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Sky-Watcher 100mm APO + Apex-L + ASI2600MC Pro First Impressions

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#1 Jim Waters

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Posted 29 November 2020 - 01:55 PM

As the title says.  I received my Apex-L and wanted to do initial testing even though the moon was 98.9% full. 

 

Physical inspection.  The included documentation is well written and covers what the user needs to know.  The Apex-L and Sky-Watcher adapter plate both come with 2ea 1mm spacer rings.  I had one area of concern when inspecting the Apex-L and Sky-Watcher adapter plate.   The male M42 threads on the Apex-L and male M48 threads on the Sky-Watcher adapter plate were only 3mm high.  Adding 1mm spacer rings on either to obtain the 58mm back focus was a concern.  That would give me 2mm to screw on the next link in the chain.  Starizona said this shouldn’t be a concern.  I disagree and think there should be more thread.  The ZWO Filter Drawer and ZWO 16.5mm spacer thread height is 4mm so I added the needed 1mm spacers there.

 

Configuration of the optical train was as follows:

Apex-L + SW Adapter Plate (1mm) + ZWO Filter Drawer (21mm) + 1mm Spacer Ring + ZWO Spacer (16.5mm) + 1mm Spacer Ring + ASI2600MC Pro (17.5mm Back Focus) = 58mm

 

I centered on Deneb to do 4-corner testing using the recommended 58mm back focus.  The figure below shows the results (screen grab).  I plan to do retesting using a back focus of 57.8mm and 58.2mm to see if the stars improve. 

 

50661917293_c81c86fdc0_c.jpgStar Four Corners Apex-L APS-C by Jim Waters, on Flickr

 

 

The figure below shows the vignetting of the above optical train (screen grab).  I am disappointed with the vignetting.  I expected much less. The figure doesn’t show an inner vignetting ring between Deneb and the start of the dark vignetting.  I realize this is a 0.65x FR/FF on a F/5.5 scope but my other FR/FF’s never showed this much vignetting for my APS-C sensors.  I am not sure if the moon being 98.9% full is a factor here.  The scope was pointed ~90 degrees away from the moon.  I will have to wait until the moon isn’t up to do more testing.  There’s too much light pollution and noise in the processed image below to come to a conclusion.

 

Vignetting

 

50662741997_cbe7fc9dab_c.jpgApex-L APS-C Vignetting by Jim Waters, on Flickr

 

 

Processed Deneb Image – 60ea 30 second subs including Darks, BIAS and Flats.  Basic PixInsight processing.  Very small crop because of Dithering.

 

50661900383_f769210e4b_b.jpgDeneb Apex-L Test by Jim Waters, on Flickr

 

 

I need to do more testing and adjusting the spacers but it looks like the Apex-L does an OK job producing acceptable stars in the corners.  I am disappointed with the vignetting using my ASI2600MC Pro.  Using the Apex-L with a 4/3 sensor or less will produce very good results...  I need to do more testing in 1 week when the moon rises later.  Lastly, its 'my opinion' that the thread height on the Apex-L and the Sky-Watcher adapter plate are too short.

 

I would like to get feedback from users that are using the Apex-L with the Sky-Watcher 100ED APO on the above vignetting.


Edited by Jim Waters, 29 November 2020 - 06:20 PM.

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#2 Jim Waters

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Posted 29 November 2020 - 02:54 PM

Forgot to include the PixInsight 'Corrected Flat'.

 

50662725107_fe35f22db0_b.jpgApex-L Flat by Jim Waters, on Flickr


Edited by Jim Waters, 29 November 2020 - 06:21 PM.

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#3 gatsbyiv

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Posted 29 November 2020 - 04:42 PM

What size is the image circle of the Sky-Watcher 100?  The reducer is going to reduce that by 0.65x, so if it wasn't much bigger than APS-sized to begin with, this may be more of a function of the scope's design than anything inherent with the reducer.  Now if there's a 'ring' effect, that's a different story.  How much dimmer are the corners than the center?  It's hard to tell here, as it's not clear if these images are linear or not.


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#4 sn2006gy

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Posted 29 November 2020 - 05:08 PM

Vignetting does show up on my 120/apex-l and calibrates out.  Do I wish it was better? yup :) but i'm generally happy. I wish they had an m63/m54 apex-L


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#5 Jim Waters

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Posted 29 November 2020 - 05:09 PM

This is all the information I have on the Apex-L.  From the Starizona Apex-L website.

 

"The Apex ED reducer features extra-low disperion glass to eliminate chromatic aberration and to provide sharp star images over a 30mm image circle, capable of working with most popular CCD, CMOS and DSLR cameras up to APS-C format."

 

I asked what the 'corrected image circle' was and they said 30mm.  At the time I was more concerned with sharp stars in the corners than vignetting.  From the posts and images I have seen it looks like it works very well on f/7 scopes.  The Flat image above is linear.

 

PixInsight FlatContourPlot.  Default settings.

