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Bad stars on an APO refractor: what's going on?

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#1 cyber

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Posted 20 August 2017 - 11:05 AM

I've never been fully satisfied with the optical performance of my TS 80mm f/6 apo. 

I'm using the recommended FR/FF for APS-C sized sensors (TSred279).

 

These aberrations bother me:

- The stars in the blue channel are always bloated compared to red and green, it looks like chromatic aberration.

- I've set the FR/FF backfocus very precisely to the recommended distance, but the field is still curved. 

- The image plane seems to be tilted diagonally from top left to bottom right.

 

The chromatic aberration is clearly visible in this image of M27:

 

get.jpg

 

What can I do to improve these?

- Are my optics not collimated correctly?

- The field curvature can probably be fixed with trial-and error adjustment of the distance

- I haven't tried to image stars without the reducer. 

Are there any other tests I can do to check the performance of the optics?

 

Honestly, I expected much more from a triplet apo, especially after using an Orion ST-80 achromat for a whole year.

 

Here's some more example data from a recent test on the globular cluster M71:

RGB 30 x 30" per filter (gain 74) -15C.

Guided with an average error of ~1" RMS.

 

Link to the full-size image:

https://astrovirusbl...-full-frame.jpg

Processing: LinearFit -> ChannelCombination -> ABE -> HT -> Saturation boost with Curves

 

FWHMEccentricity script

 

M71 full frame star analysis.JPG

 

AberrationInspector script

 

M71 RGB aberration inspector.jpg

 

I focused each channel separately, using the FWHM tool in sharp cap.

This clearly shows the star bloating in the blue channel.

The luminance channel suffers from the same problem.

 

M71 channel comparison.jpg

 

 

Am I expecting too much?

Okay, the 0.79X reducer could be too fast for these optics, and a regular flattener such as the TSFlat2 will probably improve the performance.

But I really like the f/4.8 speed of the system for imaging DSOs, that's why I bought the system in the first place. My clear sky time is extremely limited.

 

This reminds me of Firemandan's topic:

https://www.cloudyni...reducer-issues/

Except he does not experience as much chromatic aberration with his system.



#2 cometcatcher

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Posted 20 August 2017 - 11:17 AM

Yuck. I would be disappointed too. My Doublet SW ED100 has less CA than that.



#3 ChrisWhite

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Posted 20 August 2017 - 11:51 AM

I would contact TS.  I have owned and gotten rid of two TS refractors.  (Photoline 60 and ImagingStar 71).  I think that their QC is not the best on these scopes and it is really hit or miss on getting a good copy.  I sent the Star71 back for collimation and after several tries over 3 months they could not get it right and ended up simply replacing it.  By then I had moved on to a different brand so I sold it when the new one arrived. 

 

My 60 was a doublet and looked like yours does on blue.  My star71 was not bad, but could not produce a flat field.  It's possible your problem is the reducer, but if you don't have a field flattener it is tough to test that theory. 

 

One thing to check is how square is your reducer clamped into your focuser?  (I'm assuming it is a three point clamp and not thread on??) Sometimes the compression ring hits the stepdown on the reducer and tilts slightly.  It looks like your setup is pretty good in the upper left but gets worse as it radiates to the other corners. 



#4 cyber

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Posted 20 August 2017 - 12:29 PM

Thanks for the insight Chris. I've bought the scope and reducer from a local TS dealer here in Belgium.

I wanted to find out what's causing the problem before contacting them, so that's why I'm asking it here on CN.

 

The reducer is indeed clamped to the focuser, but I already checked the connection fit for tilt several times. 

So could some other optical misalignment cause the tilt?



#5 ChrisWhite

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Posted 20 August 2017 - 12:51 PM

Definitwly. There could be an element out of whack in the scope itself. Not something you could easily adjust on your own...

Can't rule out the reducer though.

Regarding the blue bloat you might be stuck with that. (Did you readjust focus for the blue channel or are you shooting with an OSC?)

#6 Peter in Reno

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Posted 20 August 2017 - 01:29 PM

TS APO scopes are "cheap" APOs and it's not uncommon that cheaper APOs don't correct for blue light as well as red or green. It's also possible that your camera may be a little sensitive to blue/violet light and so it's capturing good amount of blue/violet light and blue stars may look a little out of focused (or bloated) due to poor blue light correction from your scope. In order for a refractor to properly correct for blue light, the scope will be very expensive. Takahashi is one of the very few APOs that corrects blue light pretty nicely but they are extremely expensive.

