Optical shadow- can you help determing the cause

[sharkmelley]

Agreed, it's not flexure that is causing this.

The "embossing" pattern on this earlier image shows that the upper dust particle has "moved" in the opposite direction to the lower dust particle.

That's why I requested to see the pattern of "movements" across the whole image.

Mark

[MartinB]

4 minutes ago, sharkmelley said:

Agree, it's not flexure that is causing this.

The "embossing" pattern on this earlier image shows that the upper dust particle has "moved" in the opposite direction to the lower dust particle.

That's why I requested to see the pattern of "movements" across the whole image.

Mark

 

 

Why does that mean it isn't flexure?

It seems a shame but the OP seems to have disappeared.  All that is needed is for him to take a flat with his scope at a similar angle to when the lights were taken.  If he was able to do that I would expect a much better outcome

[andrew s]

5 hours ago, Baudat said:

The ONAG XM has a very rigid body, the connection between the imaging camera and the scope (dovetail system) is made of a single aluminum alloy profile about 10mm thick. Even under the most heavy camera/FW load (FLI Proline for instance) there is no flexure. Also having the imaging camera weight closer to the scope visual back (on top of the ONAG unit, see attached picture) reduces the torque applied to the scope connection (smaller arm) versus the on axis classical configuration for which a longer distance (arm) is required for the same optical back focus distance.

I am sure the ONGA is very rigid (I have one) but the torque I was referring to was rotational about the optic axis not along it. I am not sure that the camera would be further out without the ONAG as you would focus closer to the visual back on an SCT - at least that is what I would do. Regards Andrew

[sharkmelley]

1 hour ago, MartinB said:

 

Why does that mean it isn't flexure?

It seems a shame but the OP seems to have disappeared.  All that is needed is for him to take a flat with his scope at a similar angle to when the lights were taken.  If he was able to do that I would expect a much better outcome

I misinterpreted that image - I hadn't realise that it was an integration of flipped and unflipped images.  So the "upper" particle is one and the same as the "lower" particle.  I'm guessing this single particle has simply moved slightly between taking the lights and flats.  Without seeing the whole frame we'll never know  

Mark

[Stub Mandrel]

On 17/02/2016 at 23:27, pyrasanth said:

Very useful. I went back to the lights & stacked the red without applying a calibration & lo & behold 2 dust bunnies in the place where the donuts sit after the calibration. The calibration seems to be amplifying the donut & not removing them at least in these 2 places- any thoughts on this?

See the attached frame which is the raw uncalibrated stack with frames including the flip.

He has posted the whole frame - see above.

Note that there are multiple doughnuts which suggests to me it isn't as simple as the dust bunny moving as they would all have to move by the same amount.

[pyrasanth]

I cleaned the whole optical system. The donut was definitely caused by a large block of debris of sufficient size I suspect to have moved between the lights being taken & the calibration frames. A rough & ready set of test shots has shown the anomaly has now gone but I need to clean the system a bit more to get rid of a few more bunnies on the filters. This has been an interesting topic & shows how much image damage a tiny speck of debris can do to an optical system. I thank everybody for their valuable & considered input but I can safely now say the issue is resolved.

[carastro]

 I had this problem on one occasion in the past and it never happened before or since using the same kit.  

I confess I am not very technical but to my mind at the time I felt the flats had been over corrected and I blamed the stacking software which was DSS at the time, it never happened before or since which is strange and I am not one for cleaning my optics.  

I have since changed to a CCD camera and Astroart for stacking and not encountered the problem again.

I was using a refractor at the time. 

I just thought I'd mention this as it might take your particular scope out of the equation.

Carole 

[pyrasanth]

1 hour ago, carastro said:

 I had this problem on one occasion in the past and it never happened before or since using the same kit.  

I confess I am not very technical but to my mind at the time I felt the flats had been over corrected and I blamed the stacking software which was DSS at the time, it never happened before or since which is strange and I am not one for cleaning my optics.  

I have since changed to a CCD camera and Astroart for stacking and not encountered the problem again.

I was using a refractor at the time. 