 

50661865763_2ce9517004_b.jpgApex-L - PixInsight FlatContourPlot by Jim Waters, on Flickr


Edited by Jim Waters, 29 November 2020 - 06:02 PM.

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#6 Jim Waters

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Posted 29 November 2020 - 05:11 PM

 I wish they had an m63/m54 apex-L

Me too...  I need to wait and retest when the moon is down.


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#7 elmiko

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Posted 29 November 2020 - 11:18 PM

Very well done Jim. Your stars look good to me in the corners. 


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#8 Jim Waters

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Posted 29 November 2020 - 11:50 PM

Thanks Mike...!  PI had some issues calibrating it out.  Looking forward to a PP outing to do more testing.


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#9 Ron (Lubbock)

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Posted 30 November 2020 - 12:34 AM

I am using the Esprit 100ED and the Apex-L with a smaller sensor (16 mm) in my QSI 660-ws.  I also have some vignetting on the corners.  Who know if it is due to the reducer alone, or some combination of camera, filters, scope, and reducer?

 

I found it challenging to get the back focus "exactly" right,  and I spent quite some time fiddling with several adapters and shims.  I'm still not there, but it's quite close.  Here's a test image, 72 minutes (24 x 3  min) of minimally processed data on NGC 1579, which you can use to look at corner stars after my latest adjustments. 

 

get.jpg?insecure

 

Overall, I am happy with the reducer, because I now get 2.5 times the subs in an imaging session and a much larger field of view.  Here's a finished image taken with only 4.3 hours of data, before my latest adjustments improved the corner stars.  I only cropped out a tiny strip at the edges where dithering left a blurry border.   If I can get these results in 4.3 hours, I can live with a few wonky stars in the corners.

 

get.jpg?insecure


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#10 Jim Waters

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Posted 30 November 2020 - 12:52 AM

Hi Ron - What back-focus are you currently using?  What's the total distance?  How are you dealing with vignetting? 

 

Your images look real nice..!  Thanks for the post.


Edited by Jim Waters, 30 November 2020 - 12:53 AM.


#11 elmiko

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Posted 30 November 2020 - 01:14 AM

Thanks Mike...!  PI had some issues calibrating it out.  Looking forward to a PP outing to do more testing.

Looking forward to it too!



#12 Ron (Lubbock)

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Posted 30 November 2020 - 08:19 AM

Hi Ron - What back-focus are you currently using?  What's the total distance?  How are you dealing with vignetting? 

 

Your images look real nice..!  Thanks for the post.

When I spoke with Starizona over the phone, we decided that I needed an adapter that would give me 17 mm of spacing in order to get the 58 mm total back focus.  I purchased an adjustable adapter that provides 15 to 19 mm of spacing.  Turns out something wasn't right with the calculations.  My back focus ended up requiring a spacer that was slightly less than 14 mm, and I can't quite reach it with any of my adapters.  It's like that itchy spot in the middle of your back.  lol.gif

 

Maybe my camera's back focus deviates by a few mm from the manufacturer's specs.  It happens.  It could also be something about the back focus added by my filters (Astrodon E-series). 

 

Don't  forget the adjustable black rings on the back of the Esprit that are so useful for changing the camera rotation. By the way, those rings have fairly coarse thread, so rotating the camera with them changes the back focus enough to see some difference in the corner stars.  It doesn't take much to mess up the corner stars at f/3.6.   Just smile and be happy you aren't trying to get perfect stars at f/2.

 

 

Since my sensor is small, the only effects of vignetting are that the corners don't quite calibrate correctly when I take flats.  They end up being darker than they should.  I am able to brighten them up during processing without much trouble using standard background flattening routines. 


Edited by Ron (Lubbock), 30 November 2020 - 08:23 AM.

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#13 NeoMorpheus

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Posted 30 November 2020 - 05:16 PM

I received an Apex-L for my Esprit 120 last month, and have had pretty severe "internal vignetting rings" as the OP calls it 100% of the time, I can't get rid of it by pointing in a different place, better conditions, shorter exposures, and lower ISO. My research and experimentation tells me I'm likely exposing waaaay too much for my Bortle 9 sky, and this may help. That said, I have no internal vignetting problems at all when I overexpose just as much but without the reducer. I'd love to know how people resolve this.

 

Sample, single frame. Note the rings get much worse with calibration and integration.

 

Great_Orion_Nebula__2020_11_29__LIGHT_0067__300_00s_400__1_20__38894_00_RGB_VNG.jpg


Edited by NeoMorpheus, 30 November 2020 - 05:16 PM.

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#14 Jim Waters

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Posted 30 November 2020 - 05:56 PM

I need to do more testing at a Bortle 4 site.  If I have issues there I will try to return the Apex-L.  Thanks NeoMorpheus for the post.



#15 Ron (Lubbock)

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Posted 30 November 2020 - 07:12 PM

NeoMorpheus seems to have a problem with flats that don't exactly divide out.  I have this problem too, but much less severe.  I think that vignetting and/or internal reflections somewhere in the imaging train could be responsible.

