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WO RedCat first light: the Great Carina Nebula

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#1 whwang

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Posted 06 February 2019 - 04:21 PM

Hi,

 

I am in the middle of an astrophotographic trip to Western Australia.  A month before the trip, I decided to get a RedCat from WO.  For a guy used to various scopes from TAK, I had my doubt on this tiny scope.  But this purchase turns out to be a brilliant decision.  Although the number of clear nights is exceptionally high in Western Australia in this season, it can be windy.  For about half of my nights here thus far, my main scope had to stop imaging for a substantial fraction of the nights because of the wind.  On the other hand, the RedCat can continue to image thanks to its smaller cross-section and shorter focal length.  In those windy nights, the RedCat was my workhorse scope.  Not only that, its quality did not disappoint me.

 

Here I would like to share with you an image from its first night, the Great Carina Nebula:

Etar_Car_RedCat_small.jpg

 

Or you may use this link to download the 100% image (40MB):

https://www.dropbox....RedCat.jpg?dl=0

 

The camera is Nikon D800.  This is a stack of 14 5-minute exposures.  Usually I would apply some star size reduction so I can further bring out the faint nebulas.  In this case, I did not do it, because I want a picture that can accurately reflect the performance of the scope.  It goes without saying that there is no noise reduction nor sharpening.  There is some trailing of stars caused by tracking issues.  This is corrected in the image.  I knew the trailing was caused by the tracking after I examined every single exposure and compared them with the guide graph of PHD2.  Since the trailing is not caused by the optics, I feel comfortable correcting it in post processing.

 

The stars in the corners are quite good, better than my Sigma 180mm/F2.8.  However, unfortunately, not all pictures I took so far are as good as this one.  In some of the exposures I took on other targets, stars in one or two of the FF corners can show some distortion, and this behavior changes with the direction of pointing.  Nevertheless, even in those no-so-good ones, it's far from being bad.  For its price, I am actually quite happy with what I saw.

 

All the other images taken with this scope are tricky to process. I won't be able to present them until probably March.

 

Cheers,

Wei-Hao


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#2 sg6

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Posted 06 February 2019 - 04:35 PM

Is that a modified Nikon? Or is the Nebula that red?

Looks a very good image from the Red Cat. Which I am not sure is good or bad. $$$ expenditure posibilities.

Any info on other equipment like a flattener, reducer etc being used. Just thinking if that is Red Cat direct to sensor then very nice result.



#3 whwang

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Posted 06 February 2019 - 11:46 PM

Camera is modified. The scope has a built-in flattener.

#4 TxStars

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Posted 07 February 2019 - 03:31 AM

So this is like a non Tak "Micro-Q"..

The image looks great.



#5 the Elf

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Posted 11 February 2019 - 11:36 AM

Thank you very much for posting this. A great image. I downloaded the full res and I agree, this is decent. I have a few questions, if you don't mind:

How does the helical focuser work? Any tilt? Any tendency of moving when pointing straigt up? Does it come with the usual Bahtinov mask and if so, does it help for focus? IMHO it must be very fine to show diffraction spices on such a short focal lenght. Thank you!



#6 whwang

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Posted 11 February 2019 - 01:34 PM

Hi,

What do you mean by tilt of the focuser? As I mentioned, there could be some weak distortion of corner stars depending on where I pointed at. So something is tilted, but I don’t know what it might be. It could be the focuser, but I can’t be certain. William told me it will improve if I slightly tighten up the focuser when I focus. However, I don’t have a chance to verify this yet.

There is a tightening ring for the focuser. So there shouldn’t be any movement even if you point straight up. At least I haven’t observed any.

The built-in Bahtinov mask is quite effective. It’s a nice design and a good accessory. I can use it to focus this scope without any problems.

Cheers,
Wei-Hao

#7 AnakChan

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Posted 13 February 2019 - 08:56 PM

I believe K-Astec has an upgrade to a reinforced camera mount. I haven’t asked my wife to help translate the Japanese yet but maybe that helps any potential tilt?



#8 whwang

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Posted 13 February 2019 - 10:17 PM

I believe the brass camera mount is also made by WO, and should be the one I used.

#9 james7ca

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Posted 14 February 2019 - 02:43 AM

Interesting. The WO RedCat/WhiteCat won't be available in the U.S. until early March (at least) and none of the U.S. distributors that I am aware of have received any units.

 

Did you get this as some kind of early release direct from William Optics? In the latter case, I'd be somewhat concerned that you got a hand picked unit that may not be completely representative of the actual shipping product. What I mean is that WO probably wouldn't ship you a unit unless they knew that it was performing optimally.

 

Your results look similar to what I've seen of the full-frame samples provided by WO. There is something that looks like astigmatism in the corners and interestingly enough the classic Petzval design is supposed to suffer somewhat from that aberration. However, the aberration looks pretty balanced toward each corner which suggests good centering of the optics (collimation) which may not happen on all of the mass produced units (IMO).

 

In any case, thanks for the image sample and please post more as you process them.


Edited by james7ca, 14 February 2019 - 06:10 AM.


#10 whwang

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Posted 14 February 2019 - 04:11 AM

It's highly possible that my copy is hand-picked.  William is selling a lot to Japan, and he told me that his Japanese distributor is very strict for quality.  Because of this, he even puts the scopes through vibration tests and only allow those that passed the test to ship.  I believe mine has gone through such a procedure. I vaguely remember he said he also measures the strehl ratio of the scopes.  But this is less important for prime-focus photography.