I just thought I'd mention this as it might take your particular scope out of the equation.

Carole 

Thanks for this observation. It is a really strange issue & if I were to hazard a guess there might have been multiple factors involved like a bright moon, a larger than average particle which them moved between lights & somewhat less than optimal calibration compounding the issue but  the insights & technical info presented will help everyone so that's a good thing in a way.

[sharkmelley]

9 hours ago, pyrasanth said:

I cleaned the whole optical system. The donut was definitely caused by a large block of debris of sufficient size I suspect to have moved between the lights being taken & the calibration frames. A rough & ready set of test shots has shown the anomaly has now gone but I need to clean the system a bit more to get rid of a few more bunnies on the filters. This has been an interesting topic & shows how much image damage a tiny speck of debris can do to an optical system. I thank everybody for their valuable & considered input but I can safely now say the issue is resolved.

I'm glad the problem is now resolved.

Mark

[Jokehoba]

Forgive my stupid question, but did you take three sets of flats - one for each filter - or only one set? 

John

[pyrasanth]

On 2/22/2016 at 09:04, Jokehoba said:

Forgive my stupid question, but did you take three sets of flats - one for each filter - or only one set? 

John

Not a stupid question at all.  50 subs were taken through each filter then a 2 top & bottom clip applied then stacked- so 4 individual flats of 50 subs each were used. The sample seen in this image is the UV filter combination with the corresponding UVFLAT.

 

 

[pyrasanth]

I thought I had nailed the problem as I did not see the donut on the new set of subs which I took through the red filter after they were cleaned along with the whole imaging train. However there is light at the end of the tunnel (excuse the pun). It seems the donut in the original image causing the concern was a large lump of dust sitting in a area on the CCD which manifests itself as a black ring on the calibration frames. This dark area does not show on the target light frames being imaged and is only seen on the calibration subs which when stacked produce a dark ring. I think it is being generated from the flat panel which is placed on top of the dew corrector & the CCD imaging the flat from that position.

I'm using a typical ADU of 24K- I don't know if this is too high & is causing problems as I know saturation on the Atik 460 might be lower than this. I will try a lower ADU count & see if this help. I will report my findings but if you can offer suggestions that would be good.

[pyrasanth]

Okay- the problem is now sorted & it all boiled down to a big dust spot & bad calibration. The Atik 460 is very sensitive (ask Olly!). The calibration frames for LRGB need only be between 13-15K ADU with an average of 10K. I was using nearly twice this figure & blowing the calibration out of the water which produced all manner of weird effects.  I think the Atik saturates at 20K ADU well depth then does clever stuff beyond that but feel free to educate me if I'm wrong.

I'm glad I've got the issue sorted without having to resort to intensive pull my telescope to pieces which would not be nice

[michael8554]

Well done Peter Drew for the first correct identification 

[pyrasanth]

12 hours ago, michael8554 said:

Well done Peter Drew for the first correct identification 

Well done to everybody. I would be in tears by now if it wasn't for the forum help. So many great channels of input- I luv you all!

[ollypenrice]

Some very interesting things here. I'm still not sure that Martin's tilt might not have been playing a part in varyng the bunny.

Two other key points arise. 1) Is 1/3 of 65K always the best value for CCD flats. I think not. A discussion is overdue. 2) Can the light angle and scatter from a flat panel mess things up?

Olly

[derrickf]

Olly, they are both interesting topics and important questions - why don't you start a new thread so we can gather the collected wisdom of the members here?

Derrick

[DougAtDallington]

Olly, I'd very much welcome a thread to discuss flats.

I'm having problems with taking good flats having tried: -

i) a whiteboard on the obsy wall illuminated variously by red obsy LED lighting and/or a white LED 'bar' desk lamp, and

ii) a flat LED light pad panel on its lowest setting held against the end of the OTA (which was still too bright).

Hearing from you experienced guys would be very welcome.

[Baudat]

I am glad you solved your problem Mark.

Flat calibration is sometime challenging.

[pyrasanth]

On ‎26‎/‎02‎/‎2016 at 15:59, ollypenrice said:

Some very interesting things here. I'm still not sure that Martin's tilt might not have been playing a part in varyng the bunny.

