Well done Boyd and Reynold! I really enjoyed your approach with this review (part 1 as well). Very glad to see another UA-cam channel reviewing telescopes using the IMX455 sensor. While the corner performance with such a demanding sensor won't apply to everyone's situations, I think it is best to put these optics to the ultimate test and make inferences about smaller sensors from there. I found that trying to compare telescopes using crop sensors or full frame sensors with larger pixels, the differences were much harder to eke out.
Nico, we learn from the best! Following your example, we wrote to ZWO to request an ASI6200MC Pro review unit so we could test for chromatic aberration, as you do in several videos, but so far no luck. So we used our ZWO ASI6200MM Pro for the tests. But ultimately, though, if star images are small and round through this monochrome sensor, doesn't it seem likely that chromatic aberration will be minimal? And when we finish our first processed color image through the Cat 91, might it be useful to check this image for chromatic aberration? (Maybe not as good as a one-shot-color test, but better than nothing?) Thanks so much for your passion for the hobby, your carefully planned videos, and your encouragement and inspiration. Boyd
@@SensorSensibility Yes, I think it would definitely be useful to check the RGB image for chromatic abberation. I wouldn't advise looking at the processed image though. It would be better to combine the channels, apply an unlinked STF autostretch and look at that. The only issues I see with mono for testing CA are: 1. filter timing and sky location - could be solved by rotating through the filters with offsets but there can be systematic error there as well and 2. focus in general - there are arguments for offsets vs. focusing each filter separately and I'm not sure there can be a definitive answer on which method will give more consistent results. In any case, I think no tests are ever perfect so as long as you explain your methodology and try your best it will be very valuable to the AP community. Cheers, Nico
Great review! I enjoyed the comparison with the Cat 71. Surprising to see how blobby the Cat 71 stars are. You both have my kind of sense of humor. Happy new year and clear skies!
Thanks. We might have just gotten a bad Cat 71 unit, and other units are better. But it's what we had and we wanted to compare it with the Cat 91, which is pretty much perfect. I'm glad you appreciate our "dad" and "grandpa" jokes. And happy new year to you!
I liked your dry humour. It is a great telescope! And sold out! I have got good results with Askar 103APO so far, price/performance ratio is amazing, and I am pushing the limits of my CEM26 mount and ASI533 MC-Pro. Cat91 and 2600MM on AM5N are the combo I am craving in future... $$$ Subscribed! Way to go! Clear skies!
Great video! I have been looking forward to a review, but I was shocked to see that you already had one since they have not even shipped yet! I am looking at purchasing one for myself soon.
We just spent 6 hours last night shooting M31 with the Cat 91. We'll be posting a video on the shoot soon. Short answer? It's really amazing. I think it's a breakthrough in telescope design. It's a tall order for an f/4.9 telescope to have small, round stars all the way to the corners of a full-frame sensor.
@@SensorSensibility I have the RC 51 and Askar 107 PHQ. Great experiences with both so far, but the Cat 91 looks amazing. I have ask Santa for it but we will see.
Thanks for the review and being an early tester. I have a couple of questions for new users of the Cat 91. Does it come with, or is it possible, to use a m68 adapter for connecting larger diameter imaging trains? Also, what would be the maximum working distance between the m68 or m54 flange and the sensor plane? I’m trying to determine if my imaging train will work with this scope. I appreciate any feedback.
I don't know the answers to your questions, but Ren and I want to test the scope with a medium-format sensor and we'll need an adapter, so I plan to write to Karen Shen at William Optics (karen@williamoptics.com) to ask about this. You could certainly write to her with your question. She's been very helpful to us.
@@SensorSensibility Thanks for the reply. I will contact Karen. It’s curious to me that adapters and working distances are not commonly published specs, as it’s critical information for determining the suitability of different imaging trains. I look forward to any additional videos you guys publish. -Cheers
Thanks for the review. I have the Cat51 and might get a bigger one in the future. I have a doubt about the comparison. The Cat 91 does not require a specific back focus distance due to its optical design, but I think that this does not hold for the smaller Cats. In that case, did you consider adding the correct spacing to the Cat71 to compensate for the change in light path due to the luminance filter used in the comparison? I noticed the effect (improvement of the stars at the corners) by shifting the back focus from 55 mm to 55.65 mm with my Cat 51. And the Cat 51 should be more tolerant than the 71, given they share the same optical design.