 

Even on my high end TEC 140 APO, I get a some bright blue star bloats unless I include a TEC Field Flattener. TEC FF does a great job correcting blue light. TEC does not advertise their FF correcting blue light but many people including myself have reported TEC FF doing a great job correcting blue light. Here is a report about TEC 140 and TEC FF at: http://www.astrosurf...ent/apo140e.htm . Look for "Field Flattener (FF140)". Here is a snippet: "We also tested the TEC140 with the Field Flattener (FF140) and appreciated its very clever design : with the FF, the color balance is slightly shifted toward the blue for a better match to the sensitivity of a sensor. So, whatever the format of your sensor, the FF is a must for the best possible color correction."

 

It's possible (I cannot guarantee) that a FF might (I mean "might") help correcting blue light for your TS 80mm APO.

 

A friend of mine bought TS 130mm f/7 and got the exact same issues as your scope. He returned it for an exchange and the exchanged scope was a bit worse. So he returned it for a much more expensive TAK 130 APO and he is very happy with it and gets better results than my TEC 140 APO.

 

Peter


Edited by Peter in Reno, 20 August 2017 - 02:48 PM.


#7 cyber

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Posted 21 August 2017 - 07:21 AM

Definitwly. There could be an element out of whack in the scope itself. Not something you could easily adjust on your own...

Can't rule out the reducer though.

Regarding the blue bloat you might be stuck with that. (Did you readjust focus for the blue channel or are you shooting with an OSC?)

I found your image with the TS photoline 60, seems like you were using the same reducer.

 

get.jpg

 

My M27 image was captured with a single focus for all 3 filters (sequence: RRGGBB, dither, RRGGBB, dither,...). I used the green filter to set the focus. That's when I noticed the prominent CA.

So for the next image (M71), I used an individual focus for each filter. (sequence: Focus R, shoot 30 R subs, focus G, shoot 30 G subs, focus B, shoot 30 B subs).

 

I'll upload some more DSLR-ASI1600 comparisons tonight.

 

I bought the telescope in January 2017, when I was still using a DSLR.

The chromatic aberration has only recently become a problem, I think.

It is not so prominent in my first RGB image with the ASI1600.

I didn't record the focusing method for this one, but it was probably set through the red or green filter.

 

get.jpg

 

The blue channel is still not very good in this image though.

I kept the scope assembled on the mount the whole time, only moving out on my deck a couple of times for imaging.

 

TS APO scopes are "cheap" APOs and it's not uncommon that cheaper APOs don't correct for blue light as well as red or green. It's also possible that your camera may be a little sensitive to blue/violet light and so it's capturing good amount of blue/violet light and blue stars may look a little out of focused (or bloated) due to poor blue light correction from your scope. In order for a refractor to properly correct for blue light, the scope will be very expensive. Takahashi is one of the very few APOs that corrects blue light pretty nicely but they are extremely expensive.

 

Even on my high end TEC 140 APO, I get a some bright blue star bloats unless I include a TEC Field Flattener. TEC FF does a great job correcting blue light. TEC does not advertise their FF correcting blue light but many people including myself have reported TEC FF doing a great job correcting blue light. Here is a report about TEC 140 and TEC FF at: http://www.astrosurf...ent/apo140e.htm . Look for "Field Flattener (FF140)". Here is a snippet: "We also tested the TEC140 with the Field Flattener (FF140) and appreciated its very clever design : with the FF, the color balance is slightly shifted toward the blue for a better match to the sensitivity of a sensor. So, whatever the format of your sensor, the FF is a must for the best possible color correction."

 

It's possible (I cannot guarantee) that a FF might (I mean "might") help correcting blue light for your TS 80mm APO.

 

A friend of mine bought TS 130mm f/7 and got the exact same issues as your scope. He returned it for an exchange and the exchanged scope was a bit worse. So he returned it for a much more expensive TAK 130 APO and he is very happy with it and gets better results than my TEC 140 APO.

 

Peter

I know that a 800 euro OTA and 240 euro corrector will not perform the same as a Tak FSQ85 that costs 3 times that amount, and it would be silly to expect perfect stars.

But it's like Kevin said, even a doublet that costs way less (e.g. Skywatcher ED80 with FF is ~670 euros total) has better color correction. I see it all the time while browsing AstroBin.

