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symmetal

RedCat 51 first image - Oh No!

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Hi JimothyC,

Yes, there is significant coma (fan shape smears pointing towards the centre of the image) in the bottom right and some coma top right and bottom left. It's more obvious in the brighter stars but if you look closely the dimmer stars do exhibit similar 'smearing'. Here's the CCD Inspector plot showing the same. Ignore the curvature/tilt analysis as it's interpreting the coma as star errors due to image curvature and tilt which it isn't. Just another poorly aligned RedCat I'm afraid. :frown:

JimothyC.thumb.png.6136c282f95f04b2b69d64f500f1d874.png

Alan

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Posted (edited)

Some coma at corners in brighter stars, top left looks OK, appears to be your average RedCat going by others images so guess it comes down to what you consider acceptable.

A few crops at X 200 and centre star which looks OK at X 400.

Dave

Untitled-1.thumb.png.eed777fb9e5b8fdb0ad1307c7d2cee2c.png

 

Edited by Davey-T

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From the point of view of testing just the lens surely it would be better to use the simplest imaging train, no filters and no long exposures like 30 seconds should be adequate. Otherwise any number of the other extras could be adding issues to the image.

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Thanks all for taking a look at this. Looks like the scope has got to go back. The issues with this scope appear quite prevalent. When you couple this with the fact that the vixen dovetail is undersized and the focuser isn't all that great, it's not looking like a great product launch from WO. You would have though they would have learnt from their previous mistakes, as should have I.

I feel sorry for the retailers in all this who'll have a lot of work on their hands dealing with all the returns and will probably end up absorbing some costs. 

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9 minutes ago, happy-kat said:

From the point of view of testing just the lens surely it would be better to use the simplest imaging train, no filters and no long exposures like 30 seconds should be adequate. Otherwise any number of the other extras could be adding issues to the image.

With the ASI1600MM, I think not having a luminance filter would have caused an issue as there would be no IR and UV filtering. Also if the scope can't cope with 36mm filters on a 4/3rds sensor, then it's not really fit for purpose.  

Exposures were 60 seconds, but the guiding was excellent, in fact I don't think I've ever seen the mount so happy. Probably due to it having such a light payload. 

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On 17/05/2019 at 15:25, symmetal said:

As Dave said a single image is all you need. As the coma, if present, is usually more evident on the edges of the image, a larger sensor (as on a DSLR) will show problems which may not be apparent on a small sensor.

If there is coma you can see it yourself visually when examining the image (especially in the corners) where the stars have fan shaped smears from the star centre. If you can't see any you're lucky and have a good lens.

A fits file is the normal file format from astro cameras and the fits file header includes useful information about the image but isn't necessary. From a DSLR just convert its output file to a tif or png file, which CCD Inspector can read. I'm not sure if it can read DSLR raw files. The CCD Inspector just gives a more accurate display of the star shapes/sizes though will interpret coma as a curvature/tilt problem though that is not always the correct interpretation. The coloured CCDI plot is very useful as a quick check that the image is good or has problems.

Alan

Thx for your replies mates!

Since Im waiting for my star adventurer being returned from RMA, my father had to take test exposures. Sky wasn´t 100% clear, but maybe the shots are good enough for evaluation?

60s exp with a Canon 80D:

https://drive.google.com/file/d/1Vg9ud-6eTGZdRFO2LVh95TZfXPEl3JfK/view

Would appreciate if you could tell me, if the RC needs to be recalibrated.

Henning

 

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47 minutes ago, Nubian said:

Would appreciate if you could tell me, if the RC needs to be recalibrated

Could you post as PNG to save downloading Tif

Thanks

Dave

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Looks pretty good, bit of coma in bottom right but if mine had been like yours I wouldn't have returned it.

Try to image a brighter denser starfield if you can as it will show up any faults better.

Dave

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2 hours ago, Nubian said:

Not sure if Im getting your point. You mean because of the size? ok, I attached a compressed png

It's just that png files, like jpg, display directly on the forum so don't need to be downloaded to view them. :smile:

As Dave said, slight coma, predominantly bottom right. Better than the other RedCat images shown here. Interestingly CCD Inspector gave it a 'good' result with all four corners similar and the centre slightly out of focus. Probably not enough stars for a proper assessment as it only found 523 stars. Around 3000 or more stars are needed fairly evenly spread over the image give the best results.

IMG3142_CCDI.png.93f8351a92982d1f68f48e7952a57a99.png

Alan

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I just got notified by OPT (Sorry, @FLO, I live in the states) that mine is shipping. Keeping my fingers crossed for a good unit. Of course, considering what you all have posted, I would honestly be happy with any of them compared to what I've been using which is just a Canon kit lens.

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13 hours ago, symmetal said:

It's just that png files, like jpg, display directly on the forum so don't need to be downloaded to view them. :smile:

As Dave said, slight coma, predominantly bottom right. Better than the other RedCat images shown here. Interestingly CCD Inspector gave it a 'good' result with all four corners similar and the centre slightly out of focus. Probably not enough stars for a proper assessment as it only found 523 stars. Around 3000 or more stars are needed fairly evenly spread over the image give the best results.

IMG3142_CCDI.png.93f8351a92982d1f68f48e7952a57a99.png

Alan

That looks pretty good, especially considering it’s an aps-c sensor. Has anyone tried one with a full frame one? 