That's curious that you noticed an improvement in the stars in the corners by changing the backfocus, because all of the William Optics Cats are Petzval refractors and are advertised to not require precise backfocus. Under "Overview" on the description of the Cat 51 at williamoptics.com, it says, "Exceptional Optical Quality: Boasting a Petzval-like design and a fast f/4.9 focal ratio, this telescope offers sharp, detailed views across a wide field-perfect for stargazing and capturing celestial wonders. Enjoy hassle-free imaging without the struggle of backfocus."
Thanks for the review. Was thinking of getting the 71 until the 91 came out. I'll be getting the 91. Look forward to seeing more pictures from you two .
Great work and informative. Do you use a counterweight with the 91 and AM5? Do you use the CF tripod or some other method of supporting the am5? Thanks. Des
Thanks! We use an AM5 12-pound counterweight with the Celestron EdgeHD 9.25, but don't use the counterweight with the Cat 91. In case you're interested, we shot a setup video that shows all of the equipment in our Cat 91 rig. Ren is editing the video. Hopefully it will be up in a day or two.
Thanks for the great video. I am getting ready to pull the trigger on the Redcat 91. How do you think the scope will do with a 2600 camera? I already own the 2600MC and not sure my budget can handle the additional cost of the 6200 although they are on sale right now. Again, thanks for the video and hope to see some processed images soon. Clear skies.
The Cat 91 will do great with the 2600 camera! The main difference between using the 6200 and the 2600 with the Cat 91 is the size of the field of view, which is 4.6° x 3.1° for the 6200 and 3° x 2° for the 2600, but you will still be able to fit the Andromeda Galaxy into the field of view if you orient it corner to corner in the field of view. The camera rotator on the Cat 91 is wonderful and super convenient. There are other telescope options, though, that will give you a flat field over an APS-C sensor like the 2600, so I'm not sure that I would spend the money on a Cat 91 unless I had at least the prospect of buying a full-frame camera sometime down the road.
@@SensorSensibility Thanks so much for your advice. I will probably order both. I am somewhat concerned about future price increases since we appear headed for a war of tariffs and I have been considering the scope for some time so I might as well not wait. Thanks again.
@@marvinwhisman3333 Sounds like a plan to me! I think you'll be very happy with the Cat 91. We certainly are! The state of the astrophotography art is quite amazing to me right now, in optics, sensors, and post-processing software, making detailed images possible that were almost unimaginable just a few years back.
That's possible, but another possibility is that we received a unit with misaligned optics, and you got one that is well aligned. Or perhaps you are using an APS-C sensor? These are more forgiving of misaligned optics than full-frame sensors.
Hey guys, you are shedding light on a scope that seems to be the cream of the crop for 2025 and beyond. Let's hope there are further increases in full frame camera options with smaller pixels in the future. This telescope will shine even more then! Can you tell us more about the "backfocus" from the back of the adapter to the camera sensor? It looks like you have the OAG, FW, and camera threaded right to the back. What kind of focuser range plus and minus does this give you once you find focus?
Chen and I just posted some perspective on this question in this CloudyNights thread: www.cloudynights.com/topic/944703-william-optics-redcat-91-wifd/?p=13820800
@ I will do that and post it here. I also bought EAF, and I already use Astap. Tight spot diagrams is a high priority for me, and I was about to order the RedCat 71 when the 91 showed up. My camera is only APS-C size (ASI2600MC Air), however. BTW, for my smaller wide-field scope, it has critical backfocus, so I use Askar’s micro-adjustable backfocus with 54mm threads. It makes a difference for non-Petzval scopes, especially to compensate for the filter thickness.
@@SensorSensibility I also got beautiful round stars to the corners. But my sensor is only APS size. Chromatic aberration looks pretty good, too. I posted an IC434 image on Telescopius, if you want a look. I didn’t use calibration frames and I didn’t sharpen the image. I’m very pleased with this scope. 😊
Yeah, they are expensive! But we're hoping to use them (Antlia V series LRGB broadband and 2.5 nm ultra narrowband SHO filters) for the rest of our lives, so we're thinking of it as a long-term astronomical investment.
Hi Guys, can you send me one of your best M31 frames over so that I can compare it to my FSQ106EDX4 setup which is also on an AM5? I really would like to see whether your stars are really smaller than mine. I can also send one of mine.