I specifically saved up for a triplet to reduce CA as much as possible, and the (rebranded) chinese 80 mm FPL53 apo's seemed to offer a good deal here.

 

Thanks for the recommendations though, especially because you also advise testing the scope with a good non-reducing flattener.



#8 ChrisWhite

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Posted 21 August 2017 - 08:05 AM

Yeah, that image is what made me hate my lens.  I had horrible blue bloat... and that was even with adjusting focus for each filter.

 

At the very least you will need to do that.  Using a common focus point for filters may or may not work well.  The ZWO filters are definitely not parafocal, and even my astrodons need a little focus offset between blue/lum and red/green. (fast optics) Something else you could try, is to shorten your blue exposure so that stars don't bloat as much. Shorten to the point where stars look to be about the same size as the other filters. As long as you swamp readnoise you will be fine.   Just try to keep total integration the same for each channel (with the optimized ZWO RGB filters) and linear fit when processing....


Edited by ChrisWhite, 21 August 2017 - 08:07 AM.


#9 JukkaP

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Posted 21 August 2017 - 03:11 PM

I had similar problem whit tsred80, whit some unwanted field curvature. After hours of work, I found out that one of the reducer lens was loose. After tightening reducer to focuser the second lens was tilted. The desing is the same in tsred80, only one lens less.

Try to tap your reducer and see if there is enything loose. After I taped the reducer and shaked in my hand, I could feel the lens moving.


Also download the trial of ccdinspector to see if hou have tilt. I belive you do. Pixinsight tool is not so great whit field curvature/tilt problem.

I have only SW 80ed doublet though, so might not be the same issue.

I'am waiting for the tsred279 as replacement for tsred80. Hope that will work whit my SW 80ed. Any experiences?

#10 cyber

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Posted 22 August 2017 - 06:29 AM

Yeah, that image is what made me hate my lens.  I had horrible blue bloat... and that was even with adjusting focus for each filter.

 

At the very least you will need to do that.  Using a common focus point for filters may or may not work well.  The ZWO filters are definitely not parafocal, and even my astrodons need a little focus offset between blue/lum and red/green. (fast optics) Something else you could try, is to shorten your blue exposure so that stars don't bloat as much. Shorten to the point where stars look to be about the same size as the other filters. As long as you swamp readnoise you will be fine.   Just try to keep total integration the same for each channel (with the optimized ZWO RGB filters) and linear fit when processing....

An electronic focuser would be very convenient for this, still saving up for that smile.gif

The RGB channels in the mosaic above are already matched to the blue channel with linear fit, and the same STF is applied to all of them. 

 

I've done some research on my past images to see how the CA evolved over time.

Each of these stacks was processed in the same way: DBE -> BN -> CC -> HT -> SCNR -> Curves (saturation boost)

The panels show a 1:1 zoom with bright stars near the center of the frame.

 

First, images from my ST-80 achro with DSLR. 

The lateral chromatic aberration on this cheap telescope is very prominent.

 

ST80+DSLR.JPG

 

 

Next, an upgrade to the TS 80 mm apo with DSLR.

I can't see much CA, but the star color isn't really good either with small purple color fringes around some stars.

 

APO+DSLR.JPG

 

 

This example shows the purple fringe really well:

 

Star color artifact.JPG

 

 

Finally, the TS 80 mm apo with ASI1600MM and ZWO RGB filters

The stars in the first shot look pretty good, but it changed significantly in the following shots.

I didn't change anything in the optical train, except for a filter replacement in the FW after the second image.

 

APO+ASI1600.JPG

 

Could be the fact that the ASI1600 picks up more blue light than the Canon DSLR, the scope clearly lacks color correction near 400 nm.

Still, that doesn't explain why the first image with the ASI1600 (M17) has much less CA on bright blue stars.

 

TS photoline 80mm apo-spot.jpg

 

I had similar problem whit tsred80, whit some unwanted field curvature. After hours of work, I found out that one of the reducer lens was loose. After tightening reducer to focuser the second lens was tilted. The desing is the same in tsred80, only one lens less.

Try to tap your reducer and see if there is enything loose. After I taped the reducer and shaked in my hand, I could feel the lens moving.


Also download the trial of ccdinspector to see if hou have tilt. I belive you do. Pixinsight tool is not so great whit field curvature/tilt problem.

I have only SW 80ed doublet though, so might not be the same issue.