 

I am kind of surprised to know that it won't be available in the US until March.  WO displayed this scope as early as Nov last year, in a star party in Taiwan.



#11 james7ca

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Posted 14 February 2019 - 05:55 AM

Thanks for the update.

 

The availability in the U.S. was first announced as December, then it changed to February, and now it's early March (according to WO's own website).



#12 AstroGabe

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Posted 14 February 2019 - 10:58 AM

This is an intriguing scope.  Here's a nice image of the Witch Head Nebula. 

 

https://www.flickr.c...cs/45920590425/

 

I like the detail, but I'm worried about the huge diffraction spikes that are prominent on Rigel and Cursa.  The other fainter stars also exhibit this behavior.  I wonder where these spikes came from, and if it's a QC issue. 

 

Gabe


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#13 ciraxis

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Posted 15 February 2019 - 09:18 AM

This scope is exactly what I've been looking for in a travel scope.  I wonder how well it would work with the 1600 and filer wheel



#14 andysea

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Posted 15 February 2019 - 09:27 AM

I ordered this scope back in December. Then my preorder expired but I am on a short list of people who should get the first batch when it ships.
I ordered because on paper it looked great. However the first images that I have seen show questionable corner performance. My FS-60 with the reducer seems to produce better stars on a FF sensor. The color correction however seems to be superior to the little Tak.

#15 james7ca

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Posted 15 February 2019 - 10:39 AM

...However the first images that I have seen show questionable corner performance. My FS-60 with the reducer seems to produce better stars on a FF sensor. The color correction however seems to be superior to the little Tak.

I'm not so worried about the falloff at the edges as I am in the artifacts (diffraction?) that appear around bright stars as shown in the sample that AstroGabe just posted:

 

 https://flic.kr/p/2cXR2FF


Edited by james7ca, 15 February 2019 - 10:41 AM.


#16 andysea

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Posted 15 February 2019 - 11:19 AM

Those artifacts are strange. I wonder if the micro lenses on the sensor itself have something to do with them.



#17 AstroGabe

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Posted 15 February 2019 - 11:20 AM

The linearity and non-locality of the artifacts to me point to a diffraction effect or some internal reflections in the OTA.



#18 james7ca

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Posted 15 February 2019 - 09:46 PM

If it were a microlensing artifact then I think the artifacts would be parallel to the edges of the frame (always true, I think, unless the image is rotated during processing). But it might still be something in the camera, like some foreign object on the camera chamber window. However, in the latter case I'm not sure that the "spikes" would appear so sharp. I agree with Gabe, I think it is something in the scope itself or something that was forward of the lens (A spider web? I know that I've found spiders on the front of my scopes a few times but that only happens on warm summer nights and this image was apparently taken in the winter from Germany -- frost on the lens?).


Edited by james7ca, 15 February 2019 - 10:50 PM.


#19 whwang

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Posted 16 February 2019 - 01:16 AM

It is probably hard to tell what causes the spikes.  We need more examples.  In the particular picture of the Witch Head Nebula, there are spikes pointing to the field center, and also spikes that are more like tangential to the direction to the field center.  They are probably two different spikes.

 

In this Japanese review: http://reflexions.jp...019/02/16/8021/ , there are two photos of internal reflection of the RedCat. You can see a ring where the reflection is somewhat strong.  It can cause some flare, but it should not be in a form of spike from the bright star itself.  I hope there will be more pictures that are pointed into multiple bright stars. Then we can have much better judgement.

 

I start to slowly process my other pictures taken with RedCat.  I will post them in a different thread.  We can use this thread to discuss the scope itself.

 

Cheers,

Wei-Hao


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#20 james7ca

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Posted 16 February 2019 - 02:20 AM

Wei-Hao, thanks for the link to the Japanese review.

 

Looking at the result from the focus test with the built-in Bahtinov mask it sure looks like the color correction on this lens is quite good. But, I don't know what the image scale was in this reproduction so any deviations from "good" might be hard to see.



#21 whwang

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Posted 16 February 2019 - 02:25 AM

I tend to believe the longitudinal chromatic aberration is very well corrected.  I didn't notice any color fringes in the center of the field, and the Bahtinov focusing result is spot on (the same cannot be said on many camera lenses I had used).  On the other hand, there is indeed some lateral chromatic aberration.  It can be easily fixed with re-aligning the RGB channel though.  The SMC picture I posted in the other thread had the lateral aberration corrected this way.  The Eta-Car picture here was not corrected for this.  So you can take a look to decide whether it is a problem for you.


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#22 AstroGabe

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Posted 16 February 2019 - 10:57 AM

It is probably hard to tell what causes the spikes.  We need more examples.  In the particular picture of the Witch Head Nebula, there are spikes pointing to the field center, and also spikes that are more like tangential to the direction to the field center.  They are probably two different spikes.

 

In this Japanese review: http://reflexions.jp...019/02/16/8021/ , there are two photos of internal reflection of the RedCat. You can see a ring where the reflection is somewhat strong.  It can cause some flare, but it should not be in a form of spike from the bright star itself.  I hope there will be more pictures that are pointed into multiple bright stars. Then we can have much better judgement.

 

I start to slowly process my other pictures taken with RedCat.  I will post them in a different thread.  We can use this thread to discuss the scope itself.

 

Cheers,

Wei-Hao

Thanks for the link Wei-Hao.  I appreciate the lengthy review here.  Google translate to the rescue!

 

Here's a photo of the internal reflections Wei-Hao is speaking of

IMG_0890.jpg




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