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Why aren't images with my Redcat 51 flat?

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#26 ImNewHere

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Posted 14 July 2019 - 10:16 PM

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

I found out that if a person buys one they better get the matched WO adapter for it. A friend tried an M48-T adapter for one of his Nikon T rings and couldn't get it to focus, then went back and bought the WO adapter and it was perfect. I suspect many who are having troubles aren't using the exact matched WO adapters.



#27 rockstarbill

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Posted 14 July 2019 - 10:22 PM

Which adapter? Also, can you test a FF camera on it? Last question, do you have some unprocessed subs from the second image?

#28 ImNewHere

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Posted 15 July 2019 - 07:03 AM

I went directly to the Williams Optics site and ordered from them. I don't have it with me and won't be back home for a week and a half, but I can send some then and I can test with FF then.

All my fwhm values were in the 4.xx area, but seeing was 2/5 so I figure that's not terrible.

#29 ImNewHere

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Posted 15 July 2019 - 07:05 AM

https://williamoptic...sories/adapters

#30 spokeshave

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Posted 15 July 2019 - 11:38 AM

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?

Backfocus requirements depend on the optical design. For most scopes (including most of the various Petzval designs) focus is where focus is. In other words, if you can achieve focus, the sensor is right where it should be. Where backfocus becomes a critical issue is when a field flattener/reducer is used. These additional optical elements are designed to have the sensor some specific distance from them, and the camera as well as the reducer/flattener move as a unit when focusing. The spacing between the sensor and the reducer/flattener is critical and should be as close to the manufacturer's spec as possible. 

 

However, with the RedCat, it is possible that focus is achieved by moving the front element in relation to the rear element. That's not how most Petzval scopes work, like the TAK FSQs and the TeleVue NP scopes. For these scopes, the spacing between the front and rear elements stays fixed and focus is achieved by moving the sensor only. However, those scopes are astrographs only and are not designed for terrestrial use. The RedCat is designed for terrestrial use and can focus as close as a couple of meters. In order to do that, it may move the front elements independent of the rear elements when focusing (I can't tell by looking at the pictures). If that's the case, then the rear elements act just like a reducer/flattener and the spacing from that element to the sensor becomes critical. So, before people blame the scope, it is probably best to make sure that it is being used properly.

 

There is a very simple way to tell if the front and rear elements move independently. Take the camera off of the back and rotate the focus through its range of motion while watching the rear element. Does it move when focus changes? If so, that means that the spacing between front and rear elements is not changing and the scope should focus just like any other scope - i.e. focus is where focus is. On the other hand, if the rear element does not move when focusing, that means it is fixed relative to the front elements and must be treated liker a reducer/flattener with careful attention paid to meeting the required backfocus spec. 

 

Tim


Edited by spokeshave, 15 July 2019 - 11:40 AM.

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#31 triskelion

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Posted 15 July 2019 - 08:26 PM

Backfocus requirements depend on the optical design. For most scopes (including most of the various Petzval designs) focus is where focus is. In other words, if you can achieve focus, the sensor is right where it should be. Where backfocus becomes a critical issue is when a field flattener/reducer is used. These additional optical elements are designed to have the sensor some specific distance from them, and the camera as well as the reducer/flattener move as a unit when focusing. The spacing between the sensor and the reducer/flattener is critical and should be as close to the manufacturer's spec as possible. 

 

However, with the RedCat, it is possible that focus is achieved by moving the front element in relation to the rear element. That's not how most Petzval scopes work, like the TAK FSQs and the TeleVue NP scopes. For these scopes, the spacing between the front and rear elements stays fixed and focus is achieved by moving the sensor only. However, those scopes are astrographs only and are not designed for terrestrial use. The RedCat is designed for terrestrial use and can focus as close as a couple of meters. In order to do that, it may move the front elements independent of the rear elements when focusing (I can't tell by looking at the pictures). If that's the case, then the rear elements act just like a reducer/flattener and the spacing from that element to the sensor becomes critical. So, before people blame the scope, it is probably best to make sure that it is being used properly.

