Transforms for iTelescope

Affiliation
Royal Astronomical Society of Canada (RASC)
Thu, 10/01/2015 - 22:41

Hi

I’m new here and am using iTelescope’s t05. In April 2015 I took images of M67 in V and B in order to get the transform coefficients  for t05. I have only now got around to processing the images and running them through TG. I have obtained different values for  Tbv , tv-vb, and tb-bv than those in the AAVSO’s telescope setup for t05 as it was on 19 April 2015.

Coeff

My values

AAVSO t05

Mag

Err

R2

Tbv

1.190

0.009

1.00

1.006

Tb-bv

0.116

0.009

0.93

-0.016

Tv-bv

-0.041

0.008

0.45

-0.022

 

 

I have imaged omi  cet with the telescope and used  VPhot to determine magnitudes and compared the transformed magnitudes of the check and comparison stars using both sets of coefficients(this took a bit of effort given that I’m a newbie here) . My coeffs give values closer to the reference magnitudes for the check star and the comparison stars than those in the telescope setup.

I talked to iTelescope on the assumption that they had provided the values to AAVSO for their telescopes but they said they had not done so.

My first question is: can anyone tell me where the values came from and when they were determined?

My second question is: what else can I do to ensure that my coefficients are reliable before I submit any data?

Thanks - Patrick Ashmore

Affiliation
American Association of Variable Star Observers (AAVSO)
iTelescope Transforms

Hi Patrick:

As one of the two people who can edit the iTelescope transform coeffs in VPhot, I can state that neither Geir nor I have generated transform coeffs for these telescopes. We have entered values originally provided by iTelescope/GRAS. They may in fact be very old? I will talk to Aussie Pete or Brad about this.

A good way to check your coeffs is to repeat the transform generation with multiple sets of images. How many did you use in April? Your errors look quite small which is good. Your comparison of check and comp star transformed magnitudes and standard magnitudes with the two different coeffs is a good procedure. How different were the magnitudes? Please display this data.

Generation of iTelescope transform coeffs can be done by an iTelescope user or by iTelescope admin. Talk to Aussie Pete. Perhaps he will give you a credit for doing that for them?

Ken (MZK) 

 

Affiliation
Royal Astronomical Society of Canada (RASC)
iTelescope Transforms

Thanks Ken.

It sounds like the coefficients in the telescope setup for t05 must have been old.

In April I took six 60 second exposures for the B,V and I filters. For input to TG I used 46 stars out of 81 stars with SNR.=20.

I took 3 images of 60 seconds each of omi cet. I didn’t take more – I wanted a quick and dirty check. I then measured the magnitudes using star 118 in the chart as the comparison star and then applied the different coefficients using TA. I have attached an excel workbook showing the various magnitudes. I have included in the spreadsheet the SNR as calculated from VPhot because I believe the large difference in star 136 is due to its small SNR. In hind sight omi cet was not the best choice for checking – too few stars with high SNRs.

I didn’t understand your suggestion to confirm the coefficients by repeating the transform generation with multiple sets of images. Do you mean sets of images of different stars?

Thanks for the suggestion to contact Aussie Pete. I’m going to wait until I’m convinced my coefficients are good.

  • Patrick
Affiliation
American Association of Variable Star Observers (AAVSO)
Repeat Transform Coeff Calculation

Patrick:

Repeat the collection of multi-filter images of M67 or NGC7790 on two or three nights and calculate separate nightly transform coeffs to see how they agree. You can use TG and calculate an average of the multiple nights.

Ken 

Affiliation
American Association of Variable Star Observers (AAVSO)
Magnitude Agreement

Patrick:

I looked at your spreadsheet. I'm not sure I would have said your coeffs yielded a lot better agreement with standard magnitudes? Both went up or down +/-. No consistent bias?  Need more stars to compare? You may have just selected a few to show?

Generate coeffs over multiple nights and see what happens. If you can get a good set of transforms with TG over multiple nights, I could update what is provided in VPhot. Good luck ($$)!  frown

Ken

 

Affiliation
Royal Astronomical Society of Canada (RASC)
Hi Ken,
 Thanks for the

Hi Ken,

 Thanks for the comments.  

I agree – I need more stars to compare.

 I will redo the transforms over a 3 nights and see what I get.  I’ll use NGC7790 this time. When I’ve got the new coefficients  I’ll post them here.

 -Patrick

Affiliation
American Association of Variable Star Observers (AAVSO)
iTelescope 5

Hello,

regarding iT05 (and before iT04) - those systems have serious issues with flat fielding. There is maybe ca 20% area in the center (but not exactly dead center) where simple flatfielding yields good results. Differences about 0.1 in differential magnitudes between observations east- and westards of the meridian are common. That difference is a function of distance from "central region" and it's not always stable (from night to night). Most probably it is caused by scattered light in the system.

Also, their flats aren't always good. If you download flats from one random time period (e.g. one month) and use master flats from one fixed night to divide master flats from another nights. Besides changing dust donuts (that is normal), one will see large scale changes in flats, often different from night to night. That behaviour is pretty hard to explain.

Best wishes,
Tõnis

Affiliation
American Association of Variable Star Observers (AAVSO)
iTelescope T05 flats

I reported this to iTelescope. Here's their reply:

"

Lew I am taking fresh flats for T5 as I type this to you.

 

Should be good to go next time your on T5. 

 

I will install fresh masters on the T5 server and raws/masters will be in the FTP within 24 hrs.

 

Aussie Pete

Observatory Manager 

iTelescope.Net"