Introducing the William Optics GT81iv Refractor Telescope

We provide an introduction to the new William Optics Gran Turismo 81 (GT 81) 3-element APO refractor (478 mm focal length) and the new adjustable-length field flattener and focal reducer, the William Optics FLAT6Aiii (reduces the focal length to 384 mm). The first night out with the new scope is used to dial in the correct length for the FLAT6Aiii to provide a flat field and to identify focuser backlash and fine and coarse focuser step size s using the Pegasus Astro FocusCube 2 and the Bahtinov focus error metric.

[@chandrainsky]

Love the way you go through the details in a systematic manner. Another great video!!

[@Aero19612]

Thanks! I hope this is helpful in some way. Thanks for watching!

[@mark_egan]

Thanks for taking time to describe your gear setup and your process for setting your field flattener spacing! I purchased the WO GT81 last fall for a backcountry visual observing scope. Since then, I have slowly been outfitting it for astrophotography when components become available in the stores. Now, with the camera finally shipping, I came back for another watch. I appreciate the detail you provide. It will help me get up the learning curve faster. I am contemplating getting the Ultimate Power Box now to manage power and data cable mess I have. I remembered you had a problem with it so I wanted to understand what that was. You have inadvertently laid a trap for some of us. I can say that the textbook you used to rest the telescope in has captured some of my mental bandwidth every time I watch. I have seen it somewhere my past at work but it was not mine. I guessed it was "Advanced Engineering Mathematics" by Erin Kreyszig but it is a later addition than I own. I see you also hooked Richard Bratten below since my last visit. I find myself studying your shelf looking for other books I am familiar with which remind me of my past career. Ha, what the hell is wrong with me! :)

[@Aero19612]

Hey Mark! Thanks for checking in again. Maybe I should start putting a different textbook in each video just to drive people crazy. By all means, get a Power Box early in your astrophotography equipment acquisition phase so you don't do what I did: buy a ton of DC adapters...and then put them in a box when I got the UPBv2. You'll love the GT81 for astro. Very nice scope.

[@Patrick_McFadin]

Mr Lamb, you saved me a ton of work! I just got a Pegasus FocusCube 2 for my GT81 and I was having some really inconsistent results on autofocus. After watching this and reviewing my settings, I realized I wasn't giving it enough big step in the autofocus routine so the curve fit was flakey. Your attention to detail is enviable. Thanks for all of the work you put into your videos.

[@Aero19612]

Thanks for watching, Patrick! I'm glad it helped. Just used my GT81 with the NINA autofocus routine a couple of nights ago. Worked like a charm. Video incoming!!!

[@Patrick_McFadin]

I went back and graded a bunch of subframes I took along with the ones I did last night. The verdict was pretty clear. Autofocus gave me much more consistent FWHM than what I was getting pre autofocus. I wound up throwing out a ton of subs as a result. You got me wondering what else I may have missed in you videos and now I’m shopping around for an OAG setup. 😂

[@Aero19612]

HaHa. I know what you mean. I think I got the best focus I've ever had with the NINA autofocus procedure.

[@stuparker776]

Hi James Many thanks for this.I have just purchased the GT81 and I have the same camera so this will be very useful when it arrives. Looking forward to more images.Great stuff

[@Aero19612]

Congratulations, Stu! I'm enjoying the GT81. It's a nice scope. You will get lots of use out of it. I'll show my first images soon...lots of rain in the forecast. I've finished collecting data on my first 3 GT81 targets: M31, M33, and SH2-129 (Flying Bat Nebula).

[@stuparker776]

@@Aero19612 Thanks, James.I am from New Zealand so I cannot see any of those targets bugger it!!.Just a question thought the focus cube is this the correct one optcorp.com/products/pegasus-astro-focuscube-2-universal This will really help me if it works like your setup. Thanks for your comments

[@Aero19612]

That's the one. Good product. I know what you mean: I always see an awesome nebula or galaxy only to find out that it's only visible from your end of the planet. Oh well. Lot's to go around. Clear Skies!

