Please help with hd 189733 b !

[alpha015]

50 minutes ago, robin_astro said:

Can you post an example of your  image at gain  5, exposure 10 sec please ?

Thanks

 

Photo: https://ibb.co/bOmrSz

I attached you the file.

Capture_00031.fits

[robin_astro]

Actually it looks like the target is only just saturated in that image so an exposure of 5 sec at 5 gain could be what you need. (but check as you take the first exposures and adjust the exposure then if necessary, rather than doing a full run and then checking)

Robin 

[alpha015]

7 minutes ago, robin_astro said:

Actually it looks like the target is only just saturated in that image so an exposure of 5 sec at 5 gain could be what you need. (but check as you take the first exposures and adjust the exposure then if necessary, rather than doing a full run and then checking)

Robin 

Thanks for your answer.

Yesterday night I did another test.

 

1. exp 5 gain 1: https://ibb.co/nnZjZe

2. exp 1 gain 1: https://ibb.co/kK41Ee

3. exp 1 gain 0: https://ibb.co/iwhA7z

4. exp 5 gain 0: https://ibb.co/jaRRfK

5. exp 5 gain 10: https://ibb.co/fr8YSz

6. exp 5 gain 1 focal 4: https://ibb.co/dA8NLK

 

It seems that I obtained the best results with exposure 5 gain 1, but still the change in brightness is about 12% I think :S 

Btw you said I should get a straight line with around 1%, does it mean that most of the light points have to be between for example the values 1 and 1.01 of red_flux?

But this guy (https://www.cloudynights.com/topic/462161-detect-known-exoplanet-with-dslrtelephote-lens/) got a straight line of 6%, right? :  https://ibb.co/bCscpp

I guess i have to drastically reduce my straight line, so yes, I will try exposure 5 gain 5 and for example exposure 3 gain 1. 

 

 

[robin_astro]

Don't forget your dark and flat corrections

Robin

[alpha015]

3 hours ago, robin_astro said:

Don't forget your dark and flat corrections

Robin

Ah yes, i took 15 darks yesterday and 15 flats today, although i changed the focus a little for the flats :S

I'm still learning how to apply the master dark and flat to the data

 

Data exposure 5 seconds gain 1  : c5.rar

Master dark: dark_15_frames_13,2C_2018-09-07T00_26_07.fits

Master flat: 21_00_30.fits

 

Tomorrow there is a transit, i will try.

[robin_astro]

Your images look well exposed now and your dark looks good.  Your flat is completely saturated everywhere through ( all the pixels are 65220 counts).  Were the individual flat images ok? (You need to set the exposure for the flats to give ~30000 counts on average, use the same gain as for the star images and adjust the exposure time)

I suggest trying to put your target closer to the centre of the field. There will be less variation if the target drifts.

Robin

[alpha015]

18 hours ago, robin_astro said:

Your images look well exposed now and your dark looks good.  Your flat is completely saturated everywhere through ( all the pixels are 65220 counts).  Were the individual flat images ok? (You need to set the exposure for the flats to give ~30000 counts on average, use the same gain as for the star images and adjust the exposure time)

I suggest trying to put your target closer to the centre of the field. There will be less variation if the target drifts.

Robin

Thanks for your answer.

Ops, I didn't realize I had to adjust the exposure for the flats, I used a long exposure :S Perfect, I will do it.

I will also try to center hd 189733. 

Tonight I will try but not sure if there will be clouds :S

[alpha015]

I tried yesterday night, but cloudy :S

I was able to photograph hd 189733 for some time.

Exposure 3 gain 1 and exposure 5 gain 5 turned out to produce a light line similar to exposure 3 gain 1. 

The camera did something strange though. It sometimes moved out of the field of view without me touching anything. It never happened before..

Not sure if it was a problem with the camera or with the motor drive of the mount.

Will try again at the end of this month.

Thanks for the help and time !

[robin_astro]

Don't worry too much about the variability between the individual exposures.  Take exposures continuously and  take 2 min averages of the results (You should get ~40 in exposures in 2 min at 3sec exposure which will reduce your random variability by ~6x) and look at the variability of these. If the variability of these is around say +-2% or better then you should be able to see the transit.

[alpha015]

On 10/09/2018 at 12:41, robin_astro said:

Don't worry too much about the variability between the individual exposures.  Take exposures continuously and  take 2 min averages of the results (You should get ~40 in exposures in 2 min at 3sec exposure which will reduce your random variability by ~6x) and look at the variability of these. If the variability of these is around say +-2% or better then you should be able to see the transit.

