3 from celestron, i used to put it and after i screw a crayford focuser to put my zwo asi 294mc pro inside. The thing is could i use my f6,3 after that, screw the zwo oag and use my zwo asi 294mc pro? Or should i buy an spacer ring or so?

[@Aero19612]

Thanks for watching! A couple of things to consider: 1. Probably better to have the focal reducer in front of the OAG and the imaging camera so your OAG benefits from the larger field of view (Did I understand your question correctly?). On my system, I put the focal reducer on the SCT, then the Celestron OAG, then the filter wheel/camera. If you go with the much thinner ZWO OAG, you'll probably want to have a spacer between the focal reducer and the OAG. You may need a spacer between the OAG and the camera. 2. Consider that the ZWO OAG has a smaller prism and therefore will have a smaller field of view (more difficult to find guide stars). The Celestron OAG has a much larger field of view. I use the ZWO OAG for my 700mm refractor and the Celestron OAG for my C9.25. Let me know if I misread your question

[@niccoc1603]

Wonderful video, one question, how do you draw the light cone to the camera sensor?

[@Aero19612]

Ha. Well it may not be correct. I calculated the field of view for the sensor, say 1.5 deg, then drew a line on top rotated up by 0.75 deg and a line on bottom rotated down by 0.75 deg. It's equivalent to putting a point 1 focal length (700 mm) behind the center of the sensor and drawing lines from it to top and bottom of the sensor and extending them out. Thanks for watching!

[@riaandewinnaar5040]

Valuable info thanks!

[@Aero19612]

You're quite welcome, Riaan. Clear skies!

[@DiegoGomez-px6lh]

Thanks for sharing this information, very useful indeed!.. Quick question anyway: Could you please confirm from exactly where to where you measured the 12.25mm that you entered in the field Prisma/CCD distance in Stellarium?.. Thanks in advance!

[@Aero19612]

Thanks for watching, Diego! The 12.25 mm is the distance from the centerline of the light cone (i.e., telescope/OAG) to the bottom of the prism. To get this distance, I measured the inner diameter of the OAG and the distance from the bottom of the prism to the inner edge of the OAG. The 12.25 is the OAG-to-inner edge minus the inner diameter/2. I hope that is clear enough.

[@DiegoGomez-px6lh]

@@Aero19612 Perfect! Thanks a lot!

[@neverfox]

Love that you broke this down with CAD. Very helpful to see. But given that you have a filter wheel, which will presumably always contain filters, wouldn't you need to account for the optical impact of the filter on the total back focus requirement? I believe the rule of thumb is that it will add about half of the thickness of the filter glass to the 55mm requirement, meaning you're looking for close to 56mm optimally. That's why ZWO makes their EFW 20mm, their OAG 16.5mm, and wants you to space with two adapters totaling 13mm, for a total of 56 (considering the 6.5mm of the camera).

[@Aero19612]

Thanks for watching! Yes, there a later video that goes into filter glass effect on back focus (the rule f thumb is 1/3 the thickness of the glass). That said, you just need to be in the ballpark when adding up the contributions for all of the components. When you actually measure the total length, it is different (generally longer) than the distance you get by adding up component thicknesses because parts don’t fit exactly flat. Also there is build-to-build variation in field flatteners and so on. At the end of the day, try to get close and then do some imaging tests outside with small shims to get closer.

[@tempusfugit6820]

I have tried the Orion thin OAG using the same 2 cameras. I was never able to get focus on the 120MM. I was told the TOAG is designed to be specifically used with the Orion guide cam that has a bit of a custom shape. ZWO OAG was then recommended but I never got around to try it.

[@Aero19612]

Well, focusing will be a bit of a pain but I'm pretty sure I'll be able to focus the ASI120 based on the measurements I've made. Fingers crossed.

[@tempusfugit6820]

@@Aero19612 Have you been able to successfully use it?

[@Aero19612]

Well, I just uploaded a video for the Celestron OAG--I wanted to try that one first. I will definitely do one for the ZWO OAG soon.

[@Pipe-organ-recordings]

Well done.

[@Aero19612]

Thanks for watching!

[@ajdurrani7035]

Great channel. I just received my ZWO-OAG and it includes a set of thin plastic washer like rings. Would you know where they are used. I am using 120 mm mini.

[@Aero19612]

The thin shims are set to adjust the back focus of the imaging camera. Your OAG takes up 16.5mm. Depending on how your stars look in the corners, you may need to make small mm-level adjustments. You may not need them at all. Hope that helps!