[@Big.Ron1]

I have so much to learn. I have not seen either of the procedures before. Batinov mask and focus yes but in a comparatively crude way. Certainly not in the exacting, detailed way such as this. I think I have just found the graduate level site. Cool!

[@NeilCorke]

Really helpful review - thank you for posting this

[@Aero19612]

Thanks, Neil. Glad it was useful. I’ve been down with work and mount issues so I’m dying to get back outside with it. The stupid squid nebula ate my lunch the first time! Clear skies!

[@spike178]

Getting old my eyesight probably couldn't tell the differance from Zen81 to GT81, will save my money.

[@Aero19612]

Absolutely. "Better" is not always better.

[@febsat]

Hi James. I really enjoy your technical approach to your gear. I have the same scope & WO adjustable FF/FR. I was lucky to hit the spacing sweet spot on the first try. I did have to reach out to WO first to get some clarification on the spacing dimension needed for my DSLR imaging camera. I then set the recommended spacing using a set of digital calipers. 7.1mm. I did find that I had to tighten the locking ring quite firmly to prevent it from rotating off of the proper position. I realized this issue when I went to rotate the camera, but had not backed off the rotator locking screw sufficiently. The camera, t-ring, back end of FF/FR and locking ring all rotated as one. With those extremely fine threads, I dread overtightening that locking ring and making it impossible to change in the future, if needed, for a different camera. Every person's idea of 'hand tight' can be different.

[@Aero19612]

I really like that adjustable FF/FR. Good comment about the lock ring. I swear I can screw some of these parts together and they might as well have been welded.

[@stuparker776]

Hi James my GT81 turned up this week at last after a month wait for postage.I have just installed the field flattener and as I have an ASI1600 OXG I have set the field flattener to 8.25mm to see how good that will be compared to yours.The weather is really bad currently but I hope to try some test shots soon.I haven't added the focus cube yet I want to test out the whole system first then I will add it in. Thanks again for your tips really saved me alot of time Cheers Stu

[@Aero19612]

Congratulations, Stu! Absolutely: get familiar with a few of the systems with your new scope, then add pieces as you go. Let me know what you think.

[@wrnchhead76]

Hey niiice! I have the GTF 81 with built in flattener. I can't imagine myself ever selling it.

[@wrnchhead76]

Our rigs are almost identical looking. Same cameras, mine is the red accents. I'd pick blue but I got it long after it was discontinued and used, so I couldn't be picky. Running an AsiAir.

[@Aero19612]

Ahh. Very handy having the built-in flattener. Didn't realize they had that version. Wonder why more telescopes aren't offered in that form? Probably trying to reduce the original scope cost knowing that the typical purchaser will buy a field flattener afterward. Thanks for watching!

[@FR3EKELITE]

I read the title as GT81 Inferred Version. But great video!

[@Aero19612]

HaHa. I would not want to buy an "Inferred Version" of anything.

[@RobB_VK6ES]

Congrats on the new scope James, It will surely fill the gap between the Redcat and ED102. I am a fan of WO too. Now that you have the WO rotator/flatener it might be possible to fabri-cobble an adaptor to the ED102. Certainly within the capabilities of even a home machinist. That is some heavy shadowing from the OAG, presumably a hangover from the ED102 mounting. I bought a cheap non helical adjuster for my Orion OAG, sure beats the hell out of par focal rings and push pulling the camera. Personally I would have gone with a dedicated guide scope for the GT81 with it being a relatively short FL, no messing about recalibrating PHD everytime you rotate the imaging camera. What was wrong with the mount this time around?

