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Observing Variable Stars


Streetbob
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With the sky in my back garden now at Bortle 7, any serious stargazing is out of the question. As the moon is not always in a good position for observing, because of houses and trees, I have been at a bit of a loss with what to observe.

 

To get out there I needed to come up with a plan.  After plenty of research I came up with a couple of options; sun bothering and estimating variable star magnitudes. Now that the sun is getting more interesting I have tried some sun bothering but I also decided to try finding the magnitude of some variable stars.

 

First I tried using binoculars for visual estimates but star hopping was nigh on impossible with my level of light pollution. It meant I wasn’t certain I was looking at the correct star and also I couldn’t help feeling I was just guessing.  Perhaps I just needed more practise.

 

I then tried taking photos and then calibrating manually using graded circles. This also proved highly inaccurate even when using a spreadsheet to calculate the best fit calibration line.  Again it just felt like guessing.

 

 

I then looked at trying Astrophotometry.  After much online research I found ASTAP and AstroImagej software that can be used on my Mac.  Both free.  ASTAP can convert raw camera images to .fits format directly, the format needed for photometry, so this is the software I’ve started using.

 

I know it is made to carry far greater payloads than a DSLR with a 70-200mm lens but my Rowan AZ100 with Nexus DSC is quick to set up and a pleasure to use so it makes finding the variable stars easy.

 

IMG_0510

 

Using a Canon 500D with a 70-200mm lens set at 200mm I experimented with the camera settings and found that ISO 800, f/5.6 and exposure of 5 seconds works well.  I take 12 shots of each variable star using an intervalometer though I do need to tweak it in between shots because the mount isn’t motorised.  Photometry doesn’t need crisp images, in fact slight off focus gives better results, though I will fit motors when they become available.  I also include basic darks, flats and bias frames.

 

Using the results from ASTAP I have used a spreadsheet to start producing light curves for the variables I’ve been observing.  I added some data from the BAA & AAVSO websites to see how accurate my efforts are and I’m pretty pleased with the results.

 

Z Uma

 

 

 

AF Cyg

 

 

So now I’m waiting for clear skies to get back out to add more data to my light curves.

 

Thanks you for reading

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Interesting stuff Doug and some impressive results there.

I'm also in Bortle 7 territory at home so am usually restricted to Lunar, occasional Solar and Planets

which thankfully are my favourite things to observe.

 

I've never really spent much time with variables besides the odd look at Mira or Algol.

Have you checked out Mira in Cetus ? Its a fascinating star which varies by about 8 magnitudes over the space of a year i think.

And now is a good time to observe it as its ideally placed.

 

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Love your focusing knobs Doug.

 

As we live pretty close I also have bortle 7 skies and because of this I have turned to splitting double and triple stars, and lunar.

 

Never been interested in variable stars but so glad you are doing something different.

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1 hour ago, Bino-viewer said:

Interesting stuff Doug and some impressive results there.

I'm also in Bortle 7 territory at home so am usually restricted to Lunar, occasional Solar and Planets

which thankfully are my favourite things to observe.

 

I've never really spent much time with variables besides the odd look at Mira or Algol.

Have you checked out Mira in Cetus ? Its a fascinating star which varies by about 8 magnitudes over the space of a year i think.

And now is a good time to observe it as its ideally placed.

 

Mira is hidden by trees at the moment but I’m hoping it will rise above them soon.

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55 minutes ago, Doc said:

Love your focusing knobs Doug.

 

As we live pretty close I also have bortle 7 skies and because of this I have turned to splitting double and triple stars, and lunar.

 

Never been interested in variable stars but so glad you are doing something different.

They were a birthday present and I prefer them to the originals.

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Interesting stuff. Bortle 8 here. And no view apart from a small 20 ° area overhead. 

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2 hours ago, Doc said:

Love your focusing knobs Doug.

 

As we live pretty close I also have bortle 7 skies and because of this I have turned to splitting double and triple stars, and lunar.

 

Never been interested in variable stars but so glad you are doing something different.

 

I think these are slow motion knobs for the alt, az axes Mick rather than 'focusing knobs' Mick.

Correct me if i'm wrong.....

 

The 70-200 lens is an impressive beast as well Doug.

 

Nice set up there Doug. More pictures of the AZ100 in action would be welcome.

Never tire of looking at them.....🙂

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Interesting stuff Doug, I must admit variables are something I've neer really given much time to, I prefer viewing doubles.

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38 minutes ago, Bino-viewer said:

 

I think these are slow motion knobs for the alt, az axes Mick rather than 'focusing knobs' Mick.

Correct me if i'm wrong.....

 

The 70-200 lens is an impressive beast as well Doug.

 

Nice set up there Doug. More pictures of the AZ100 in action would be welcome.

Never tire of looking at them.....🙂

 

Thanks Rob that's what I meant.

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10 hours ago, Bino-viewer said:

The 70-200 lens is an impressive beast as well Doug

I bought it used from eBay it doesn’t have image stabilisation so was a sensible price. For what I want it for I don’t need IS.

 

10 hours ago, Bino-viewer said:

I think these are slow motion knobs for the alt, az axes Mick rather than 'focusing knobs' Mick.

You’re right Rob.

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