[@wanderingquestions7501]

I also have the zwo oag. I am using the 120mm-S guide camera. I’m still waiting for a 2600mc-P which has the 55 mm back focus but in the meantime i’m testing w/ a 178mc. (12.5mm back focus). The OAG does not have everything one needs for this combination in the box. There are not enough spacers to achieve common back-focus. If one uses 1) the nose-piece collar and the 178mc nose piece on the observing end the guide camera still needs 1 more mm of inward travel to focus. Item 2) is a 5mm space which appears integrated due to its tightness. I put the guide scope holder/pedestal assembly in the freezer about 30 minutes and the space unscrewed easily. However, to focus ‘out’ the guide/prism pedestal w/o 5mm spacer it was so far out that one cannot tighten the pedestal. In addition using the 178mc nose piece w holder collar is incorrect because the nose piece bumps into the prism. The solution? I removed the 5mm collar from the guide-scope pedestal. Screwed on the 120mm-S body. I ordered Stellarvue 10mm and 8-12 mm adjustable spacers. I put this on the observing port and screwed the 178mc body onto that. For simplicity I focus the telescope on the guide camera first, them adjust the Stellarvue adjustable spacer to bring the observing camera into position and iteratively tweak. Note: I also have the zwo helical focuser which will be great when the 2600 arrives. To use the helical w the 178mc I would need about 30mm more to account for extra length introduced into optical path by the helical. So far working w this kit I believe the helical focuser to be a very handy connivence.

[@Aero19612]

I saw that ZWO helical focuser. I was very tempted to order it at the time, but there was a manufacturing/shipping delay at that time. So far, I'm getting along without it. I think ZWO does a pretty decent job of providing the spacers with their cameras, filter wheel, and OAG to a attach their cameras. I also have the Celestron OAG which has several spacer rings. It's all about mix and match. Good work!

[@blueobject]

Great video James. When you added camera to filterwheel were you lucky to have sensor in landscape? I had to add spacers to get it to show up close to landscape for the OAG as you only about 30 degrees of freedom. But how do you deal with the random position of the camera to filter wheel connection?

[@Aero19612]

Hey Marty. I'm not sure I follow your question, so try me again if I missed your point. It is true that the camera screws into the filter wheel and ends up where it ends up. Of course, the filters are round so that doesn't affect the light getting to the camera sensor. The OAG can only fit on the filter wheel in one orientation (+/- a few degrees) because of interference. That means you can't independently adjust the imaging camera orientation (for target framing) and guide camera orientation (for finding a guide star). The good thing is that I only use the ZWO OAG on my shorter focal length telescopes (< 700mm) so there's always a guide star to choose from when I set the imaging camera orientation. Does that answer your question?

[@blueobject]

@@Aero19612 yes thanks James! I'm finding a lot of people use the shims to help make sure the camera ends up perpendicular to the OAG. But that affects back focus so need to adjust for that on other side. Lot of people want ZWO to allow rotation of the camera. It's random where the sensor lands

[@AstroPinoy]

Thanks James, I just got the 174mini and ZWO OAG, do they work good an not blocking the imaging train?

[@Aero19612]

The 174mini should work just fine. It's a good guide camera. You will not be able to make use of the entire sensor area because the OAG prism is smaller than the sensor. This probably won't be a problem unless you're imaging at a large focal length (> 1500 mm, say). You can adjust the height of the prism to prevent it from blocking light to the imaging sensor. I raised mine by 4 mm to avoid the ASI1600's sensor (long axis). Good luck and thanks for watching!

[@AstroPinoy]

@@Aero19612 Thx, I use the long edge of the sensor so it wont interfere and yes I can only select stars inside the circle because of the prism hole is small, but seems to work fine last night

[@Aero19612]

@@AstroPinoyxcellent! I am planning to try the Celestron OAG on the SCT tonight. We'll see what happens.

[@tonycarillo5790]

Regarding the two camera holder rings (M48 and M42), I believe the three 'lugs' must be placed under the three thumbscrews in order to lock the assembly. If the lugs are positioned between the screws as demonstrated, then the adapter/camera is not held in position and will drop out. So, the available rotation is confined to three separate sectors defined by the relatively narrow lugs. I wonder why ZWO designed three lugs rather than a 270 - 300 degree flange? Clearly that would provide greater and more convenient axial rotation (similar to the Lumicon OAG for Meade LX200). Maybe for Version 3?

[@Aero19612]

Yes that is correct. Perhaps the video is not clear, but there is no way to assemble the parts so that the lugs miss the screws. The prism column fits between two lugs and serves as a rotational stop that prevents you from rotating the lugs out from under the screws. Also, with my filter wheel, there is only one way to attach the filter wheel to the OAG. Thanks for watching, Tony!