[@Aero19612]

Thanks, Rob. Yeah, I'm not sure I needed to fill that gap, but the WO scope design sucks me in way too easily. I did briefly go on a web hunt for a male 68x0.75 to male 48 adapter that would work with the ED102. But I'm too lazy to move the Flat6Aiii back and forth between scopes (and re-tune each time) and too cheap to buy a second Flat6Aiii. I know what you mean by re-calibrating PHD2 each time I change the orientation of the camera, but I do like the convenience and "low profile" offered by the OAG. I will generally stay with a single orientation for many targets. Not sure anything was terribly wrong with the CGEM. The guys did find that both encoders were firmly attached (my concern), but did find the RA gear support was a bit loose. They re-tightened the screws, re-meshed the gears, and cleaned the electrical connectors. Seems to work fine. Clear skies!

[@wanderingquestions7501]

I have a WO GT71 and I’m completely satisfied w the product. With the GT71 if you want to use the case you have to disassemble/refigure body. Nothing convenient about it. It’s only practical for travel as far as I’m concerned. I also use my GT71 (420mm) as a guide scope for my 2032mm FL SCT. It’s far better then OAG. In large temperature swings (75 deg day/below 32 dogs at night) the WO can loose focus; watch those focus lock screws.

[@Aero19612]

Thanks for the feedback! Not surprised at all that you’re satisfied with WO gear. Love their products. And I totally agree about the case. Travel-capable is not a big consideration for me so the case will sit on a shelf, probably unused. Wow! A GT71 guide scope?! Haha. I’ve totally converted over to the OAG dark side now. I’ll be totally impressed with you if you use your SCT as the guide scope for your GT71.

[@marvinwhisman3333]

Are you still happy with the GT81? I am getting ready to pull the trigger on ordering one. Thanks for your thoughts and the nice video.

[@EMAINC3]

Just discovered that I may need to use the same approach with my GT81iv - that is - to mount the Vixen plate onto a Losmandy style plate. Just curious how you specifically made this connection in terms of the method/bolt size/dimension that accomplished this. Thanks for your very helpful videos!

[@Aero19612]

The Celestron dovetail plate I'm using has a number of holes for 1/4-20 screws. I'm using two of the countersunk holes to connect to two of the threaded holes in the William Optics base with sockethead cap screws. I can't remember what the thread size is on the WO plate, but I think it's M6, which works great with the 1/4-20 holes in the Celestron plate.

[@southbronxny5727]

Love the setup but would'nt a quadruplet help avoid those issues with corner stretched stars since its, supposedly, perfectly matched and built?

[@Aero19612]

Yes. The William Optics Redcat series is a four-element design. However, most of the APO refractors are three-element designs that require a field flattener to deal with the outer perimeter of the image.

[@bjdiss]

Great video as awlays! I see you have the Pegasus Astro FocusCube 2? DId you get the Celestron mount for it?

[@Aero19612]

Hey Ben. I've got two FocusCubes for my two refractors. No I didn't get the SCT mounting. Funny thing. When I was considering buying a focuser for the SCT, I had that version of the FocusCube2 in the "basket" ready to purchase and then I happened across someone mentioning the Celestron focuser which was news to me. So I went that direction instead. oops.

[@foxglovemead]