 

Based on my results after taking flats three different ways, which did not help, I decided that correcting the erroneous flats is purely a post-processing issue.  You need to find a way to subtract out the background in whatever software you use.  Gradient removal techniques that rely on multiple background points will work reasonably well.  I have processed out worse problems with flats than what is shown above.  Even dust donuts can be deleted with good technique.


Edited by Ron (Lubbock), 30 November 2020 - 07:13 PM.


#16 rockstarbill

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Posted 30 November 2020 - 07:36 PM

The vignetting is due to the imaging circle change. Scope is 40mm, reducer is 0.65x, so now the image circle on the scope is 26mm. Diagonal of the APSC chip is 28.6mm (IIRC). Seems to be an expected result no?

Also there is an amount of expected light falloff that mistakenly gets called vignetting around here.

Edited by rockstarbill, 30 November 2020 - 07:38 PM.


#17 Jim Waters

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Posted 30 November 2020 - 08:01 PM

The image circle size of the SW 100mm ED APO is 40mm?

 

And Yes - "there is an amount of expected light falloff that mistakenly gets called vignetting around here"



#18 rockstarbill

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Posted 30 November 2020 - 08:07 PM

The image circle size of the SW 100mm ED APO is 40mm?

And Yes - "there is an amount of expected light falloff that mistakenly gets called vignetting around here"


The Esprit 100 is 40mm. Is this a different scope?

#19 NeoMorpheus

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Posted 30 November 2020 - 08:35 PM

The vignetting is due to the imaging circle change. Scope is 40mm, reducer is 0.65x, so now the image circle on the scope is 26mm. Diagonal of the APSC chip is 28.6mm (IIRC). Seems to be an expected result no?

Also there is an amount of expected light falloff that mistakenly gets called vignetting around here.

This is one thing I thought might be the problem. I set the camera to DX mode which is APS-C sized, and changed shape to square so I get the max amount from my 30mm image circle. Pretty sure I did this correctly, and if so can rule this out as a cause of the rings, if that makes sense.



#20 NeoMorpheus

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Posted 30 November 2020 - 08:38 PM

NeoMorpheus seems to have a problem with flats that don't exactly divide out.  I have this problem too, but much less severe.  I think that vignetting and/or internal reflections somewhere in the imaging train could be responsible.

 

Based on my results after taking flats three different ways, which did not help, I decided that correcting the erroneous flats is purely a post-processing issue.  You need to find a way to subtract out the background in whatever software you use.  Gradient removal techniques that rely on multiple background points will work reasonably well.  I have processed out worse problems with flats than what is shown above.  Even dust donuts can be deleted with good technique.

I also thought Flats might be the problem, so I did a bunch of experimentation on my light frames without any stacking so that I can exclude the effect of flats or my (likely poor) PI technique. Since I see the rings in the unstacked frame, probably not flats related I think. Unless we expect flats would correct these rings (I can't tell you how much worse they get after processing) ....

 

What might cause "internal reflections"? I have a quality rig - SW Esprit + Starizona Apex + DSLR (ok the DSLR isn't ideal for this purpose....) and it seems to be set up pretty standard.


Edited by NeoMorpheus, 30 November 2020 - 08:41 PM.


#21 Jim Waters

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Posted 30 November 2020 - 08:41 PM

The Esprit 100 is 40mm. Is this a different scope?

No



#22 rockstarbill

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Posted 30 November 2020 - 08:42 PM

This is one thing I thought might be the problem. I set the camera to DX mode which is APS-C sized, and changed shape to square so I get the max amount from my 30mm image circle. Pretty sure I did this correctly, and if so can rule this out as a cause of the rings, if that makes sense.


The ring is a reflection. Check your adapters you use for spacing. Lots of them are shiny on the inside. Also check that your draw tube is well blackened.

#23 Jim Waters

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Posted 17 December 2020 - 10:09 PM

This is a follow-up post. 

 

I tested the Apex-L with my Sky-Watcher 100ED APO at a Bortle 4 site and the results can be seen below.  Processing out the vignetting was somewhat difficult. IMHO - If you intent to use the Apex-L at a more light polluted site you will get vignetting zones and rings which can not be processed out.  I decided to keep the Apex-L even though I am disappointed with the vignetting.  I will only use it at dark sites.

 

https://www.cloudyni...po-apex-l-frff/


Edited by Jim Waters, 17 December 2020 - 10:10 PM.


#24 Xentex

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Posted 19 December 2020 - 07:26 PM

I'm using a 44mm sensor (asi6200) on the esprit 100 and there's full coverage without much vignetting. Corner stars are not great shape, but plenty bright.

#25 Jim Waters

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Posted 19 December 2020 - 07:39 PM

I don't understand this based on what I have seen.  Starizona even says it only delivers a 30mm image circle.  I will have to try my 5DMKIV camera.




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