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Weirdly different flats on Newton vs EdgeHD

Celestron EdgeHD 11" Acquisition Calibration Deep Sky Troubleshooting
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Arny avatar
Arny avatar
Troubleshooting some flats issues on my EdgeHD 11 I noticed that I get a weird ring like structure on the raw flat. 

So I compared with flats from my Skywatcher 203pds, which show no ring like structure. 

Is there a problem with my Edge image train - or do SCT flats look different in general?

SCT: EdgeHD 11 flat 30ms, 100gain, luminance


Newton: Skywatcher flat 30ms, 100gain, UVIR cut


equipment
- Asi2600 cooled cam at -10C
- AsiAir plus auto preview of flat

Arny
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andrea tasselli avatar
andrea tasselli avatar
Yes, it is kinda weird. Is it a sky flat or something else? Although you might get this weird results if there is condensation on the sensor (and thus this is good a reason NOT to switch on the cooler!).
Arny avatar
Arny avatar
andrea tasselli:
Yes, it is kinda weird. Is it a sky flat or something else? Although you might get this weird results if there is condensation on the sensor (and thus this is good a reason NOT to switch on the cooler!).


I get it on sky and LED flats equally.
Just checked the camera and filters last week to remove mots and spots, but had the effect before and after cleaning on the Edge.

Calibrating with flats does eliminate parts of the effect, but some gradients remain. 

Do you know if this could be specific to SCT's?
Could it happen if the camera is not at the perfect backfocus distance using an additional crayford focuser?
andrea tasselli avatar
andrea tasselli avatar
Arny:
I get it on sky and LED flats equally.
Just checked the camera and filters last week to remove mots and spots, but had the effect before and after cleaning on the Edge.

Calibrating with flats does eliminate parts of the effect, but some gradients remain. 

Do you know if this could be specific to SCT's?
Could it happen if the camera is not at the perfect backfocus distance using an additional crayford focuser?


Typical of SCTs no. And I haven't seen anything like that except in conjunction with a light leak.
Mike Hamende avatar
Mike Hamende avatar
Hi Arny, 

That does look different from the flats I get with my Edge but I'd bet if you look at the actual pixel values between the ring and the center the difference in illumination is only a few percent.  The most important thing is that the flats correct well.  Your flats are very short.  Any time I've had an issue with flats the most common solution has been to dim the panel, use more layers of printer paper or t-shirt to make the flats longer.  I shoot for a minimum of 8 seconds for LRGB, SHO tends to be a bit more forgiving but I like at least 3 seconds.

Mike
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John Hayes avatar
John Hayes avatar
That is almost certainly stray light reflecting from the inside of one of your adapter tubes…or (unlikely) from the inside of the primary baffle tube.  Does that ring “print through” into your calibrated frames?  If so, that’s definitely a stray light effect.

John
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Arny avatar
Arny avatar
John Hayes:
That is almost certainly stray light reflecting from the inside of one of your adapter tubes…or (unlikely) from the inside of the primary baffle tube.  Does that ring “print through” into your calibrated frames?  If so, that’s definitely a stray light effect.

John



Thanks John, very good to know its not normal SCT behavior.
Now I just need to understand where the reflection might occur, as all image train components are antireflective:
- Baader Steel Diamond focusser
- ZWO EFW 7x2“
- ZWO L-OAG

Or could it be the ZWO RGBL and NB Filters itself, creating a halo around the center?
Arny avatar
Arny avatar
Mike Hamende:
Hi Arny, 

That does look different from the flats I get with my Edge but I'd bet if you look at the actual pixel values between the ring and the center the difference in illumination is only a few percent.  The most important thing is that the flats correct well.  Your flats are very short.  Any time I've had an issue with flats the most common solution has been to dim the panel, use more layers of printer paper or t-shirt to make the flats longer.  I shoot for a minimum of 8 seconds for LRGB, SHO tends to be a bit more forgiving but I like at least 3 seconds.

Mike


Hi Mike,

I will try to see if substantially longer flats (seconds vs milliseconds) will help improve - great idea!

May I ask what kind of panel you use for your flats?

Arny
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Mike Hamende avatar
Mike Hamende avatar
Arny:
Mike Hamende:
Hi Arny, 

That does look different from the flats I get with my Edge but I'd bet if you look at the actual pixel values between the ring and the center the difference in illumination is only a few percent.  The most important thing is that the flats correct well.  Your flats are very short.  Any time I've had an issue with flats the most common solution has been to dim the panel, use more layers of printer paper or t-shirt to make the flats longer.  I shoot for a minimum of 8 seconds for LRGB, SHO tends to be a bit more forgiving but I like at least 3 seconds.

Mike


Hi Mike,

I will try to see if substantially longer flats (seconds vs milliseconds) will help improve - great idea?

May I ask what kind of panel you use for your flats?

Arny

I'm currently using a flat panel I made from this project: https://github.com/jlecomte/ascom-flat-panel

Before that I used a simple LED tracing panel and simply added printer paper until I got the desired brightness.

Same results with both, the ascom panel is just a little easier and faster.
John Hayes avatar
John Hayes avatar
Arny:
John Hayes:
That is almost certainly stray light reflecting from the inside of one of your adapter tubes…or (unlikely) from the inside of the primary baffle tube.  Does that ring “print through” into your calibrated frames?  If so, that’s definitely a stray light effect.