@@Aero19612 The SCT FocusCubes are supplied for particular models. I have the 68925 version. There is also a dual-motor focus controller and Motor Focus Kit with Universal and SCT variants. The DMFC includes a rotary "infinity" dial for micro and mini focusing. Once mounted, the motor is locked to the scope focuser. A benefit is it acts as a "brake", the disadvantage is there is no "manual" focus. For simple "non-computer" visual observing, the knob provides "manual" focusing. The advantage of DMFC is that it can be shared amongst two (or more) attached motors - which lowers the cost and it has manual focusing too. Hindsight is a great thing e.g. you know in advance that you want to share motors! The SCT version is specific to the scope and fits like "Mecano" e.g. just works. The Universal was a bit more fiddly to fit (on my scope) depending upon where the available mounting holes are. The coupling has four grub screws and if you use all four, the point of contact is only the grub screws. Providing the spindles are a good fit, it is better to only use one pair of grub screws on one side. This provides a grub screw into the D (or flat) and contacts with the whole of the other side. IMHO, this would be better because it ensures things are parallel. Like an eyepiece, you don't use screws on opposite sides if you don't want the EP to rock. On my scope, there was difficulty getting access to the grub screw through a hole in the focuser (YMMV) and it is here that an (optional) "ball" Allen key works well because you have the option to access the grub screw in difficult places. On Amazon/eBay there are "flexible" couplers for $5 a pair. It would be interesting to try these? The "key" to a smooth-running motor, is to align each axis as it goes together and a few minutes to get this "right" pays off. DMFC refers to use with other motors e.g. Lakeside motors. Overall Pegasus are pretty good focusers and the cube or motor kit uses an EEPROM to store its "count" after 10 seconds. So next time it is connected it knows where it is independently of software or a controller. Unlike some motors, it is not possible to forget where it is positioned and it doesn't allow any slip. So it is "ideal" for the task. The backlash is estimated at 30-40 steps (mine is about this) and should be minimal because it is using the (direct) mini focus shaft, but will be scope focuser dependent. Some controllers use the micro shaft and this introduces additional backlash. Perhaps you can clarify if the backlash step is a difference between the same position approached from the other direction or the "minimum" amount the motor has to "reverse" and recover when changing direction to reach the same position e.g. anything more than the "minimum" is all that is required. Simon

[@oryanastrophotography3450]

very nice scope! im hoping to get a frac for my star adventurer at some point

[@Aero19612]

Thanks! There a lots of good refractors out there. I like both of my William Optics refractors and my Explore Scientific. But there are lots of others.

[@oryanastrophotography3450]

@@Aero19612 I think for what my mounts capable the best would be the sharpstar 61edph ii

[@Aero19612]

That's a good lightweight, widefield scope. Should be fun to use and provide great results. Clear Skies!

[@johnparker9524]

James, first off great video. I have the same scope and flattener as you do. When I make the adjustment as you mentioned the gap in the flattener was 7.00mm and when I made the 1 1/4 turn adjustment it became 7.6mm. Does this match your gap. Also I noticed your focuser was at about 27. Is this where your auto focuser normally finds focus? Thank you

[@Aero19612]

Hey John, My focuser is still at about 27mm after numerous nights imaging/focusing. Your field flattener setting seems about right. We could be a little different because of the spacers/shims I use in the rest of the optical train back to the sensor that may be different from yours. Thanks for watching!

[@johnparker9524]

@@Aero19612 One last question, when I look at the WO's web site it shows that the focal plane is 55mm. With your setup do you still maintain the 55mm focal plane?

[@Aero19612]

@@johnparker9524 Yes. As measured from the camera end of the Flat6A, then you add on the 7mm(ish) extension in the Flat6a (as you have done, to get to the 62mm total back focus. The 55mm is a standard. That same 55mm is required with the non-adjustable field flattener on my Explore Scientific ED102. I just unplug the camera (plus OAG, filter wheel, and spacer) from it and screw it onto the WOGT81.

[@bobellis1219]

Fun to watch as I recently acquired the same scope and camera. I'm not as scientific as you are. So, I set the FLAT6Aiii to 8 mm, figuring I needed to add half the thickness of my filters to the back spacing. Using a ZWO EAF I use NINA's overshoot AF with backlash set at 100. After checking autofocus worked and the stars were round in the corners of a test image looked good I took advantage of the best seeing I've had and got 6 hours on Andromeda on my first night out with the GT81iv. I'm still tweaking the processing, but the subs are incredibly sharp. I like the perforated plate mounting idea, I plan to steal that once I install my OAG. Is that a hardware store item?

[@Aero19612]

Great, Bob! Let me know how things go. Your Flat6Aiii starting point is certainly reasonable. You may have to experiment a bit with the extension to dial in the best result. It’s not too painful. Windows updates, on the other hand, are. I updated earlier today, and now my system seems to lose contact with the USB devices. A perfect night for imaging is a wrap...