 

There is a very simple way to tell if the front and rear elements move independently. Take the camera off of the back and rotate the focus through its range of motion while watching the rear element. Does it move when focus changes? If so, that means that the spacing between front and rear elements is not changing and the scope should focus just like any other scope - i.e. focus is where focus is. On the other hand, if the rear element does not move when focusing, that means it is fixed relative to the front elements and must be treated liker a reducer/flattener with careful attention paid to meeting the required backfocus spec. 

 

Tim

Thank you very much for that explanation. That actually makes a whole lot of sense. I did the suggested test and the front and rear elements move together, which should mean my setup should be okay, from my understanding. Also in the manual that the Redcat came with it did not mention backfocus once and all I had to go off of was the diagram WO had on their website, which tells me they didn't expect people to worry about it too much. Someone mentioned to me that the issue might be tilt (a term which I had never heard of before). Could that also be causing what I am getting?


Edited by triskelion, 15 July 2019 - 09:29 PM.


#32 GraySkies

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Posted 16 July 2019 - 05:34 PM

Here is my latest RedCat image, Andromeda. 4 Hours worth of data.

http://www.emberskym.../andromeda-2-0/

I need to learn to take "Flats" but otherwise I'm pretty happy with how flat it seems to be, I'm sure if I start Pixel Peeping the very corners I'll find something...



#33 triskelion

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Posted 17 July 2019 - 12:20 AM

Here is my latest RedCat image, Andromeda. 4 Hours worth of data.

http://www.emberskym.../andromeda-2-0/

I need to learn to take "Flats" but otherwise I'm pretty happy with how flat it seems to be, I'm sure if I start Pixel Peeping the very corners I'll find something...

Beautiful shot! Its hard to see how flat it is because I can't see full resolution. Are you using the WO adapter or a different M48 one?



#34 james7ca

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Posted 17 July 2019 - 04:14 AM

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/

The general star shapes look good, but I notice that this was taken with an APS-C format camera and it appears to have been cropped or reduced in size (a little, the camera specs say 4928 x 3264 pixels, the image on Astrobin is 4109x2434, a change from 15.7 megapixels down to 9.8). However, the strong halos around the stars are a little disconcerting, and they appear even in the center of the field. Some of this could be from the processing. Did the OP use a star mask or a masked stretch during any of the histogram adjustments?

 

That said, there is also evidence of an off-axis component to the halos and flare, which is also commonly seen in camera lenses. This along with the helical focusing makes me think that the RedCat may just be a repurposed camera lens. A nicely redesigned (for astrophotography), fairly high-quality and sharp camera lens, but it MAY not be something that was formulated specifically as an astrograph (pure conjecture, as I don't really know).

 

Honestly, however, I have never owned a scope that produced flawless images over something as large as an APS-C format. I tried two different AT65EDQs (neither very good), a Tele Vue NP127is (not bad but certainly not pinpoint with perfectly round stars), and a Stellarvue SV80ST2 with a matched 0.8X Stellarvue reducer (again, not that bad, but certainly not flawless). I also have an EdgeHD that produces nice images at f/10, but less so with either of the reducers that I've tried (both an Optec Lepus and Celestron's reducer for the EdgeHD series).

 

What I've finally concluded is that you have to be lucky to find any scope that will cover an APS-C format and there may be very few if any that can do full-frame. Of course, you can always crop or resample the image to something smaller (meaning either a smaller field or a larger number of arc seconds per pixel). The sampling (arc seconds per pixel) is also critical, since you can combine a large format and large pixel CCD with a good number of high-end astrographs to produce good looking images (which is certainly a valid approach, although it makes it difficult to compare such results with what can be output using today's relatively small-pixel CMOS cameras).

 

Wei-Hao’s image of the Small Magellanic Cloud taken with the RedCat and a Nikon D800 (full-frame) does definitely show some off-axis aberration, but it’s still a very good image (it was selected as an "Image of the Day" on Astrobin). However, the D800’s native pixel size is 4.88 microns and the largest image on Astrobin is “just” 4800x3200 pixels when the camera’s native size is 7360x4912. So, it’s obviously either been cropped or reduced in size rather significantly. In fact, the Astrometry.net solve says that the image scale is 6.1 arc seconds per pixel which is eight times larger than what I get with my Tele Vue NP127is when using a camera like Sony’s IMX183 (or three times larger than I'd get with the IMX183 and the RedCat).