Ren and I would absolutely love to collaborate on this! Here's a link to a 3-minute FITS luminance frame of M31 taken with the ASI6200MMPro / Cat 91 combination under a moonless Bortle 2 sky: usu.box.com/s/ktrgqo7cdtdin7xxskf6qfjkpcsqzrwn. The cores of M31 and M32 are blown out in this image - I'm thinking that 3 minute exposures are too long on this bright object. A comparison between Tak 106 and Cat 91 frames will be most useful if the camera angle is approximately the same and if the exposure length is identical, because the longer the exposure, the bigger the stars. If you don't have a 3-minute exposure at approximately the same camera angle, please send me what you have and I'll try to match it - it looks like I might get a break in the clouds during the next few nights. What camera are you using? Can we continue the collaboration via email at boyd.edwards@usu.edu?
Ok hear me out. Why should I care about these aberrations (as long as they are not egregious) because blurX can easily take care of it? I can spend that extra money towards other things.
Yes, BlurX can fix star aberrations if they're not too egregious. We had some nasty star aberrations from our Cat 71 (maybe we got a bad unit) that caused us trouble in post processing even with BlurX and StarX, and we're interested in processing images that are as clean as possible from the start. I'm not so sure that BlurX can fix aberrations in nebulosity (that also come from imperfect optics) as well as it fixes aberrations in stars. Hence the 91, which we're happy with. But each astrophotographer will find their own sweet spot in the price/performance continuum, and it appears that folks can get round stars with a good triplet APO and BlurX.
If you don't expect truly excellent results from your equipment, then they are massively overpriced. Yes, you can spend less to get reduced quality if you choose. lol
How does the 'entertainment value' of this video affect the results they got in their photos? lol Do you believe that a less entertaining video means their evidence is less valid? lol
Well done Boyd and Reynold! I really enjoyed your approach with this review (part 1 as well).
Very glad to see another UA-cam channel reviewing telescopes using the IMX455 sensor. While the corner performance with such a demanding sensor won't apply to everyone's situations, I think it is best to put these optics to the ultimate test and make inferences about smaller sensors from there. I found that trying to compare telescopes using crop sensors or full frame sensors with larger pixels, the differences were much harder to eke out.
Nico, we learn from the best! Following your example, we wrote to ZWO to request an ASI6200MC Pro review unit so we could test for chromatic aberration, as you do in several videos, but so far no luck. So we used our ZWO ASI6200MM Pro for the tests. But ultimately, though, if star images are small and round through this monochrome sensor, doesn't it seem likely that chromatic aberration will be minimal? And when we finish our first processed color image through the Cat 91, might it be useful to check this image for chromatic aberration? (Maybe not as good as a one-shot-color test, but better than nothing?) Thanks so much for your passion for the hobby, your carefully planned videos, and your encouragement and inspiration. Boyd
@@SensorSensibility Yes, I think it would definitely be useful to check the RGB image for chromatic abberation. I wouldn't advise looking at the processed image though. It would be better to combine the channels, apply an unlinked STF autostretch and look at that. The only issues I see with mono for testing CA are: 1. filter timing and sky location - could be solved by rotating through the filters with offsets but there can be systematic error there as well and 2. focus in general - there are arguments for offsets vs. focusing each filter separately and I'm not sure there can be a definitive answer on which method will give more consistent results. In any case, I think no tests are ever perfect so as long as you explain your methodology and try your best it will be very valuable to the AP community. Cheers, Nico
@@NebulaPhotos Thanks, Nico. That's good advice.
Great review! I enjoyed the comparison with the Cat 71. Surprising to see how blobby the Cat 71 stars are.
You both have my kind of sense of humor. Happy new year and clear skies!
Thanks. We might have just gotten a bad Cat 71 unit, and other units are better. But it's what we had and we wanted to compare it with the Cat 91, which is pretty much perfect. I'm glad you appreciate our "dad" and "grandpa" jokes. And happy new year to you!
I liked your dry humour. It is a great telescope! And sold out! I have got good results with Askar 103APO so far, price/performance ratio is amazing, and I am pushing the limits of my CEM26 mount and ASI533 MC-Pro. Cat91 and 2600MM on AM5N are the combo I am craving in future... $$$
Subscribed! Way to go! Clear skies!
Wow, amazing scope and images guys, thanks for sharing!
Great video! I have been looking forward to a review, but I was shocked to see that you already had one since they have not even shipped yet! I am looking at purchasing one for myself soon.
You won't regret it.
What a winner. I’m in my early days (both for Astro & UA-cam) and am still using telephoto lenses for astrophotography - this cat looks nuts!
Yeah. It is really impressive. I consider it to be a breakthrough in telescope design.
Great Video. Looking forward to more.
We just spent 6 hours last night shooting M31 with the Cat 91. We'll be posting a video on the shoot soon. Short answer? It's really amazing. I think it's a breakthrough in telescope design. It's a tall order for an f/4.9 telescope to have small, round stars all the way to the corners of a full-frame sensor.