Perfect, thanks !

[alpha015]

I tried again yesterday night.

2 doubts if someone can please answer me.

 

1. The field of view was suddenly moving a lot every several minutes during the whole transit without me touching anything. It was going to the left, then right, then middle, etc.. always the same exact positions. Not sure if there is a problem with the camera or motor. 

Here 2 different shots:

https://ibb.co/kr9eKK

https://ibb.co/nvkKKK

 

2. I took several darks and the program did the master dark. The light photos show some vertical thin white lines, but the master dark doesn't. Is this right?:

https://ibb.co/kD8Zmz

 

Thanks in advance!

[robin_astro]

7 hours ago, alpha015 said:

 

1. The field of view was suddenly moving a lot every several minutes during the whole transit without me touching anything. It was going to the left, then right, then middle, etc.. always the same exact positions. Not sure if there is a problem with the camera or motor. 

Here 2 different shots:

https://ibb.co/kr9eKK

https://ibb.co/nvkKKK

 

 

I think it is the camera/software. I have just bought one of these cheap ZWO ISI 120MM cameras as a guide camera and have already seen the same problem a couple of times. (you can also see a sudden change in  background brightness across the frame which shifts in position.)  I am not sure what the cause is yet but it is a very popular camera so someone on  the forum here probably knows, perhaps on the planetary imaging section ?  

[alpha015]

12 minutes ago, robin_astro said:

I think it is the camera/software. I have just bought one of these cheap ZWO ISI 120MM cameras as a guide camera and have already seen the same problem a couple of times. (you can also see a sudden change in  background brightness across the frame which shifts in position.)  I am not sure what the cause is yet but it is a very popular camera so someone on  the forum here probably knows, perhaps on the planetary imaging section ?  

Damn..  so the data i gathered is useless?

Thanks for answering.

[robin_astro]

It seems to be a common problem with this camera, for example here on the ZWO forum and on many other forums as well

 https://bbs.astronomy-imaging-camera.com/viewtopic.php?f=29&t=7409

Now I know why it was cheap ?

[alpha015]

2 hours ago, robin_astro said:

It seems to be a common problem with this camera, for example here on the ZWO forum and on many other forums as well

 https://bbs.astronomy-imaging-camera.com/viewtopic.php?f=29&t=7409

Now I know why it was cheap ?

thanks for the link 

damn...

so, there is not way to overcome the problem? should I send the camera back to ZWO for repair?

I think part of my data is good , i get most of the light curve except the end , so i was thinking in modifying the .fits files by moving part of the images to the right place and leaving the other side for example in darkness.

I'm trying this software to see if i can modify the .fits files: https://heasarc.gsfc.nasa.gov/ftools/fv/

[alpha015]

I think I can do it with Astroimagej actually. I'm gonna move the field of view manually so the images overlap while keeping their size.

Tedious task lol but i want to get the light curve yes or yes. 

I guess I will ask ZWO to repair the camera or send another one also.

Does this graph looks right? is that the transit?

https://ibb.co/idjnpK

That graph shows very few of my data. But I think, please correct me if i'm wrong, that the star normal state was at 1.02 and it went down to around 0.99, implying a 0.03 change which equals to a 3% drop. Am I right ? 

[robin_astro]

8 hours ago, alpha015 said:

Does this graph looks right? is that the transit?

https://ibb.co/idjnpK

 

Perhaps. It is difficult to be sure without more observations after the transit.  What were the predicted transit start and end times ?

[alpha015]

2 hours ago, robin_astro said:

Perhaps. It is difficult to be sure without more observations after the transit.  What were the predicted transit start and end times ?

ingress : 0.414583 

regress: 0.490278

indeed, i  have data for 30 min after the transit but i still have to overlap the images

in total i have 300 images, only 100 are showed in that graph

will show u the final graph asap

[alpha015]

Ok i already overlaped manually most of the data:

https://ibb.co/hjOWmz

I still have to overlap 50 measurements more, those at the end.

 

[Whirlwind]

Hi

A few things that might help identify your data. 

1) Bin the data points (i.e. average an equal number, say 5 and average the result).  Even professional raw observations can look a mess before binning.
2) Plot the comparison stars.  A good indication of whether you have the transit is if the comparison stars show the same features (usually you divide the target star by the comparisons as that consistent effects) - Not sure if the software is doing this
3) Only plot the points (not lines).  Lines are for spectra not photometry.  Lines can lead your eye as to whether you have a feature or not.