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On 24/05/2019 at 23:26, symmetal said:

It's just that png files, like jpg, display directly on the forum so don't need to be downloaded to view them. :smile:

As Dave said, slight coma, predominantly bottom right. Better than the other RedCat images shown here. Interestingly CCD Inspector gave it a 'good' result with all four corners similar and the centre slightly out of focus. Probably not enough stars for a proper assessment as it only found 523 stars. Around 3000 or more stars are needed fairly evenly spread over the image give the best results.

IMG3142_CCDI.png.93f8351a92982d1f68f48e7952a57a99.png

Alan

Hi Alan,

i took some more test exposures tonight. EOS 700D, 20s, ISO 800

Still got some issues with the star adventurer, but I hope this one has enough stars for a proper analysis.

What do you think?

IMG_5869.png

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Hi Nubian,

For an APS-C sensor that looks pretty good. There is slight coma in the bottom corners but unless you're really fussy not enough to worry about. The CCDI plot makes it look worse than it is in reality and is an indication of where the problems are but the actual figures displayed don't mean much. It found 2918 stars which is good for the check. Hope you're happy. :smile:

IMG_5869.png.f5a29fd536fd7cfc7763422c98d3c62e.png

Alan

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The Redcat is a Petzval lens.

Some frantic googling suggests that the design lacks a flat field, whioch can be corrected with a field flattener, at the expense of introducing coma.

As the Redcat is marketed as being flat full frame, perhaps some  residual coma is an inevitable consequence of the design?

This is a 'pixel peep' of the bottom right corner of one of the (stunning) example Redcat images on the Williams Optics website. The coma looks worse than some examples above...

image.png.ce20b071c241277a284341fcb5002fd5.png

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1 hour ago, Stub Mandrel said:

This is a 'pixel peep' of the bottom right corner of one of the (stunning) example Redcat images on the Williams Optics website. The coma looks worse than some examples above...

Don't mind a bit of coma in the corners but mine was in the centre :grin:

Dave

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13 hours ago, Nubian said:

Hi Alan,

i took some more test exposures tonight. EOS 700D, 20s, ISO 800

Still got some issues with the star adventurer, but I hope this one has enough stars for a proper analysis.

What do you think?

IMG_5869.png

I'd be happy with that if I could get my scopes and flatteners that good! The FOV is wide enough and after stacking the edges would need cropping back anyway.

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Got the RedCat back from Es yesterday, managed to snap the same stars last night through a bit of high cloud, before and after looks a lot better but need some clear skies to test properly.

Dave

Before

IMG_0863SCROP.png.cb45b215cc831cf0f256bfdfb9b554ed.png

After

1805693814_RCat-test-image-300-per-cent.thumb.png.fe5760cd0145707e05625d7da8faf57a.png

 

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Cheers Dave, your stars are indeed better. 

The coma issue seems solved, but there is a slight horizontal elongation. Guiding or a tad of astigmatism? 

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Posted (edited)
22 minutes ago, FaDG said:

Cheers Dave, your stars are indeed better. 

The coma issue seems solved, but there is a slight horizontal elongation. Guiding or a tad of astigmatism? 

Just a quick setup unguided on the Star Adventurer so probably not PAd very well.

Dave

Edited by Davey-T

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Got my WhiteCat back after Es' has had it on his workbench. It's certainly better than it was, though there is still a hint of coma in three of the corners with the bottom left being the worst. I've decided to keep it and see how it looks with real stacked images. :smile: Here's crops of the edges. Click for full resolution.

WhiteCat_L_20s_Crops.thumb.png.ca8f402ab90ccf3bfeef73e946305e5f.png

Alan

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Here's my WhiteCat edge crops with an APS-C sensor. It doesn't look much different and no worse than it did with the 4/3 ASI1600 above. The bottom left is the worst. I'm happy to live with it though. :smile: Click for full size

WhiteCat_EOS_15s_Crops.thumb.png.b73c4dab1cfc0b1f3c0230be22f6df0f.png

Alan

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Looking good Alan, had mine back but apart from a quick pic through some clouds not tried it again, had a couple of good nights but I was fighting with other stuff :grin:

Dave

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My Redcat just came in this week. Had it out for the first time last night. I've been following the issues here and on Cloudy nights. Just wanted to add another another reference point. This is about 6 hours of HA. There's very slight coma at the corners. FWHM and Eccentricity look ok. I'd be happy to share data if someone wants a closer look. The larger stars have a very strange look to them, like pinched optics. Someone over on Cloudy Nights thought it might be an internal reflection. I've used this exact imaging train on other scopes and not seen the issue, so I suspect the problem lies in the scope itself.

1668085109_fwhmcopy.thumb.jpg.7a24489e76f73e52b4ca05f1dd20f525.jpg

large-star.jpg.2cb52703bf1256b344727539489c6d95.jpg

arberration-inspector.thumb.jpg.72d19b17ae145b103c1db44d6487a6d1.jpg

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Hi Lead_weight,

As you say, slight coma in all the corners but not excessive. I was surprised that my RedCat produced coma on Ha very similar to that seen on L. As the blue seems to contribute the bulk of the coma when viewing RedCat colour images, I'd have thought that Ha images being at the other, more 'tolerant' end of the spectrum would show significantly less coma but that doesn't seem to be the case unfortunately.

As to your odd spikes on the bright star it does look similar to pinched optics but others may have a more knowledgeable assessment.

Alan

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