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First Long term observing list

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marlin's picture
Joined: 2010-08-31

I have selected 8 stars to experiment with for data collection that are accurate given my city light conditions and aperture size. S Boo V Lib RU Lib Y Lib U Lib SS Her W Sco RS Lyr I need a mathematical formula to determine integration times for ACP control. I use a Paramount ME with a Meade 16" SCT, SBIG ST-10 And Maxim DL. So far my attempt is to image where max ADU is at 20,000. Is this a correct target ADU? Y Lib at 300 seconds estimated magnitude is 14-15 from AAVSO list gives me a count of 18,232. SS Her at 9 seconds measured mag 10.2 MaximDL counts 9333. I presume I should increase integration time. Is there a formula or is it guess and check? S Boo 10 sec mag 9.5 gives 57,606 with no bleeding pixels. 65,000 is the well limit and my camera tests linearly at least as far as 50,000 ADU. Ru Lib and V Lib test similarly large. The others are washed out by the Moon so I will add them in a week or so. I have not calibrated yet for a presentable image, just getting my legs for a good set to start so I can match dark frame integration times. Anyway is there a formula to lead me to guess a proper integration time? Arnie told me to contact Jerry Foote for advice, so could I have a link to him? Marlin Costello

First Long term observing list.
PKV's picture
Joined: 2010-07-26


You are on the right track.  You have a nice setup and have tested for linearity, so you know your limits.  You can contact Jerry by going to: and then using the "Contact" tab to email Jerry directly through the AAVSO system.  As far as a mathematical formula goes, I do not have an answer.  Multiplying the exposure time by a factor of 2.5 should get you a magnitude deeper in theory, but with factors as moonlight, seeing, air mass and other variables, you may have to experiment.  I hope this helps.  Kevin - PKV

marlin wrote: " I use a
roe's picture
Joined: 2010-07-25

marlin wrote:

" I use a Paramount ME with a Meade 16" SCT, SBIG ST-10 And Maxim DL."

With that setup you could do dozens and dozens of fields each night.  Have you considered scripting?  (I think ACP will do that, but, there are other ways, too.)

Regarding exposures, I use S/N of the target and comp stars.  Usually, we say try for at least a S/N of 100.  MaximDL will show you that easily.  Looking at a given exposure for a particular star, to double the S/N requires 4X the exposure.  You might want to shoot for a larger S/N on a good night which will carry you through some nights with light clouds and still get your magic 100.  If you are having trouble doubling and doubling the exposures, try stacking.  MaximDL will handle that easily, too.

S Boo is now mag 8.5V...cover
KTC's picture
Joined: 2010-12-08

S Boo is now mag 8.5V...cover it with a 16-inch scope?  Too bright.

You must master your equipment and be knowledgable of your sky/seeing conditions.

Take some (for example) 30-second images of a known star field at low air mass...during good seeing/guiding/tracking/calm wind.  Make note of those stars that have max ADU at about 50% of the top of your linearity limit.  Let's assume those stars are at mag 10.0.

Set up a spreadsheet in 1/2 magnitude steps (that's a factor of 1.585 for every step) to define your 'saturation limit'

Time   Saturation Mag

30   10.0

48   10.5

75   11

119   11.5

...and so on.

Do that for each filter.

Why set the 'saturation limilt' at 50% of your linearity limit?  you may have nights of very calm air and good seeing and the drive works well...sharper images with tighter stars will have higher ADU than you planned for...but you set yourself up for success with that safety margin and you don't have saturated stars on those rare good nights.

Also extend your spreadsheet in 1.5 mag steps in this manner:

Saturation is at mag 10? (for a given exposure)

Then SNR = 200:1 at 11.5,    and SNR100 at mag 13,    and SNR 50 at mag 14.5, and limiting magnitude (SNR = very low) at mag 16.0  Note that these are approximate, but give you a good idea of what performance you'll get for various stars on a given exposure.  You need to know that because your target and comparison stars will be at different magnitudes.

See attached spreadsheets that I use as rough planning guides for various scopes/filters here.  Note that they have several for each scope.

If I need to optimize performance, I will check the images the next morning and adjust exposures accordingly.  (Red variables, like Mira's are tough to plan for R and I filters...they can be 100 times brighter in I than in B.  That also makes it hard to find one comparison star that will work in B and I for red variables.)

A little experience with long term variables
marlin's picture
Joined: 2010-08-31

Hi folks,

Here are this week's observations:

Star             SNR         Integration time seconds     ADU's        FWHM     Flatness (tracking)

RR VIR         170.4      300                                       26,694      4.4          0.346 so-so

RS Lyr          16.9 bad  65                                        3351         15(bad focus)  0.7 bad tracking  I will try 560s.based on a previous run of 360s. at 13,000 ADU's

A previous run showed 360s @ 18412 ADU's.  Iwill try that time again

RY Her          743.0      15                                        26,078      2.2 good  0.074 good tracking

SV Dra          42.6 bad 30                                        9418         2.8            0.192 so-so Previous run of 60 s @ 13521 ADU's will be retried.  I should have increased time not reduced. I will try 90s.  

RT Oph         49.7        700                                       25,022 All stas were badly trailed so measures are not reliable.  I will try again at 700s


RZ Her         4.8           180                                      16,093     3.7           0.4 oval.  330s. calculates that I should get 30,000 ADU's.  Will this bring up my SNR?

Should I reduce and report RR Vir and RY Her?

I will increase integration times for RS LYR, SV Dra, and RZ Her

I have eliminated a number of candidates that are too bright and will continue to experiment with these values.  After honing it to proper integration times , I will image this set on a two week basis as suggested by Arne.  How many images of each should I take during a single run?  ACP will do the heavy lifting for me.  I have to work out the guiding issue which came up recently.

Marlin Costello

Please show us your images
KTC's picture
Joined: 2010-12-08

Please show us your images.  Why do I ask for them?

For SV Dra you mention SNR = 42.6 while max star ADU 9418  ???

Something is not right, and I don't want to play guessing games.  Please make images available.  An image is worth a thousand words.

Last week I recommended you take some test exposures and build a planning tool/exposure spreasheet.  What results did you get from that test?

Why did you have a focus problem with RS Lyr?  Tell us about your focusing rig...hardware and software.

Thanks in advance.

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