[@Dennis-tf2cs]

You mention the SCT OAG, what one are you using? I'm getting ready to setup my SCT and purchased the ZWO Large OAG with a 12x12mm prism. Do you think the ASI174 would be the best fit for this?

[@Aero19612]

I'm using the Celestron OAG. It has a 12mm prism and a large optical length, which is useful for the large backfocus required for an SCT. It comes with lots of spacers and adapters as well. I didn't know ZWO had come out with a large OAG-will have to take a look. I like the ASI174 because of the large sensor area it has. No point in having an OAG with a large prism if the guide camera can't make use of it. Yes, I think the ASI174 is a good match (kinda pricey though). Thanks for watching, Dennis! Good luck!

[@DeveshPande]

Hi James! Another excellent video. Many thanks. Is the 12.25mm distance that you input in Stellarium the depth of the prism inside the light path?

[@Aero19612]

The distance to the prism is measured from the center of the light cone (the optical axis of the scope) to the bottom of the prism. Thanks for watching, Devesh!

[@DeveshPande]

@@Aero19612 Thank you James! Any quick way to physically measure it? I have the Celestron focal reducer so i just need the radius of the back element and add it to the distance of top edge of the reducer to prism bottom?

[@Aero19612]

There are probably several ways to go about it. I used a pair of digital calipers to measure the inside diameter of the OAG (call that number "D"). Then I measured from the inner surface of the OAG (directly below the prism) to the bottom of the prism (call that number "H"). Then the number you're looking for is: H - D/2.

[@cheeky_devil]

do you have any tricks to get those threads undone? I didn't use oil :( Also can you confirm what you are saying is the distance from the prism to the sensor should be the same distance from the prism to the dslr's sensor?

[@Aero19612]

Ah, threads. I have tried oil. I have tried lithium grease. Neither is a sure-fire way to prevent sticking. Heck, I'm not even sure they help at all. A strap wrench is a good tool but it may not work with spacers that are too thin to get a grip on. ZWO has a thread converter ring (M42 on the inside and M48 on the outside. Those things might as well be welded to the M42 adapter they are screwed on to. Yes, the axial distance from the middle of the OAG prism to the DSLR sensor should be the same as the off-axis distance from the center of the OAG prism to the guide camera sensor.

[@ajdurrani7035]

I have one more question. My TAK flattener TK00582 has 52M and the OAG has 48F on the scope side. Did you use an adapter to connect the two or your flattener had 48M to match the OAG?

[@Aero19612]

In my case, the thread coming off of the scope is 48mm, so I didn't have that issue. You can get adapter rings to convert one thread to another. I use a lot of the Blue Fireball spacers/adapters from Agena Astroproducts, but there are other manufacturers out there as well.

[@jimtaylor5802]

You hit all of the issues and corrective actions that have befuddled me (and likely many others) in using the OAG - what’s been your experience using the helical focuser?

[@Aero19612]

Thanks for watching, Jim! I don't have the helical focuser for the ZWO OAG. Honestly, I find focusing by ever so slightly moving the camera in/out works very well. Had to focus it just a couple of nights ago on a star field. Worked fine. Good enough for a guide star anyway.

[@gloubiboulgazeblob]

For those who possess a SW Esprit 120ED, with flattener in place, and the ZWO OAG placed just after the flattener (I think it's the best place if one wants to place the OAG prism as far as possible in the main cone light of the refractor and the main camera sensor), the flattnere M48 adapter doesn't screw enterely because it is a wee bit too long (something like 0.5mm) and hits the prism shaft. I can image though, but maybe, for the more demanding astronomers, it may be an issue...

[@gabrieljaimes9491]

Hello James Sorry I would like to Know your opinion for what is the best Setup for RedCat 51, At first I was thinking on following: - WO RedCat 51 - ZWO 1600 MC - IOptron Skyguider Pro Furthermore, without considering filters, I would like to know also about accessories such as Barlows, Guiding Camera, Off Axis Guiders or other elements for Deep Sky Astrophotography that could be effective and made good results for you. I would appreciate details for what would be best equipment in your opinion

[@Aero19612]