[@bobellis1219]

@@Aero19612 Stars in the corners are round at 8:1, so I'm pretty happy. I guess I need to go to PI and measure eccentricity, but if they look good at 8:1, I don't really care too much. It's incredibly easy using this scope after learning with an Orion 8" Astrograph. Somewhere in the future is a longer focal length for smaller targets.

[@richbratten]

I just opened up this review and the first thing I noticed was that the scope is sitting on a book which I instantly recognized as the "Engineering Mathematics" textbook I had in college, LOL!

[@Aero19612]

Ha. Bingo! Sorry for the painful(?) reminder. Thanks for watching the first 2 sec, Richard!

[@richbratten]

@@Aero19612 Haha! I made it through the entire video and it was great. I happen to be looking at this Williams Optics scope and that's how I found your video. I just watched another of your videos, which I just commented on, and am subscribing to your channel. Great work!

[@dr637]

Hi. Very nice. Would you be able to tell me the thread size of the three holes that are on the mounting rings, and the distance between them? Thanks

[@Aero19612]

Looks like M6 screws with hole to hole spacing of 15 mm

[@dharialmastaki]

nice telescope! whats the threaded flange size ( focuser side ) because i'm willing to buy one and fix nite crawler in it any idea ?

[@Aero19612]

Hi Dhari, I'm not sure what that thread is. It's not mentioned on WO's website and they don't send technical information with the scope. Maybe send a question into WO? The Nitecrawler is a very nice focuser. Consider that an electronic focuser at 10x lower cost works perfectly...Thanks for watching!

[@SH_space]

Quick question. Instead of getting a bigger dovetail, would it also be ok to rotate the telescope inside the rings so that the focusing gear is on top, allowing for more dec balance space?

[@Aero19612]

You can certainly rotate the telescope 180 deg. In my case, I need the dovetail plate to install the scope on my Celestron mount saddle. Also, I need additional real-estate on the plate to attach the L-bracket for the Focuscube. If you are using the manual focus or the ZWO electronic focuser that attaches to the OTA focuser, then you are free to rotate the scope in the rings.

[@SH_space]

@@Aero19612 Ok, good to know, thanks for the reply.

[@richardneel6953]

James, just stumbled across this amazingly helpful video! I'm am also a newly minted GT81 IV owner with the Flat6a III. I'm currently only connecting my ASI2600MC-pro directly to the flattener - no FW or OAG. I can't quite get the flattener adjusted so the edges are giving me non-distorted stars. Could you possibly help make a suggestion on my flattener adjustment settings for a simple setup like I have?

[@richardneel6953]

I failed to mention that I have setup the current backfocus at 55mm as per the ZWO instructions using the spacers supplied with the camera.

[@Aero19612]

Hey Richard. Congratulations! You'll enjoy the scope a lot. Even though you don't have the filter wheel or the off axis guider, you have spacers that essentially create the same back focus I have: 55 mm. The Flat6Aiii requires 62 mm for our scope, so that leaves 7 mm. I set my initial extension on the Flat6A to 7 mm and then took pictures at that extension, then shorter, then longer, then a tad bit longer, and eventually settled final extension just by how the stars looked in the corners. So the steps are: 1. Set the extension on the Flat6A 2. Focus on a star field 3. Take a short-duration picture (we don't want guiding/seeing effects) 4. Inspect the star shape in the corners 5. Go back to (1) until you like what you see in (4).

[@richardneel6953]

@@Aero19612 I think I have it nailed James! Again, thanks for this video. Now, the next question, could you perhaps show me how you have the Pegasus focuser attached to your GT81?

[@Aero19612]

Hey Richard. Check out this video: kzbin.info/www/bejne/eaWmi3qvqMx9ndU I show the attachment in more detail with that scope and it's identical to the GT81.