 

The RedCat/D800 combination should produce an image scale of 4.03 arc seconds per pixel, so taking that and the 6.1 arc seconds reported by Astrometry.net you get a reduction in size to 66%. Now applying that to the native pixel count on the D800 (7360x4912) and you get an estimated pixel dimension of 4865x3247, which isn't too far from the Astrobin size of 4800x3200. So, it looks like Wei-Hao’s image is resized but not cropped (much) from the full frame.

 

Now, if I wanted similar looking performance with my IMX183 and the RedCat I'd have to reduce the image to about 33% of its original size (to produce the same 6.1 arc second sampling). That would mean going from the sensor's native 19.7 megapixels down to approximately 2.15 megapixels (5496x3672 pixels down to 1814x1212). That's certainly usable (about HDTV) but probably not something that could be considered a "grand vista." However, the IMX183 is a smaller format sensor (less than APS-C), and it should be certain (hopefully) that the center-field performance on the RedCat would be notably better than at the edges of the full-frame D800. Thus, the needed reduction in size/scale would probably not be as great as the aforementioned 33% (meaning less reduction and a greater number of pixels in the final, "acceptable" image).

 

You can simulate the field coverage that you'd get with the smaller format IMX183 by cropping the center from Wei -Hao's image, that crop would just cover the Small Magellanic Cloud itself, without 47 Tuc or any of the surrounding field (from the top, bottom, left, or right). With that crop and at the same 6.1 arc second per pixel image scale the image quality looks nearly flawless (using Wei-Hao's sample image).

 

The compromise that I've finally settled on is to use a smaller format camera with a shorter focal length lens (to maintain a fairly large field). In the latter case, I had some hopes that the RedCat might be an ideal solution but I've yet to see much that would seem to confirm that possibility (at least not conclusively). Note that this approach (smaller format sensor with a shorter lens) is really not that much different than using a large format, large pixel camera with a longer focal length lens (except that you might sacrifice some resolution with the assumed reduction in aperture size and perhaps with a similar sacrifice in imaging speed if trying to match image scales against a potentially larger aperture).


Edited by james7ca, 17 July 2019 - 11:05 PM.

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#35 GraySkies

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Posted 17 July 2019 - 10:07 AM

Beautiful shot! Its hard to see how flat it is because I can't see full resolution. Are you using the WO adapter or a different M48 one?


I’m using the WO F-mount t-adpeter right to a FTZ adapter which is mounted on my camera (check out the page on my gear and you’ll see the imaging train). Pretty straightforward.

#36 timmbottoni

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Posted 17 July 2019 - 02:31 PM

Hi all,

 

I will post test star shots from my RedCat 51 and Canon 60D as soon as we get a clear night that's on a weekend, and isn't ridiculously hot out.  For that, I typically just use Vega or another bright star, and focus in the middle, and then move the mount so that Vega is in each of the 4 corners without refocusing. 

 

Lousy year here for weather - wow.   I could do some indoor test shots too, I suppose, but I fear they wouldn't show field flatness as well as stars.

 

Timm

 

P.S. I posted a number of terrestrial shots earlier in the Refractor Forum threads, and a moon shot, and while quite sharp, they won't be able to show if stars are sharp out to the edges. 



#37 GraySkies

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Posted 17 July 2019 - 03:25 PM

Hi all,

 

I will post test star shots from my RedCat 51 and Canon 60D as soon as we get a clear night that's on a weekend, and isn't ridiculously hot out.  For that, I typically just use Vega or another bright star, and focus in the middle, and then move the mount so that Vega is in each of the 4 corners without refocusing. 

 

Lousy year here for weather - wow.   I could do some indoor test shots too, I suppose, but I fear they wouldn't show field flatness as well as stars.

 

Timm

 

P.S. I posted a number of terrestrial shots earlier in the Refractor Forum threads, and a moon shot, and while quite sharp, they won't be able to show if stars are sharp out to the edges. 

So something like this...
RedCatCorners.jpg


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#38 timmbottoni

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Posted 18 July 2019 - 01:13 PM

So something like this...
attachicon.gif RedCatCorners.jpg

I was going to do what I did with the WO ZS61 review in this post - https://www.cloudyni...come/?p=8086444




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