John



Thanks John, very good to know its not normal SCT behavior.
Now I just need to understand where the reflection might occur, as all image train components are antireflective:
- Baader Steel Diamond focusser
- ZWO EFW 7x2“
- ZWO L-OAG

Or could it be the ZWO RGBL and NB Filters itself, creating a halo around the center?

Without looking at your system, I can't say where the reflection is coming from but the evidence is there.  Most antireflection black paint isn't very black and sometimes the edges of lenses aren't blackened at all.  When you are trying to achieve photon level sensitivity with your system, it doesn't take very much stray light to screw things up.  My recommendation is to remove the camera, point the scope at a bright scene during the daytime and put your eye about where the sensor is to look backwards through the system.  Sometimes it's possible to spot a bright edge or other source of the problem.  Turning marks on anodized tubes can be particularly troublesome if they aren't painted a very flat black.  Pay attention to the smallest things and do some experimenting.  You should be able to find the problem.

John
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Sean Mc avatar
Sean Mc avatar
I’ll vote for too bright, and getting reflections back from the filter. Of course I’m just guessing though. That shape looks like the corrector plate.
Brian Puhl avatar
Brian Puhl avatar
While I can't say much for your SCT flat, though I might agree theres a light leak issue, maybe dew….    I find your Newt flat rather concerning as well.    Is your focuser protuding inside the tube causing the sharp light falloff on that corner?
Arny avatar
Arny avatar
Mike Hamende:
Hi Arny, 

That does look different from the flats I get with my Edge but I'd bet if you look at the actual pixel values between the ring and the center the difference in illumination is only a few percent.  The most important thing is that the flats correct well.  Your flats are very short.  Any time I've had an issue with flats the most common solution has been to dim the panel, use more layers of printer paper or t-shirt to make the flats longer.  I shoot for a minimum of 8 seconds for LRGB, SHO tends to be a bit more forgiving but I like at least 3 seconds.

Mike



So I took some longer flats of approx. 1.6seconds - here is the absolute value across the masterflat from 30 flats in WBPP showing K values
- dark corner 0.4082
- bright ring 0.4777
- darker center 0.4166

2Dplot with STF turned on


2D plot without STF
Arny avatar
Arny avatar
Brian Puhl:
Is your focuser protuding inside the tube causing the sharp light falloff on that corner?




Hi Brian,

the dark corner is an effect of the TS Optics OAG not fitting perpendicular onto the ZWO 5x2" filter wheel.
Something I need to address, probablby by getting another ZWO OAG as the TS Optics suffers from poor quality (the prism dropped out once ...)

The potential light leaks on the Edge still puzzle me ...

Arny
Arny avatar
Arny avatar
Sean Mc:
I’ll vote for too bright, and getting reflections back from the filter. Of course I’m just guessing though. That shape looks like the corrector plate.



So you'd think dimming the panel and thus extending the exposure times would improve the flats?
Dan Watt avatar
Dan Watt avatar
John Hayes:
That is almost certainly stray light reflecting from the inside of one of your adapter tubes…or (unlikely) from the inside of the primary baffle tube.  Does that ring “print through” into your calibrated frames?  If so, that’s definitely a stray light effect.

John

My guess would be light leaking in the gap between the focuser drawtube and the focuser itself. Try putting a blanket or something over the whole backside and see what happens. 

If it's a light leak, increasing the panel brightness and decreasing exposure time will make any light leak from ambient light dimmer. 

Worth putting the scope in a dark room and hit different spots with a flashlight while looping exposures.
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V avatar
V avatar
That appears to be a light leak. I've had a similar issue on my Edge HD 8 caused by my flat panel having a gap or hole inbetween it and the tube. Also had this from my light source not being positioned correctly. I now like to use my phone flashlight at the end of the dew-shield, positioned directly in the center, works the charm every time.

Heres what an Edge HD flat should look like (Ignore the color, it's NIR, and ignore the OAG shadow and corner Vignette; the 8 is not optimized for Full-Frame.)

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Sean Mc avatar
Sean Mc avatar
Damn, that’s better than I thought from full frame.
Arny avatar
Arny avatar
John Hayes:
Arny:
John Hayes:
That is almost certainly stray light reflecting from the inside of one of your adapter tubes…or (unlikely) from the inside of the primary baffle tube.  Does that ring “print through” into your calibrated frames?  If so, that’s definitely a stray light effect.

John



Thanks John, very good to know its not normal SCT behavior.
Now I just need to understand where the reflection might occur, as all image train components are antireflective:
- Baader Steel Diamond focusser
- ZWO EFW 7x2“
- ZWO L-OAG

Or could it be the ZWO RGBL and NB Filters itself, creating a halo around the center?

Without looking at your system, I can't say where the reflection is coming from but the evidence is there.  Most antireflection black paint isn't very black and sometimes the edges of lenses aren't blackened at all.  When you are trying to achieve photon level sensitivity with your system, it doesn't take very much stray light to screw things up.  My recommendation is to remove the camera, point the scope at a bright scene during the daytime and put your eye about where the sensor is to look backwards through the system.  Sometimes it's possible to spot a bright edge or other source of the problem.  Turning marks on anodized tubes can be particularly troublesome if they aren't painted a very flat black.  Pay attention to the smallest things and do some experimenting.  You should be able to find the problem.

John



1. This is an image taken into the tube from the camera end through the Baader Focuser with the scope pointing at the sky - showing a lot more reflection than expected, but on camera a lot more than what the bare eye would see. I wonder if that is different from other EdgeHD‘s …?


2. and here taken with a flashlight to Illuminate the baffles of scope and baader focuser - does not look like parts of the blackening dropped off …