Spectra Problems
Posted: Sat Dec 09, 2017 6:58 pm
Hi All,
i am sorry if this is a repeat post. thought I posted this earlier today but it did not appear in the forum. So I am making a second attempt.
I have recently noticed a big change in my BeSS spectra that I am hoping someone can help me solve. A few weeks ago my spectra started having an undulating continuum along with some recurring big bumps at around 6546A and 6626A. After many hours of attempting to narrow down the cause, I have concluded that it has something to do with my flat image and/or with the processing routine.
My equipment consists of a 12” SCT, Lhires III with 2400 grating, and SBIG ST8mxe camera. Image acquisition is done with Maxim DL and image processing is done with ISIS v8 (and I have recently upgraded to v 9.2). I have always reduced my images using darks, flats, bias, and instrument response images. My issue is described as follows:
Up until a couple of weeks ago, my processing methodology described above has produced spectra that have been validated by the BeSS staff. When I recently sent in about 20 spectra, they were not validated due continuum undulation and bumps as described above. Previously, my specs used to have smooth continuum and no bumps. Important: all of those previous images were flat fielded and validated. Now something has caused a reversal in my results. When I flat fielded my recent spectra, they appear as in my archived specs when no flat was used (undulating continuum with bumps). However, if I now do not use a flat, the continuum is not undulating and the bumps are gone, just like the earlier specs that were flat field corrected. I tried without success to include these images as attachments, but ARAS apparently does not accept fits files or other extensions. I even tried including them in the body of this email with no success.
In attempt to narrow down the exact cause, I pulled up some spectral images taken a couple of years ago. Those specs were perfect and were processed with ISIS using my flat image, IR, bias, and dark. The continuum was smooth and there were no bumps. However, when I ran them through again using the same flats, darks, and IR curve used when I originally processed them, the results showed undulating continuum and bumps. Without the flat field the continuum was level and there were no bumps.
These results tend to make me believe something has changed in my processing routine since the archived specs that were originally submitted showed no bumps and undulating continuum when flat fielded. Those imperfections now disappear when the flat field is left out of the processing routine, just like my newer specs. So next I tried processing with different versions of ISIS but the results are the same.
After many hours of trying to figure this out, I now have two questions:
1) Does anyone have any idea how such a reversal in my flat fielded results could have occurred and what a remedy might be?
2) I have seen different opinions on the value of using flats. Since I am now getting much better results when not using flats, would not using them leave a spectrum that would be imperfect in any way that would render them unusable? I would prefer to use flats as I always had success using them. But this recent bizarre occurrence has me so perplexed I cannot figure out what is happening and think not using them is the only way around this situation.
Thank you in advance for any suggestions you might have that would rectify this. BTW, I would deeply appreciate it if anyone has a suggestion as to how I can get my spectra attached. Pictures would illustrate what I am experiencing much better than only my description.
Cheers,
Keith Graham
i am sorry if this is a repeat post. thought I posted this earlier today but it did not appear in the forum. So I am making a second attempt.
I have recently noticed a big change in my BeSS spectra that I am hoping someone can help me solve. A few weeks ago my spectra started having an undulating continuum along with some recurring big bumps at around 6546A and 6626A. After many hours of attempting to narrow down the cause, I have concluded that it has something to do with my flat image and/or with the processing routine.
My equipment consists of a 12” SCT, Lhires III with 2400 grating, and SBIG ST8mxe camera. Image acquisition is done with Maxim DL and image processing is done with ISIS v8 (and I have recently upgraded to v 9.2). I have always reduced my images using darks, flats, bias, and instrument response images. My issue is described as follows:
Up until a couple of weeks ago, my processing methodology described above has produced spectra that have been validated by the BeSS staff. When I recently sent in about 20 spectra, they were not validated due continuum undulation and bumps as described above. Previously, my specs used to have smooth continuum and no bumps. Important: all of those previous images were flat fielded and validated. Now something has caused a reversal in my results. When I flat fielded my recent spectra, they appear as in my archived specs when no flat was used (undulating continuum with bumps). However, if I now do not use a flat, the continuum is not undulating and the bumps are gone, just like the earlier specs that were flat field corrected. I tried without success to include these images as attachments, but ARAS apparently does not accept fits files or other extensions. I even tried including them in the body of this email with no success.
In attempt to narrow down the exact cause, I pulled up some spectral images taken a couple of years ago. Those specs were perfect and were processed with ISIS using my flat image, IR, bias, and dark. The continuum was smooth and there were no bumps. However, when I ran them through again using the same flats, darks, and IR curve used when I originally processed them, the results showed undulating continuum and bumps. Without the flat field the continuum was level and there were no bumps.
These results tend to make me believe something has changed in my processing routine since the archived specs that were originally submitted showed no bumps and undulating continuum when flat fielded. Those imperfections now disappear when the flat field is left out of the processing routine, just like my newer specs. So next I tried processing with different versions of ISIS but the results are the same.
After many hours of trying to figure this out, I now have two questions:
1) Does anyone have any idea how such a reversal in my flat fielded results could have occurred and what a remedy might be?
2) I have seen different opinions on the value of using flats. Since I am now getting much better results when not using flats, would not using them leave a spectrum that would be imperfect in any way that would render them unusable? I would prefer to use flats as I always had success using them. But this recent bizarre occurrence has me so perplexed I cannot figure out what is happening and think not using them is the only way around this situation.
Thank you in advance for any suggestions you might have that would rectify this. BTW, I would deeply appreciate it if anyone has a suggestion as to how I can get my spectra attached. Pictures would illustrate what I am experiencing much better than only my description.
Cheers,
Keith Graham