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Why aren't images with my Redcat 51 flat?

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#1 triskelion

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Posted 14 July 2019 - 02:18 PM

Hello,

 

I just recently obtained a William Optics Redcat 51. I have loved everything about it so far, except I can't get a flat field. I have achieved a backfocus of 55 mm on it with my ZWO 1600 MM (camera+electronic filter wheel + 16.5 mm spacer), but I notice that the image still isn't flat. Does anyone have any idea why this might be the case? The scope is supposed to not require a field flattener. I have attached a recent image I took as an example. 

 

https://imgur.com/a/qIAgfX4

 

Thanks!


Edited by triskelion, 14 July 2019 - 02:19 PM.


#2 rockstarbill

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Posted 14 July 2019 - 02:21 PM

I've been hunting down every image I can taken with the Redcat and haven't found any perfect images in terms of the field yet. Coma has been mild to severe in most of them.

Even the images on their site suffer from it.

Edited by rockstarbill, 14 July 2019 - 02:27 PM.


#3 Ballyhoo

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Posted 14 July 2019 - 02:30 PM

Hello,

 

I just recently obtained a William Optics Redcat 51. I have loved everything about it so far, except I can't get a flat field. I have achieved a backfocus of 55 mm on it with my ZWO 1600 MM (camera+electronic filter wheel + 16.5 mm spacer), but I notice that the image still isn't flat. Does anyone have any idea why this might be the case? The scope is supposed to not require a field flattener. I have attached a recent image I took as an example. 

 

https://imgur.com/a/qIAgfX4

 

Thanks!

 

I've been hunting down every image I can taken with the Redcat and haven't found any perfect images in terms of the field yet. Coma has been mild to severe in most of them.

Will this allow you to return it?   

 

Same thing with my EON 85mm, but the way they word it means they have a loophole in my case :

 

OPT:

What does this mean for you, the consumer? With its two fluoro-crown ED glass elements and fully multi-coated optics, the Orion EON 85mm ED-X2 refractor provides superior correction of chromatic and spherical aberrations. By suppressing these aberrations so effectively, this triplet "apo" telescope produces impeccable visual views and razor-sharp astrophotos without any fringing or loss of clarity.

 

But this thread is not about my Eon.  If a manufacturer promotes a flat field, then they really need to deliver one. in my case I think the adverting was misleading but when I do an astrobin search of redcat I find quite a few images where it would take a discerning eye to catch the coma:

 

https://www.astrobin.../full/412639/E/


Edited by Ballyhoo, 14 July 2019 - 02:31 PM.


#4 rockstarbill

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Posted 14 July 2019 - 02:39 PM

Your EON wasn't misleading. Your understanding of imaging on a refractor was.

Back to the RedCat.

https://www.astrobin...391087/?nc=user

This is an excellent image from a seasoned imager. It is also taken with a full frame DSLR camera. However it still shows star problems at full resolution. While not a game breaker for everyone, the scope doesn't live up to WO's promise and marketing of a perfectly sharp instrument.
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#5 Traveler

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Posted 14 July 2019 - 02:46 PM

RedCat is a nice name for a telescope in a nice design...on paper. Now the real astroworld test...and it won't be me as i am not an early adapter/buyer...lessons learned long time ago.



#6 rockstarbill

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Posted 14 July 2019 - 02:49 PM

RedCat is a nice name for a telescope in a nice design...on paper. Now the real astroworld test...and it won't be me as i am not an early adapter/buyer...lessons learned long time ago.


There is plenty of data available, and many threads on the Redcat 51. Most buyers got duds with severe coma, some folks got decent scopes with minor issues, I'm sure a few people got fairly close to the design goal. The variance in scope quality is disappointing. I was hoping this scope would live up to its billing....
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#7 RogeZ

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Posted 14 July 2019 - 02:49 PM

And you are having issues with a small sensor, install a FF and now things are even worse. It takes an incredibly well made scope to perfectly correct a full frame sensor.
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#8 rockstarbill

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Posted 14 July 2019 - 02:51 PM

Maybe we'll get a Redcat 2! Lol

#9 einarin

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Posted 14 July 2019 - 02:58 PM

Boy, demanding customers ;). I would be quite happy with that - only 100% shows it.

Triskelion, how about if you add 1mm to that 55mm ? The recommended distance is not always the absolute truth.



#10 kyle528

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Posted 14 July 2019 - 03:03 PM

Will this allow you to return it?   

 

Same thing with my EON 85mm, but the way they word it means they have a loophole in my case :

 

OPT:

What does this mean for you, the consumer? With its two fluoro-crown ED glass elements and fully multi-coated optics, the Orion EON 85mm ED-X2 refractor provides superior correction of chromatic and spherical aberrations. By suppressing these aberrations so effectively, this triplet "apo" telescope produces impeccable visual views and razor-sharp astrophotos without any fringing or loss of clarity.

 

But this thread is not about my Eon.  If a manufacturer promotes a flat field, then they really need to deliver one. in my case I think the adverting was misleading but when I do an astrobin search of redcat I find quite a few images where it would take a discerning eye to catch the coma:

 

https://www.astrobin.../full/412639/E/

You're thinking of field curvature. 


