# Transform questions

Affiliation
American Association of Variable Star Observers (AAVSO)
Mon, 12/04/2023 - 18:40

To those who are more experienced,

Recently I started transforming my photometry data on VPhot with transform data from NGC 7790.  As I looked at my data and analyzed for accuracy, I was somewhat pleased with some results, but not so well pleased with others.  Let me ask a question first, or at least I'll tell you my assumption.  It's relatively clear that the KREFMAG is the magnitude of the check star from the AAVSO database, and likewise KMAGTRAN is the transformed magnitude estimate of the check star.  It also appears that KMAGSTD is the untransfored magnitude estimate of the check star.  Is my assumption correct?

The problem I'm running into is that the untransformed mag for the check star is often closer to reality than the transformed magnitude.  Below is an example of where the untransformed mag for R Tri is at most around 0.02 from the actual mag of the check star, but the transformed B band for instance would be about 0.165 off reality which is not comforting.  Prior to starting to transform data, the check stars difference was rarely above 0.1m and often much closer.  A guess is that I need new transform data.  Note also that the exposure time for the B band is around 8x the time for the R band.  I thought that should be expected when photographing M class.  Note that I've been primarily looking at Mira variables and there has not been much interest in the Mira variables that I can see from my back porch, so no other results to compare..

R Tri results in VPhot, 10 exposures of each:
V Band
KREFMAG= 11.814, KMAGTRAN= 11.770, KMAGSTD= 11.801

R Band
KREFMAG= 9.929, KMAGTRAN= 9.878, KMAGSTD= 9.908

B Band
KREFMAG= 12.582, KMAGTRAN= 12.417, KMAGSTD= 12.593

My transform data was created with 6 exposures of each band of NGC 7790 and a 130mm APO w/0.8 reducer, ASI2600mm, 120s exposures, zero gain with each filter.  Should the exposure time possibly be lengthened or shortened dependent upon the band?

Clear skies,

Steve

Affiliation
American Association of Variable Star Observers (AAVSO)
Coeffs

Steve:

Have you generated coeffs on another night?  2-3 nights averaged is useful.

Ken

Affiliation
American Association of Variable Star Observers (AAVSO)
Coeffs

Hi Ken,

Thanks.  Tried to upload the coeffs but the only way I can see to do it is by editing the original message and adding a file.  When I try to add a file(click on Browse) I get the red circle with the hash through it.  Confused.

Steve

Affiliation
American Association of Variable Star Observers (AAVSO)
Copy and Paste Text

Steve:

Can you not just copy and paste the coeffs as text into the comment box as shown below?

[Setup]
description= TG - Version 6.9, Telescope= HYP, Time created (UT) = 2023_09_04_15:21:58
[Coefficients]
Tbv= 1.042
Tb_bv= 0.073
Tbr= 1.069
Tb_br= 0.049
Tb_bi= 0.038
Tbi= 1.062
Tv_bv= 0.039
Tvr= 1.118
Tv_vr= 0.074
Tr_vr= -0.037
Tri= 1.029
Tr_ri= -0.049
Ti_ri= -0.067
Tvi= 1.072
Tv_vi= 0.040
Ti_vi= -0.031
Tr_vi= -0.023
[Error]
Tbv= 0.009
Tb_bv= 0.009
Tbr= 0.005
Tb_br= 0.006
Tb_bi= 0.005
Tbi= 0.005
Tv_bv= 0.003
Tvr= 0.007
Tv_vr= 0.005
Tr_vr= 0.004
Tri= 0.007
Tr_ri= 0.006
Ti_ri= 0.006
Tvi= 0.004
Tv_vi= 0.003
Ti_vi= 0.003
Tr_vi= 0.003

Ken

Affiliation
American Association of Variable Star Observers (AAVSO)
Text Coeffs

Transform coefficients:

[Setup]
description= TG - Version 6.9a, Telescope= 130mm EON, Time created (UT) = 2023_11_18_22:21:07
[Coefficients]
Tbv= 1.203
Tb_bv= 0.215
Tbr= 1.203
Tb_br= 0.124
Tv_bv= 0.038
Tvr= 1.236
Tv_vr= 0.075
Tr_vr= -0.118
[Error]
Tbv= 0.009
Tb_bv= 0.008
Tbr= 0.006
Tb_br= 0.004
Tv_bv= 0.002
Tvr= 0.009
Tv_vr= 0.005
Tr_vr= 0.005
[R Squared Values]
Tbv= 0.995
Tb_bv= 0.883
Tbr= 0.998
Tb_br= 0.932
Tv_bv= 0.654
Tvr= 0.992
Tv_vr= 0.652
Tr_vr= 0.812

Affiliation
American Association of Variable Star Observers (AAVSO)
Unusual?

Steve:

Despite how small the errors are, the coeffs are larger than I would have expected. What camera/filters (manufacturer/model) did you use?

I assume that you deactivated many of the standard stars on the individual plots? Unfortunate you can't add one in the forum posts. How many std stars did you end up with?

Ken

PS: Do you have a cloud storage location? You could store some of the plots and provide the link in your post.

Affiliation
American Association of Variable Star Observers (AAVSO)
Camera and filters

Hello Ken,

First let me thank you for the help.  My camera is a ZWO ASI2600mm and I'm using Baader Planetarium Bessel filters.  Sort of doubt the problem is the filters or the non transformed results wouldn't be so close, but it could be.  Yes the stars out of range were deactivated.  There were around 110 stars for the color transformation and over 160 for magnitude transformation.

Unfortunately, I don't have a cloud storage location but it's obvious that I need one soon.  I'll talk to my neighbor who has one.

Think I'll shoot a couple new sets of transfer coefficients tomorrow or Wednesday night as it is to be clear.

Clear skies,

Steve

Affiliation
American Association of Variable Star Observers (AAVSO)
Color Coeffs

Steve:

The fact that the three color coeffs are all about 1.2 is 'strange'? Most Baader color coeffs I've seen before are not that far from 1.

You certainly had plenty of std stars and great errors!

Trying coeffs on several nights and averaging is always good practice.

Ken

Affiliation
American Association of Variable Star Observers (AAVSO)
A quick search of the forums…

A quick search of the forums shows other user's values very comparable (deviation from 1), including Arne's for the QHY268m.

Keith

Affiliation
American Association of Variable Star Observers (AAVSO)
Thanks for checking

Keith:

Thanks for checking. I obviously didn't. Were all three color coeffs much the same at 1.2?

Ken

Affiliation
American Association of Variable Star Observers (AAVSO)
Here are some of Arne's…

Here are some of Arne's results as an example:

Tbv = 1.269

Tvr = 1.242

Tri = 0.868

Tvi = 1.037