Two other key points arise. 1) Is 1/3 of 65K always the best value for CCD flats. I think not. A discussion is overdue. 2) Can the light angle and scatter from a flat panel mess things up?

Olly

I put the question to Atik last year about what ADU for the Atik 460 would be appropriate as at the time I asked the question I was even more clueless than what I am now!

They said to aim for an ADU of 30K. If I use a 30K ADU on the C11 Edge with the Atik 460 at F7 I get a black ring, as you know, on the calibration frames. This produces an area of darkness on the flat when stacked. This totally vanishes on an ADU of 15K max & the images appear to be fully corrected. I used Maxim DL6 to evaluate the flat so I'm not sure if this software gives the most accurate result but most people say its very good at what it does.

I checked again with Atik, pointing them to this thread, the answer came back that you should use whatever ADU setting works for your system. I guess from this response that there is no best ADU setting only your experimentation to establish what works for your system. If you are struggling just bracket a number of ever increasing exposures until your system appears to be corrected. That is how I worked this problem.

If your panel is too bright you can use a ND gel to cut down the intensity of the light. These are fairly cheap on eBay as they are used in the film industry to attenuate lighting and they work well for bringing down the intensity of your flat panel.

I find the taking of real sky flats difficult as the light changes rapidly so have never really had much success as I like groups of at least 50 exposures to build the flat. This allows you to use sigma clipping to improve the quality of the flat and the result is smoother. Unless you have a Sony chip camera with ultra low noise you may as well make some darks as well. I built a whole set for the Atik 460 from 1-20 minutes- it only took me 2 days to do! (sets of 20 images to stack). You don't really need separate bias as the dark carries the bias. I'm not sure If I have ever seen any difference by using or not using a bias calibration but it may be applicable for some imaging devices.

Good luck- I have learned much from this thread & the people who have contributed.

Clear skies to you all.

[ollypenrice]

I treat a bias as a dark for flats. When struggling with flats on one camera (and still am after four years) I tried dedicated darks for flats instead of master bias as dark for flats but it made no difference.

Olly

[ultranova]

2 hours ago, ollypenrice said:

I treat a bias as a dark for flats. When struggling with flats on one camera (and still am after four years) I tried dedicated darks for flats instead of master bias as dark for flats but it made no difference.

Olly

I struggled with flats on my little WO 71 ed  zenith star, what it boiled down to in the end was

two things really, I had the light panel way to close to the end of the scope, moving 

back a foot helped a bit, but what nailed it for me was cutting some flat black thin card

making it snug fit the inside of dew / light shield, which I would place in there for taking

flats, they are more even now,

I mean why would a Telescope manufacture paint the inside of the dew / light shield white,

the out side yes as a visible reference to where the end of the scope is in the dark but why

the inside as any kind of stray light will be reflected off to some degree and as soon as you

put a light panel at the end of the scope, your going to get a load of extra reflected light off 

the inside of the Dew / light shield , so you could be over or under sampling your flats

depending on how far away you light panel is, even taking flats of the sky is not without problems

depending on your F/L

[pyrasanth]

Interesting what ultranova said. I'm going to blacken the inside of my dew shield even more than it is now & see if it makes a difference.

[pyrasanth]

16 hours ago, ollypenrice said:

I treat a bias as a dark for flats. When struggling with flats on one camera (and still am after four years) I tried dedicated darks for flats instead of master bias as dark for flats but it made no difference.

Olly

Does this work for the Atik 11000?- I know that if I take increasingly longer darks the hot pixels become quite apparent & I would image the same would hold true for the light frames- let me know your mileage with this.

[pyrasanth]

I'm going to try an experiment- bear with me- the details might be boring.

I'm going to stack up a typical image with no calibration correction. I'm then going to measure the intensity of that image using Maxim DL. I'm then going to create a calibration master based on the same ADU as this image. I'm then going to calibrate the stack with this new master & see if it makes a difference to the result. I get a feeling that we should be building the flat to suit the target & not just using a generic library to suit all. I will let you know how I get on.