@@SensorSensibility I have the RC 51 and Askar 107 PHQ. Great experiences with both so far, but the Cat 91 looks amazing. I have ask Santa for it but we will see.
Thanks for the review and being an early tester. I have a couple of questions for new users of the Cat 91. Does it come with, or is it possible, to use a m68 adapter for connecting larger diameter imaging trains? Also, what would be the maximum working distance between the m68 or m54 flange and the sensor plane? I’m trying to determine if my imaging train will work with this scope. I appreciate any feedback.
I don't know the answers to your questions, but Ren and I want to test the scope with a medium-format sensor and we'll need an adapter, so I plan to write to Karen Shen at William Optics (karen@williamoptics.com) to ask about this. You could certainly write to her with your question. She's been very helpful to us.
@@SensorSensibility Thanks for the reply. I will contact Karen. It’s curious to me that adapters and working distances are not commonly published specs, as it’s critical information for determining the suitability of different imaging trains. I look forward to any additional videos you guys publish. -Cheers
Thanks for the review. I have the Cat51 and might get a bigger one in the future. I have a doubt about the comparison. The Cat 91 does not require a specific back focus distance due to its optical design, but I think that this does not hold for the smaller Cats. In that case, did you consider adding the correct spacing to the Cat71 to compensate for the change in light path due to the luminance filter used in the comparison? I noticed the effect (improvement of the stars at the corners) by shifting the back focus from 55 mm to 55.65 mm with my Cat 51. And the Cat 51 should be more tolerant than the 71, given they share the same optical design.
That's curious that you noticed an improvement in the stars in the corners by changing the backfocus, because all of the William Optics Cats are Petzval refractors and are advertised to not require precise backfocus. Under "Overview" on the description of the Cat 51 at williamoptics.com, it says, "Exceptional Optical Quality: Boasting a Petzval-like design and a fast f/4.9 focal ratio, this telescope offers sharp, detailed views across a wide field-perfect for stargazing and capturing celestial wonders. Enjoy hassle-free imaging without the struggle of backfocus."
@SensorSensibility optical excelence yes, but I have read the hassle-free comment only for the ones that are quintuplet.
Nice hardware!
Yes!
Awesome video. I love the humour.
Thanks.
Thanks for the review. Was thinking of getting the 71 until the 91 came out. I'll be getting the 91. Look forward to seeing more pictures from you two .
Good call! We're almost finished taking a deep dive into the Andromeda Galaxy with the 91, and should have our image out within a few days.
Great work and informative. Do you use a counterweight with the 91 and AM5? Do you use the CF tripod or some other method of supporting the am5? Thanks. Des
Thanks! We use an AM5 12-pound counterweight with the Celestron EdgeHD 9.25, but don't use the counterweight with the Cat 91. In case you're interested, we shot a setup video that shows all of the equipment in our Cat 91 rig. Ren is editing the video. Hopefully it will be up in a day or two.
Thank you very much for the video. Greetings from germany😊
You're welcome, and thanks for watching!
Thanks for the great video. I am getting ready to pull the trigger on the Redcat 91. How do you think the scope will do with a 2600 camera? I already own the 2600MC and not sure my budget can handle the additional cost of the 6200 although they are on sale right now. Again, thanks for the video and hope to see some processed images soon. Clear skies.
The Cat 91 will do great with the 2600 camera! The main difference between using the 6200 and the 2600 with the Cat 91 is the size of the field of view, which is 4.6° x 3.1° for the 6200 and 3° x 2° for the 2600, but you will still be able to fit the Andromeda Galaxy into the field of view if you orient it corner to corner in the field of view. The camera rotator on the Cat 91 is wonderful and super convenient. There are other telescope options, though, that will give you a flat field over an APS-C sensor like the 2600, so I'm not sure that I would spend the money on a Cat 91 unless I had at least the prospect of buying a full-frame camera sometime down the road.
@@SensorSensibility Thanks so much for your advice. I will probably order both. I am somewhat concerned about future price increases since we appear headed for a war of tariffs and I have been considering the scope for some time so I might as well not wait. Thanks again.
@@marvinwhisman3333 Sounds like a plan to me! I think you'll be very happy with the Cat 91. We certainly are! The state of the astrophotography art is quite amazing to me right now, in optics, sensors, and post-processing software, making detailed images possible that were almost unimaginable just a few years back.
I have the original 71 and I don’t think I have elongated stars at all. Could that be on the wifd model only?