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#11 rockstarbill

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Posted 14 July 2019 - 03:09 PM

Boy, demanding customers wink.gif. I would be quite happy with that - only 100% shows it.

 

IMHO it is not demanding to ask the product to do what the manufacturer tells folks it will do, which is a flat field across a full frame sensor.

 

In this example, the sensor is a 4/3. Not even close... 


Edited by rockstarbill, 14 July 2019 - 03:17 PM.

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#12 Ballyhoo

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Posted 14 July 2019 - 03:13 PM

You're thinking of field curvature. 

oh yes I was. I am such an amateur.   I have been eyeing this little telescope though. i am personally very surprised to read of the variation in quality. One would expect much more from WO.  No?

 

edit, but in the case of eon they mentioned: "spherical aberrations".  Is that not field curvature?


Edited by Ballyhoo, 14 July 2019 - 03:14 PM.


#13 rockstarbill

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Posted 14 July 2019 - 03:20 PM

edit, but in the case of eon they mentioned: "spherical aberrations". Is that not field curvature?


No it's not. That's off topic though.

Go read this: https://en.m.wikiped...ical_aberration
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#14 kyle528

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Posted 14 July 2019 - 03:21 PM

oh yes I was. I am such an amateur.   I have been eyeing this little telescope though. i am personally very surprised to read of the variation in quality. One would expect much more from WO.  No?

 

edit, but in the case of eon they mentioned: "spherical aberrations".  Is that not field curvature?

Field curvature is when the focal plane is not flat. Spherical aberration is when the light is not focused to a single point, but rather along a line. 


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#15 Ballyhoo

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Posted 14 July 2019 - 03:23 PM

Field curvature is when the focal plane is not flat. Spherical aberration is when the light is not focused to a single point, but rather along a line. 

Thank you I should know this. 



#16 triskelion

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Posted 14 July 2019 - 03:48 PM

Wow! Great discussion. What a fun way to get introduced to this forum.

 

I think something I am still confused about is how backfocus plays a role with this design. I have read on other threads that as long as you can achieve infinity focus with this Petzval design the image should be flat. Can someone confirm this? From my understanding, the sensor needs to be 59.7 mm behind the rotator (based on the product page), so with my 55 mm setup I have to use the helical focuser to achieve that additional ~5 mm. If I am mistaken on this (and actually need to have that 59.7 mm) then any criticism for the lack of flat field in the image above should be towards me. Can anyone provide insight for a newbie like me?



#17 rockstarbill

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Posted 14 July 2019 - 03:51 PM

Yeah as long as you're in focus, the field should be flat. Should being the operative word. This scope isn't living up to its promise at this point.

#18 triskelion

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Posted 14 July 2019 - 04:02 PM

Yeah as long as you're in focus, the field should be flat. Should being the operative word. This scope isn't living up to its promise at this point.

Thank you for clarifying! Yes. That is quite disappointing then. 

 

With respect to being too picky of a consumer, I think for me both the cost and the promise of the flat field make that simply not the case. Not to mention, as discussed above, I am not using a full frame camera. There are a lot of options out there and after already sacrificing things like autofocus capability, I just expected to get what was promised.


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#19 rockstarbill

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Posted 14 July 2019 - 04:08 PM

There are a few things that are upsetting with the RedCat, but maybe the manufacturer plans to fix them. You may want to contact them and see what they have to say. 

 

A good widefield well corrected lens/scope like this, was very alluring to people. I would hope they would step up and address the problem. 


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#20 RogeZ

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Posted 14 July 2019 - 06:39 PM

I have also seen some examples that show significant CA on this scope. The more I ponder about scopes, the more I realize that I’m going to end up with an AstroPhysics 130GTX one day.


Edited by RogeZ, 14 July 2019 - 07:39 PM.

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#21 ImNewHere

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Posted 14 July 2019 - 09:11 PM

I bought the Pentax  K mount adapter for mine and the field seems really good with my astro modded K5 and my full spectrum K-01. I have not tried my K1ii on it yet. https://www.astrobin...412567/?nc=user

That being said on the full res pic you can see elongated stars, but that was due to me not getting my guiding working at all that night.



#22 rockstarbill

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Posted 14 July 2019 - 09:13 PM

Would be interesting to see some images without the guiding issue. Kind of hard to grade this with the trails.



#23 ImNewHere

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Posted 14 July 2019 - 09:18 PM

Would be interesting to see some images without the guiding issue. Kind of hard to grade this with the trails.

Here's some from Friday night with a bunch of 30s exposures. I set it and forgot it. Didn't expect a ton with the moon like it was!

https://www.astrobin.com/415392/


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#24 rockstarbill

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Posted 14 July 2019 - 09:21 PM

Thanks. That is better than the others I have looked at. 



#25 tonyt

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Posted 14 July 2019 - 09:47 PM

Everyone has different standards of course; provided the centre stars/collimation are good a little edge of field curvature won't be an issue for me (got one incoming).  




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