Hello! I started imaging AA TAU after a break of 3 years. I have the same error bars in V and B now as I had then - roughtly 1/2 mag and a full mag respectively. I filter has always been close
I am wondering if the comps are off, and I would like guidance on how to check this.
For example, comp 14.4 (000-BKM-137) has B = 15.436, but my UCAC3 and MPO catalogs show B = 14.443. V = 14.425 and 13.586 while I is close at 12.893 and 12.674.
Similarly, comp 12.0 (000-BKF-953) has B of 12.773 vs. catalog value of 12.319, V of 12.001 vs. 11.580, and I of 10.82 vs 10.771.
Of note, I use an 8" LX200 with a 0.63 focal reducer and and SBIG ST-402. use MPO Connections and Canopus to obtain images and reduce them. I have calculated the transforms for my system from a number of Landolt fields.
Is it possible that the comps are off in B and V? How might I go about checking this?
Thank you and best regards.
Mike
I believe that the UCAC catalogues were primarily 'astrometric' rather than 'magnitudinal' so not sure how much reliance should be placed on the mag values. Also AA Tau is in an extensive obscuration region caused by dark nebulae so that probably affects measurements made in various wavelengths.
Thank you for your note!
I am note sure how to reduce the error in B and V. I've tried stacking multiple 60-second exposures, to no avail. My measurements on the other stars I follow have error ranges that are good.
Thank you for guidance. Best regards.
Mike
Mike,
Both of the comps you mention are APASS stars, but for comp 120 no uncertainty estimate is given for the V magnitude. For comp 144, neither the V nor the B magnitudes have uncertainties. I think this means that there was only one APASS observation in those filters.
There could be more APASS observations for these stars now. I'd suggest you ask for an updated comp sequence.
Phil
Mike,
Just as a FYI, when observers have issues with a sequence the recomended procedure is to use the chart error reporting tool, aka CHET:
https://www.aavso.org/apps/chet/
No need to do this with this target at this time in as much as I am a member of the Sequence Team and will see what I can do for you; unfortuanly I am getting ready to leave for the day so will be this weekend.
We have access to the next release of APASS data (DR10) which is not an official release yet but I will compare that data with what you are reporting and let you know whether or not I will be updating the exisitng data. Please send me your email address: tcarchcape@yahoo.com
Tim
Hello! Thank you for your help. The APASS values for 12.0 and 14.4 are listed as very close to the values in the current chart. In my image, in I, SNR = 50; in V, SNR = 12; and in B, SNR = 3
In my CCD image, I have comps 12.0, 14.4, 14.2, and 15.2. Using these stars, the error bands are
I = 0.042, V = 0.819, B = 0.526
I eliminated 15.2 and I = 0.042, V = 0.134, B = 0.526. 15.2 only had V values, so this appears to be the major source or error in V.
So, using 12.0, 14.2 and 14.4 gives reasonable errors in I and V. B is still high. I played around with elimiating 14.2 and then14.4 as well as making each comp star the target. The values were reasonable.
It looks like eliminating 15.2 and improving the SNR of the image would likely improve my error range.
I'll try stacking 10 images and seeing how the SNR and error ranges change.
Thank you all for your help. Best regards.
Mike
Sorry, the message got uploaded before I had finished.
After elimiating comp 15.2, the error ranges in B,V, and I appear consistent with the SNR of the image. Given the consistency of the mags among the comps stars, I suspect that I should be able to significantly reduce the error range of the images by improving the SNR and eliminating comp 15.2 in future reductions.
Of note, previouisly, I had tried stacking mutliple images of the same duration - 60 sec in this case - but that did not seem to impove the error range of the images. So, I'll increase the exposure length (hopefully, my LX200 mount will allow unguided images of 90 sec and 120 sec) and stack multiple images and see how the error range changes.
Thank you again everyone for your help. Best regards.
Mike
Hi Mike
Are you trying to use an ensemble?
If so, I thinkyou have some issues. Your brightest star and your dimmest star have a 3.2 mag separation. That's a lot. Unless you are very careful, you can be saturting the 12.0 star and/or getting a very high SNR in the 15.2 .
Beyond that, the 15.2 star has a high error bar and no value for B. And it isn't clear what the source is. On the other hand it is close to the target.
The 14.4, and and 12.0 stars have no errors. This means that in the APASS catalog they have an error of zero. They are probably OK but we normally don't use them unless we have to.
This is a pretty rough sequence. tt looks like there isn't a lot to choose from.
I suggest you find a single comp, close to the magnitude of AA Tau.Try to get a SNR of at least 100 and see what that gives you.
JJI
You could also try to find Sloan magnitudes and convert them to UBVRI. Of course when those stars have measurements (i.e. usually that means "not too bright"). Uncertainties of conversion equations are around 0.01-0.03 mag. See e.g: http://www.sdss3.org/dr8/algorithms/sdssUBVRITransform.php
Compstar 144 has e.g. (SDSS Photometric Catalogue Release 12):
RA DEC Name u u_err g g_err r r_err i i_err z z_err
68.851933 24.434962 J043524.46+242605.8 16.989 0.007 14.936 0.003 14.004 0.004 12.176 0.002 13.192 0.003
Hope that helps
Tõnis