[@jaredroberts6409]

did you ever figure out a way to get the william optics batinov mask to work better. I was out last night and the lines were to illuminated to get a measurement on batinov grabber, the pixel readings were bouncing all over the place and I noticed the line created were very illuminated. I was thinking about painting the clear part black to reduce reflecting light. I will probably use gaffer tape first to see if that helps but do you have any other suggestions for the batinov grabber routine. I tried different ISO, time, stars, and such and could never get a good reading. I was in 1:1 on APT when doing this. Thank you for the great videos.

[@Aero19612]

Hey Jared, Yes. I stopped using it. HaHa. The NINA autofocus routine has been working so well, even with the narrowband filters, that I've decided it's not worth the effort to define filter offsets. I'll just do an autofocus on filter change and call it a night. Meanwhile, I've been using the Redcat 51 (manual focus with its acrylic Bahtinov mask) and the diffraction spikes are awesome (see my latest video on Bahtinov mask focusing in NINA and APT). Is it just that GT81 mask? Focal length? Operator error (no way)? Not sure. Good idea with the tape, but try to maintain the sharp edges so you get crisp diffraction spikes. As a last resort, could try a "normal" Bahtinov mask: farpointastro.com/shop/farpoint-82mm-camera-filter-bahtinov-focus-mask/ Let me know how things go!

[@jaredroberts6409]

@@Aero19612 Thank you for the help. I am at an influx in my setup right now where I am not able to use nina for capture because I am still on DSLR and nina just does not have the same level of control on DSLR as APT does. Also I am not using mono so my goal was to figure out the backlash of the focuser/telescope combination. I just ordered a black plastic batinov mask to test out; after trying to use the tape idea it was difficult to get a sharp edge and I think painting my current one might not be the best idea given the affordable cost of the one I got.

[@Aero19612]

Ahh. Ok. Since you're not interested in filter offsets, using the Bahtinov mask to determine the backlash should work OK-that's the way I got my backlash. Where I got into "trouble" was trying to get consistent filter offset numbers (a much smaller number than the backlash: 10 steps VS 80 steps for example). I'll be interested to hear how the good ol' fashioned Bahtinov mask works for the same scope.

[@danjensen9425]

Hey another great video about a telescope I would actually buy . I’m getting the asi 294mc pro when they get here , money is camping out at the bank . So what nice setup , OAG . On the guiding part I have the cgemll and sent it in for hypertuning at deep sky products . What king of guiding do you get with the cgem and have you hypertuned it .

[@Aero19612]

Hey, Dan. I'm enjoying the GT81. I'm imaging with the Explore Scientific ED102 now (well, as soon as the clouds move out), then it'll be back to the attitudinal C9.25 for galaxy season. I've had an eye on the 294. Looks like a winner! Are you going to get a tri- or quad-band filter for it? I have had my mount hypertuned. Made balancing more precise. Not sure it improved guiding (must have, at least at the margins). Tough to say. Lots of variables...seeing being the biggest. I tend to have a lot of DEC backlash, which is problematic (trying some things out, video to follow). Also, the RA gear harmonics show up in guiding performance. That said, I can get guiding around 0.8 RMS arc-sec, which is good enough for my SCT. I also get guiding above 1 RMS arc-sec frequently which is a problem for the SCT but not so much for the GT81. PHD2 has a "predictive periodic error compensation" algorithm that does a decent job with some of the RA gear harmonics. I've been playing with the DEC backlash compensation in PHD2 and am pleased with its contribution (or was it just good seeing that night?). So many questions. So few answers. Have fun with the new camera!

[@danjensen9425]

I have a spacecat and a C8 with the 6.3 focal reducer corrector. One of the reasons I liked your video so much . Not many will say great things about imaging with a sct but I have images of m101 and m51 plus the lagoon nebula that are really good . So far the spacecat images of Pleiades , Andromeda and Orion and the running man are really awesome. A middle of the two scopes is next . I start exploring Nina when I get my mount back . I started watching you when I wanted information on dew shield limits and dew heaters . Im able to drive to bortle 3 but at 4000 foot winter is a no go and at other times dew point happens quickly. Star alignment in a bortle 3 is challenging. the first two anyway. Im thinking Nina will change things . I use a polemaster . Looking forward to next video . Better than watching current events at this point . Oh i read in cloudy nights , turn declination backlash on and set the maximum to 5000 or more in phd2 . Also set the declination guide speed to 90% instead of 50% with the hand control for the mount . Not sure about where in phd2 the first is done but read in the cgemll manual how to do that . That was mentioned by Michael Covington.