I'am waiting for the tsred279 as replacement for tsred80. Hope that will work whit my SW 80ed. Any experiences?

Hmmm, I'll have to disassemble the imaging train to check the reducer for a loose lens, thanks for the tip. Seemed pretty solid last time.

 

My setup definitely has tilt, easy to see in the FWHMeccentricity plots. Solving the issue probably requires buying a separate tilt adjustment adapter.

According to the datasheet, the TSred279 can be used on refractors with focal lengths between 350 and 800 mm, so it should work with your SW 80ED (600 mm FL).

The recommended backfocus distance is 58 mm for a 600 mm FL scope.

I have a small amount of vignetting (~10%) at the edge of my medium-sized sensor (ASI1600MM-C), removed entirely with flats.

 

@Peter in Reno: Josh Smith also mentioned your tip about the TEC flattener in a video from The Astro Imaging Channel. Images with that setup look amazing smile.gif

https://www.youtube....h?v=sZ2IH-x3pFw


Edited by cyber, 22 August 2017 - 07:08 AM.


#11 ChrisWhite

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Posted 22 August 2017 - 06:49 AM

Blue can be a tough channel.  Optics tend to bloat it more, as well as atmospheric conditions.  Haze or high altitude clouds (that you can't even tell are there) will cause this.  Low altitude vs high altitude targets...  It's possible that you are seeing differences between targets from these kind of variables. 

 

I do think it would be worth trying a shorter exposure for blue to see if you can get the channels to overlay better.  As far as the tilt, I wish you luck man.... it can be a real PIA, but something many have been able to sort out.



#12 JukkaP

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Posted 22 August 2017 - 09:43 AM

"My setup definitely has tilt, easy to see in the FWHMeccentricity plots. Solving the issue probably requires buying a separate tilt adjustment adapter"

I would fix this first. The tilt can effect in numerous ways.
There is real problem whit fast optics and the microlenses on the sensor.
The blue light can hit the microlens in angle and bloat/defocus at the sensor surface.

So basic things first:
1. Check for pinned optics, loosen if needed.
2. Center your optics/focuser.
3. Collimate your scope(yes you need to collimage refractor too)
4. Fix tilt
5. Check for light leaks
6. Make sure there is no slipping on the focuser.

I can not say to master this subject and I can be wrong. But the tilt in your image train is mostly causing the problem IMHO.

And the seeing conditions Chris said is valid point aswell. When you compare dkfferent exposures I would like to see some stacked image to avarage seeing conditions.

EDIT:
By the way, I'am using baader clicklock, that caused my tsred80 to malfunction. And that was first ok and in time the lens inside the reducer was more and more pinned.

Edited by JukkaP, 22 August 2017 - 11:29 AM.


#13 cyber

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Posted 22 August 2017 - 03:37 PM

I never collimated my refractor before, so I did a couple google searches on the subject.

Apparently, collimating a triplet lens cell isn't trivial and likely requires an optical bench.

My TS dealer does telescope repairs and might have the proper hardware for refractor maintenance though.

 

If I remember correctly, the out of focus star image isn't symmetrical during the focusing procedure in SharpCap.

Here's a simple drawing, it looks like the image drawn on the right, which indeed indicates a collimation error.

 

Collimation.png

 

But can such an image differentiate between collimation of the primary cell and elements of the reducer?

Anyway, I should probably take a few images of the out of focus stars, with and without the corrector.



#14 JukkaP

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Posted 23 August 2017 - 12:49 AM

If it's easy to take the OTA to your dealer, I'am sure they are more then willing to check the tilt and collimation issue for you.

If the collimation is not something you would do, it is much easyer to take the OTA straight to dealer and then you are done whit this problem.

They have artificial star they can bench your tube.

We dont have much clear nights in Finland so I would not want to waste more nights just for understanding the problem.

I have not seen eny telescope that dont need some adjustmemts if you want to be sure your gear is working flawlesly.

So its not so much quality issue.

#15 cyber

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Posted 30 August 2017 - 05:06 PM

So, I've talked to my dealer and he said that collimation of the scope is likely fine (based on my images).

He also suggested testing the scope without the reducer, because the performance of an f/6 system is already pretty critical for blue light.

 

I've spent a couple nights on the same object (M71), to compare different setups. 

This shows blue bloat in the ASI1600MM-C images, both at native FL and with the reducer.

However, I also performed this test using my old Canon 450D at native FL (just to eliminate effects from the reducer) and the stars look fine.