Hey Gabriel. I love the RedCat! I'd be happy to share some thoughts, but I won't claim there is a "best" arrangement. Of course, the RedCat has a short focal length (from an astrophotography perspective) and is best used for large nebulae (plus the Andromeda galaxy). The wide angle view and low weight mean you won't have too much difficulty guiding and the Skyguider should be fine from a payload weight perspective. You can use short exposures (30 sec?) and not guide at all, but you'll probably want to use longer exposures and guide. Some people use a 50 mm guide scope. I've gone to OAGs for my three scopes. I use the ZWO OAG with the RedCat. I use a ZWO ASI290mini for the guide camera. It has a small sensor - about the same size as the OAG prism so it pairs well with it. With the wide angle view, there is never a problem finding a suitable guide star with the OAG. I only use a Barlow (Celestron X-Cel x2 - not sure how it compares to other manufacturers) when I'm imaging the planets through my C9.25 SCT. I suppose there's no harm in using it for DSOs, although I prefer not to (you do lose some image quality). The one "concern" I might have about your equipment list is the ASI1600MC. I'm sure it's a great one-shot-color camera, but the RedCat is "meant" to chase nebulae and they are best photographed with Ha, Oiii, and Sii filters (i.e., with a monochrome camera). You can use a OSC camera, but your images will mostly be red-toned. And some nebulae may not show up at all. You might check Astrobin for examples of nebulae taken with the 1600MC. I hope that helps a little. I offer full refunds on advice. Thanks for watching!

[@gabrieljaimes9491]

@@Aero19612 this is so useful! thank you James!

[@2badger2]

What type of oil do you use? Thanks

[@Aero19612]

I used the general purpose “3-in-one” oil. Just a drop. Now, after using it, I can’t say that it solves all sticking problems. I’ve also tried white lithium grease. Same opinion. Get a strap wrench, you’ll probably need one. Thanks for watching!

[@RobB_VK6ES]

Strange ZWO made the OAG so thin (*cough* Orion TOAG copy) as all of their image trains are based on 55mm back focus. If they had incorporated the 11mm used by the spacer into the main OAG body there would be no need for the segmentation of the adapter similar to Celstron's version and at the same time allowed for a larger guider port using a larger stem. The other upside as far as they are concerned it would require a mini guide camera so potentially another sale. The 174 camera is not cheap, did you buy before the release of the 290 which is half the price with a marginally smaller sensor or for other reasons? The damn Corona virus is delaying delivery of my 2600 OSC. Like a mug I pre ordered in Dec with an estimated Jan delivery. Between ZWO's over optimism and the virus outbreak I am now looking at a late Mar delivery. :(

[@Aero19612]

Yeah, not sure what led ZWO to select the dimensions they did. It's got to fit on a number of different camera configurations with different sensor setback distances. For example, your 2600's sensor is set back 17.5 mm, whereas my 1600's sensor is set back only 6.5 mm--hence the need for a removable 11 mm spacer. You won't need the spacer and can screw your OAG right onto the 2600. And then there's the 16.5 mm filter wheel thickness that you may not need--there's a 16.5mm spacer that you'll use instead. The 55 mm back focus seems to be common for field flatteners and such.The small prism is problematic for long-focal-length optical trains. At the time I bought the ASI174, I had the (original) ASI120 which has a small sensor (4.8mm x 3.6mm)--much smaller than the Celestron OAG prism. The ASI174 has a fairly large sensor (11.4mm x 7.1mm)--4.6 times the area--and is much better suited to the larger Celestron prism. I did buy an ASI290 later for the ZWO OAG because it has a small sensor (5.6mm x 3.2mm, pairs well with the small ZWO prism). You might be comparing "resolution" when you say the 174 sensor is slightly smaller than the 290 sensor. The 174 has larger pixels (brings in more light) and therefore has a 4.4 times larger field of view than the 290 -- good for high focal lengths where stars are few. That's crazy that the Corona virus is affecting delivery so much! Can't wait to hear what you and others say about the new ASI2600--very tempting. Guess I won't hear anything until March.

[@RobB_VK6ES]

@@Aero19612 senior moment with the sensor sizes. I was comparing the two at work today and I seem to have drawn the wrong conclusion. It was a second hand retail outlet source of specs so they may have published erroneous info but more likely a senior moment:)

[@Aero19612]

The only moments I have are senior moments

[@hypersphereengineering6015]

Where did you get the 11.55mm spacer? Is that a ZWO part?

[@Aero19612]

That spacer came with the Celestron off-axis guider. Thanks for watching!