[@Aero19612]

Thanks, Dan. I ran PHD2 Guide Assistant. The manual says to turn up the guide rate to try and get backlash delay less than 3 sec. it told me I have 3.8 sec with a guide speed of 80% (have only used 50% rate in past). Anyway, I set the pulse length to 3800 ms and things seem to work ok. Going to experiment with a small amount of DEC backlash comp in mount to “help”. We’ll see.

[@gunnarjensen5910]

I just got my new WO Zenith81 and tried to focus on my Canon 5D and Canon M50 but failed. Can you advice ?

[@Aero19612]

Hey Gunnar. If you're focusing on a star or the moon, you can estimate where the camera's sensor should be by measuring back from the telescope objective by the focal length (I think it's 559 mm without a focal reducer). If the focuser is fully extended and you can't get the camera's sensor 559 mm back from the objective, then you won't be in focus and will need additional adapters to get more back focus. Just a thought...

[@BrokenPik]

ive been looking for this video for a long time.. no matter what i do my Planostar on the TMB 115 MM F7 EDT just will not come to good stars in the corners. im in bad need of a Variable 12-18MM , M42 0.75 pitch spacer. Real hard to find all i need is 56.5 MM .. Any ideas.? my Pegasus has 200 steps of backlash and 125 steps on 9 points in SGP ...

[@Aero19612]

Hey Mitch. I have a similar problem with my Explore Scientific ED102 and its field flattener. ZWO is good about providing thin shims with the imaging cameras and filter wheel. That said, the adjustability of the WO Flat6Aiii is VERY nice. I have heard of adjustable spacers and may have seen a few, but have never owned one. Check out the line of Blue Fireball adapters at Agena Astro, such as: agenaastro.com/blue-fireball-m48-thread-variable-spacer-ring-set-s-set2.html

[@BrokenPik]

@@Aero19612 Dang all my spacers are M42 not M48 maybe they have those also.

[@Aero19612]

Also, ZWO provides (and sells separately, I believe) a double sided threaded ring with M42 on the inside and M48 on the outside that will convert M42 spacer (male thread) to M48 (male thread). Beware: once it's on there, there's no taking it off! But there's lots of Blue Fireball spacers and M48 is not that odd.

[@mrtelecaster009]

I have an FLT91 and about to attatch it to a WO Losmandy plate in the same way you have on the Celestron Losmandy plate. I wanted to ask what bolts you used to connect the two together safely please? Thank you.

[@Aero19612]

Hey, Andy. They are either 1/4-20 or M6. My Celestron plate has clearance holes for 1/4-20, but the threaded holes in the WO base are probably M6. Just need a minimum of 2 screws. Hope that helps.

[@mrtelecaster009]

@@Aero19612 Hi James, thank you for that, it is a big help

[@Astro-George]

Hi James, What nonreducing field flattener do you use with this scope?

[@Aero19612]

Hi George. I’m using the WO Flat6Aiii which is also a 0.7x reducer which drops the focal length from 478mm to 385mm. If you’re trying to maintain the 478mm, you might look up the Hotech flattener: Hotech 2" SCA Self-Centering Field Flattener - SCA-FFT58 It’s supposed to work with focal ratios from 5 to 8 (GT81 = 5.8). Ask Hotech support or the seller if this flattener is compatible with this scope. Thanks for watching!

[@astrophotonics9470]

Was going to go for this until i seen W.O.'s soon to be released 90mm FLT.

[@Aero19612]

Wow. That's a good aperture. I might have considered that if it was available at the time. I like their scopes quite a bit. Enjoy and thanks for watching!