[alpha015]

Thanks Whirlwind. Indeed, the comparison stars show graphs totally straight without any change in brightness.

The final (partial) light curve: 

https://ibb.co/nvCuwz

The transit cleary starts at the ingress and shows the 2.6% difference in brightness.

No data for the egress because the camera took useless photos during that part of the transit.

I will soon make a video about how I did it (you guys are masters but I think most of youtube audience would like to learn it)

Thanks for all your help and support !

PS.: This is the solution ZWO gave for the problems with the camera: Please try to turn down USB bandwidth in software to 40.

 

[alberto91]

On 01/09/2018 at 13:00, robin_astro said:

I recommend running for as long as possible before and after the transit. The more data you collect the more obvious the transit will be. 10 min is too short to get a good reference level.

Looking at the camera specification I would recommend running at a lower gain (say 10) so  you can use more of the dynamic range of the sensor 

The camera sensor is capable of recording up to ~14000 electrons (14.5k FWD) but the 12 bit ADC can only record 4096 counts.  A gain or 10 corresponds to a gain of  2.2e-/pixel which would allow you to record up to ~9000 electrons which should still be in the linear region of the sensor.  (The increased read noise does not matter. we are measuring bright stars with long exposures here, the read noise is insignificant)

  (Note the capture software you are using stretches the 12 bit camera output, multiplying it by 16 to make the image look like 16 bits.  The saturated 65520 counts you see in the image are really 4095 counts from the camera (2^12-1)

This should also help with your over exposure problem but you might still need to reduce the exposure time from your 10 sec to avoid any saturation problems

Don't forget, you dont need to wait for a transit to practice.  You can measure the star outside transit any time, trying to get a flat line with as little scatter in the measurements as possible.  

Robin

Hi,

Sorry may I ask you another doubt please?

It is about the ADU.

Do I need to ideally get around 30,000 in ADU for any given star, regardless of its apparent magnitude? 

For hd 189733 (8 mag) I got 30,000 with exposure 5s and gain 1, but now I'm trying a 12th mag star and I get the 30,000 with exposure 30s and gain 30 (out of 100), for example.

With this 12th mag star, when I use exposure 30s and gain 10, I get an ADU of 10,000 only. Is 10,000 enough to detect transits or should I always try to get at least 30,000 ADU?

The variation in brightness I'm getting with this 12th mag seems to be right. The dip of the transit would be 4%.

Thanks in advance.

Regards.

[alberto91]

According to Bruce Gary:

Quote

 

The following best practice procedure conducted immediately prior to the actual observing session is recommended to determine the optimal exposure time:

1. At the target star’s current sky position, take test images at various exposure times. First, select the exposure time with the greatest SNR for the target star where the peak ADU count is less than 75% of the saturation point of the imaging camera’s detector. Note: this 75% limit may have to be reduced if the camera is expected to reach non-linearity much earlier.

2. If the star will be rising toward the local meridian, reduce the exposure time such that the detector will not reach saturation.

3. If the star will be falling from the local meridian, increase the exposure time such that the peak ADU count of the target star is equal to 75% of the saturation point of the detector.

 

 

[robin_astro]

Yes  Bruce Gary's advice is correct (Not surprising as  he who was one of the first amateurs involved in this sort of work and effectively wrote the rules.) For best results, you should check what the maximum count for your camera is before saturation and  regardless of the star, expose as long as needed to get near to ~75% but not over this, keeping in mind the star will get brighter as it rises. (You must not change exposure during the run).  You may still be able to detect transits using less exposure but if you want to detect the smallest transits it is best to follow Bruce Gary's advice as much as  possible (4% isa very large drop and easy to detect. Most transits are much shallower 

Robin

[alberto91]

28 minutes ago, robin_astro said:

Yes  Bruce Gary's advice is correct (Not surprising as  he who was one of the first amateurs involved in this sort of work and effectively wrote the rules.) For best results, you should check what the maximum count for your camera is before saturation and  regardless of the star, expose as long as needed to get near to ~75% but not over this, keeping in mind the star will get brighter as it rises. (You must not change exposure during the run).  You may still be able to detect transits using less exposure but if you want to detect the smallest transits it is best to follow Bruce Gary's advice as much as  possible (4% isa very large drop and easy to detect. Most transits are much shallower 

Robin

Thanks for your answer !

Regards.