[@hypersphereengineering6015]

@@Aero19612 thought as much. Cheers

[@DamianVines]

I'm not sure how big of a difference this would make but... according to a users guide written in 2019 by George Zotti and Alexander Wolf for ver. 0.19.0 page 177, They state that the measurement for "Prism/CCD distance" in the Off Axis guiding setting in the Sensor area, should be "between the center of the prism/CCD of Off-Axis Guider and optical axis of telescope in mm." I've heard many say measure from the bottom of the prism to the axis of the telescope. Again no sure it would make a huge difference. :) Here's the link the the manual I'm referring to. www.academia.edu/38617778/Stellarium_0_19_0_User_Guide Another question I have is... when you set your camera rotation angle to 90 degrees, why is still tilted at an angle? Again great stuff James!

[@Aero19612]

That's interesting, Damian. The tool tip that pops up when you hover your mouse over that entry box says the distance is measured from the optical axis to the bottom of the prism. My star field as seen through the off-axis guider appears to confirm this. For camera and OAG angles, you might take a look at about 8:00 into this video: kzbin.info/www/bejne/ZpWodnaarKmCY5Y

[@kyredormangara4298]

you have a new subscriber ;) I'll chekc if you have done a video about how to adjsut the prism to not overcome in the field of the light. Some friends told me they were doing it by daylight.

[@Aero19612]

Thanks for the sub!

[@netmaster78]

One quick question here, at 10:39 you show the OAG in detail. May sound stupid, but isn´t the prism in that graphic the wrong way? Shouldn´t the flat part of the prism face the telescope? If not, that may be the reason my attempt to focus my ASI120 sucks...

[@Aero19612]

Yes, when you look at the prism, the flat face points toward the telescope. I don't show that face in the drawing--I'm showing the sloped mirror and a vertical face on the back (camera) side that may or may not be there for some OAGs. My drawing is confusing. Good observation. Thanks!

[@brianalsum7706]

Can you share the cad file for the OAG?

[@Aero19612]

Hey Brian. I’ll load it up on Google Drive and reply back with a link.

[@Aero19612]

Sorry for the delay, Brian. Here's a link: drive.google.com/file/d/1b6wquP6-dZyWfDu97uAOTu68ud3b0Tdc/view?usp=sharing

[@r.fortner4661]

anti-sieve compound is less messy.

[@Aero19612]

I’ll give that a shot. Thanks!

[@gregorymckenna6609]

Awesome instructions! ZWO makes great products but their instruction manuals are terrible. Perhaps ZWO could hire you to do English instructional videos for all their fine equipment. Many thanks, Greg Toronto Canada

[@Aero19612]

Thanks for watching, Greg! Yep, I'm a big fan of the ZWO products myself. I'm anxiously waiting for them to make a proper mounting bracket for my ED102 so I can buy their Electron Automatic Focuser.

[@RobB_VK6ES]

@@Aero19612 If you can get some dimensions of the focuser and the EAF unit being a machinist I could make you a decent bracket as I did for my WO ZS126 and carbon RC8

[@Aero19612]

Dude! That's a very nice offer. Since I recorded that video, I came across the Pegasus Astro FocusCube 2. It has a bracket that's deep enough to attach to my extended dovetail plate (instead of the focuser). Works really good. Thanks for the offer!

[@RobB_VK6ES]

@@Aero19612 It would have been a pleasure and a small thankyou. The technical insight and analyses shown in these videos is way beyond any other channel out there and you do it for selfless reasons. I am so over you tube creators trying to squeeze a dollar or begging on Patreon and in doing so sell themselves out. It may be coincidence but every long term you tuber I have subscribed to that then turned professional has been dropped from my subscribe list. You never know if what they say is truth or sponsored deceptions.

[@marekmiszczak3266]

Don't get all that backfocus calculations. Can't you just adjust it by the focusing wheel?

[@Aero19612]

Hey Marek. There are a couple of issues here. 1. Let's say we have a scope with no field flattener and no off-axis guider. The scope has an ideal back focus, but you just need to be close and then, like you say, adjust the focus knob to put the sensor at the location location. Over night, as the temperature drops, the tube changes length and you'll have to adjust focus to keep up. 2. Now, let's add a field flattener. In this case, the flied flattener requires a specific back focus of about 55 mm and since that part is attached to the camera, you can't use the focus knob to adjust that distance. You will still need to adjust the knob to achieve focus, but you'll need spacers/shims between the camera and the field flattener to nail down that 55 mm. 3. Now let's add the off-axis guider. You'll do exactly as discussed in (2) to achieve focus and a flat field for your imaging camera, but you'll have to adjust the location of the guide camera sensor to achieve focus for the guide camera. Once again, you can't use the focus knob to do this because it is set for focus of the imaging camera. Hope that clears things up. Thanks for watching!

[@marekmiszczak3266]

@@Aero19612 Ok gotcha

[@woody5109]

Really dragged this out…