 

M71 star comparison.jpg

 

 

 

 

Some CCD users have reported severe blue bloating in high-end refractors (e.g. Tak FSQ106).

https://www.cloudyni...140trius/page-3

They solved the issue by using an additional Lum filter with a different UV cut-off in front of the filter wheel.

 

My case appears to be similar:

 

The ZWO blue and lum filters pass light down to about 385 nm.

https://astronomy-im...imised-asi1600/

 

ZWO_RGBL_Narrow_curve.jpg

 

But the the TS 80 mm apo is only corrected to about 450 nm.

http://www.teleskop-...rd-focuser.html

 

TS photoline 80mm apo-spot.jpg

 

On the other hand, my Canon 450D's blue band pass stops around 410 nm according to one website.

https://maxmax.com/canon_450d_xsi.htm

The Canon UV/IR filter actually goes down to 380 nm according to Gary Honis, but that doesn't matter when the sensor can't register the signal between 380-410 nm.

http://dslrmodificat...elmod450d8.html

 

Canon_450D_Spectral_Response.jpg

 

That would explain the difference in my latest test images.

The ZWO combo registers all the blue and violet wavelengths below 410 nm, which are not focused well by my optics, while the 450D's sensor barely picks up any of this light.

 

Something like a Baader 2" UV/IR cut filter, which has a 400-680 nm band pass, should therefore improve the appearance of my Blue and Lum subs.

http://www.baader-pl...--l-filter.html


Edited by cyber, 31 August 2017 - 03:00 AM.


#16 rockstarbill

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Posted 30 August 2017 - 05:58 PM

I have an FSQ106ED, Astrodon LRGB filters, and it has issues with blue bloat. They are significantly less severe with the reducer on the scope. I have not looked at solutions to solve the problem yet, although it seems putting a filter in front of the wheel is known to help at times...? 



#17 cyber

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Posted 31 August 2017 - 03:13 AM

Thanks for the info Bill, how bad is the blue bloat on your Tak?

The Astrodon's Gen2 E Blue filter stops at 400 nm, do you see a difference with between the QSI690 and ASI1600? Their QE near 400 nm is quite different.

 

Also, Peter in Reno and Josh Smith have a TEK 140 with TEK field flattener, and this flattener is designed to correct the blue of the 140's objective.

Might be similar with the reducer on your Tak FSQ106ED?



#18 rockstarbill

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Posted 31 August 2017 - 01:33 PM

Thanks for the info Bill, how bad is the blue bloat on your Tak?

The Astrodon's Gen2 E Blue filter stops at 400 nm, do you see a difference with between the QSI690 and ASI1600? Their QE near 400 nm is quite different.

 

Also, Peter in Reno and Josh Smith have a TEK 140 with TEK field flattener, and this flattener is designed to correct the blue of the 140's objective.

Might be similar with the reducer on your Tak FSQ106ED?

Well, I am troubleshooting some issues that may be related to collimation, but the bloat isnt significant (but it is there) and using the reducer on the scope seems to help. I have yet to use the ASI1600 with the FSQ. That isnt a bad idea though and something I should test for both the ongoing issues I am working on as well as the blue bloat.

 

A flattener wouldnt work to solve the blue bloat issue, the FSQ106ED is a flatfield refractor out of the box.



#19 cyber

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Posted 23 October 2017 - 07:08 AM

Update after 2 months of testing and bad weather:

I've installed a Baader 2" UV/IR-cut filter in front of the FF/FR.
This filter's transmission is 420-680 nm, so the B and L channel will collect less poorly-corrected violet light.

Compared to the image in my first post, the performance of my scope has improved considerably:
- Blue halos are reduced to a minimum.
- Corner stars have less distortion.

A test on M15 below, calibrated with PhotometricColorCalibration in PixInsight.
This shows good color distribution in the cluster and the surrounding bright stars have only minor color fringes.

Center from L-RGB 2X drizzle:

M15_LRGB_drz_small.jpg

AberrationInspector on pure RGB stack:

M15_RGB_mosaic.jpg

FWHMEccentricity script on Luminance filter:
Still some residual field curvature, probably solved by adjusting the backfocus distance.

M15_L_62x30s_FWHM.jpg
M15_L_62x30s_eccentricity.jpg

Thanks for the help everyone, I'm really happy with the improved setup! laugh.gif

Edited by cyber, 23 October 2017 - 08:49 AM.



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