That's possible, but another possibility is that we received a unit with misaligned optics, and you got one that is well aligned. Or perhaps you are using an APS-C sensor? These are more forgiving of misaligned optics than full-frame sensors.
Hey guys, you are shedding light on a scope that seems to be the cream of the crop for 2025 and beyond. Let's hope there are further increases in full frame camera options with smaller pixels in the future. This telescope will shine even more then!
Can you tell us more about the "backfocus" from the back of the adapter to the camera sensor? It looks like you have the OAG, FW, and camera threaded right to the back. What kind of focuser range plus and minus does this give you once you find focus?
Chen and I just posted some perspective on this question in this CloudyNights thread: www.cloudynights.com/topic/944703-william-optics-redcat-91-wifd/?p=13820800
Hmmm.... Idk. I like your Seestar S50 much better. This delivery was subpar. Hope the best.
The jokes were….. how can I say it politely……keep your day jobs guys.
Mine arrived today!!!! But it’s raining… 😢
I would be interested to see if your corner stars are as round as ours, as a check on the quality control of the Cat 91 production.
@ I will do that and post it here. I also bought EAF, and I already use Astap. Tight spot diagrams is a high priority for me, and I was about to order the RedCat 71 when the 91 showed up. My camera is only APS-C size (ASI2600MC Air), however. BTW, for my smaller wide-field scope, it has critical backfocus, so I use Askar’s micro-adjustable backfocus with 54mm threads. It makes a difference for non-Petzval scopes, especially to compensate for the filter thickness.
@@swagonman Fantastic!
@@SensorSensibility I also got beautiful round stars to the corners. But my sensor is only APS size. Chromatic aberration looks pretty good, too. I posted an IC434 image on Telescopius, if you want a look. I didn’t use calibration frames and I didn’t sharpen the image. I’m very pleased with this scope. 😊
@@swagonman That's fantastic! Congratulations!
Ohhh! very nice :)
But don't forget the spare kidney you'll need to buy the matching 2" filter set! ;)
Yeah, they are expensive! But we're hoping to use them (Antlia V series LRGB broadband and 2.5 nm ultra narrowband SHO filters) for the rest of our lives, so we're thinking of it as a long-term astronomical investment.
@@SensorSensibilityGreat choice of filters! In have the 2.8nm and love them!!!!
Hi Guys, can you send me one of your best M31 frames over so that I can compare it to my FSQ106EDX4 setup which is also on an AM5? I really would like to see whether your stars are really smaller than mine. I can also send one of mine.
Ren and I would absolutely love to collaborate on this! Here's a link to a 3-minute FITS luminance frame of M31 taken with the ASI6200MMPro / Cat 91 combination under a moonless Bortle 2 sky: usu.box.com/s/ktrgqo7cdtdin7xxskf6qfjkpcsqzrwn. The cores of M31 and M32 are blown out in this image - I'm thinking that 3 minute exposures are too long on this bright object. A comparison between Tak 106 and Cat 91 frames will be most useful if the camera angle is approximately the same and if the exposure length is identical, because the longer the exposure, the bigger the stars. If you don't have a 3-minute exposure at approximately the same camera angle, please send me what you have and I'll try to match it - it looks like I might get a break in the clouds during the next few nights. What camera are you using? Can we continue the collaboration via email at boyd.edwards@usu.edu?
What a purr-fect thumbnail.
Thanks to you!
Ok hear me out. Why should I care about these aberrations (as long as they are not egregious) because blurX can easily take care of it? I can spend that extra money towards other things.
Yes, BlurX can fix star aberrations if they're not too egregious. We had some nasty star aberrations from our Cat 71 (maybe we got a bad unit) that caused us trouble in post processing even with BlurX and StarX, and we're interested in processing images that are as clean as possible from the start. I'm not so sure that BlurX can fix aberrations in nebulosity (that also come from imperfect optics) as well as it fixes aberrations in stars. Hence the 91, which we're happy with. But each astrophotographer will find their own sweet spot in the price/performance continuum, and it appears that folks can get round stars with a good triplet APO and BlurX.
...gave up watching after the first few bad jokes. Nice job
These scopes are massively overpriced.
If you don't expect truly excellent results from your equipment, then they are massively overpriced. Yes, you can spend less to get reduced quality if you choose. lol
@derekbaker3279 bs
Pretty dry video, maybe just get an 80mm PHQ and save
How does the 'entertainment value' of this video affect the results they got in their photos? lol Do you believe that a less entertaining video means their evidence is less valid? lol