11 replies to this topic

[patrisha]

What equipment would I need to do photometry that would actually be scientifically useful with a 10" Dob?

[lwbehney]

Thanks for this post. I can think of some uses for photometry for the amateur astronomy community. Since you have a mirrored telescope, the energy received from the mirrors and sent to the focal plane is not compromised by chromatic aberration.  Perhaps you could test eyepieces to determine their true relative transparency to different wavelengths. As you may have likely noticed, some observers characterize various eyepieces as warm or cool or neutral. I aesthetically appreciate the blue tints of the O and B spectral class stars and would like to own  eyepieces that can pass the most energy possible in the visible spectrum close to the 435 nm limits. I have read that published transmission percentages can be misleading, because these values do not include light lost due to reflection, which may vary from one proprietary coating to another. 
Another example of a project might be to see if you can detect a change in the luminosity/wavelength ratios over time in bright planetary nebulae. The rings should be cooling as they expand and maybe the rates of cooling could be later correlated with ring diameters to calculate ages. 
Or perhaps measure the changes in the brightness of specific lunar features or Neptune or Uranus as a function of the solar 10.7 cm flux, which is a marker for solar activity. 

[robin_astro]

What equipment would I need to do photometry that would actually be scientifically useful with a 10" Dob?

Just your eye :-)

 

(A non tracking Dob is not really suitable for photometry using a camera)

 

The AAVSO, BAA  or other similar organisations have programs for visual observers and here is the AAVSO manual for visual observers 

https://www.aavso.or...Manual-2013.pdf

 

Cheers

Robin

[robin_astro]

Here is an example of what a visual variable star observer can do. BAA member Gary Poyner has made over 350 thousand visual magnitude observations. 

https://garypoyner.w...m/variablestars

One area he works in  is Cataclysmic Variables which increase dramatically in magnitude at unpredictable intervals. Alerting the amateur and professional community to these outbursts as quickly as possible allows them to be studied in detail.

 

Cheers

Robin

[noisejammer]

A Mk 1 eyeball, a really good star chart and  notebook are all you need to get to within 0.1 magnitudes. This allows reasonably good light curves of - say - Mizar - to be produced. Digital setting circles will help you find more obscure targets. Realisitically, I'd not try to estimate the brightness much deeper than 10th magnitude under a dark sky. A V-band filter would be a good idea. See the AAVSO tutorials.

 

If you're after results that will be scientifically useful, you need to achieve a noise floor of about 1 milli-mag (at least 100x better than the eye can do) and make calibrated measurements in different photometric bands. This is fairly difficult but the big deal is pointing and guiding your scope.

 

1. Calibrating your instruments - for a ccd or cmos imager this means calibrating the entire field to achieve as high a dynamic range as possible

2. Finding the object of interest and enough reference stars of similar spectral class in the field to allow you to correct for atmospheric extinction.

3. Integrating the exposure so that you can achieve a useful signal to noise level.

4. Processing & reporting your data.

 

As a start, I would look for a relatively low resolution ccd camera. The old SX HX516 and SBIG ST-8 cameras are capable of delivering noise floors of about 1 milli-mag. (both are 20+ years old!)

 

I would avoid a single shot colour camera. Although they were frowned on, chips that compress the dynamic range are actually much easier to use if you limit the exposure to about 80% of the maximum charge.

 

Finding the object - here it's all about the mount. Ditto for integration. Sorry.

 

Processing - The Handbook of Astronomical Image Processing (Berry & Burnell) contains a lot of useful information on photometry. The software that comes with it - AIP4Win is now freely available. The AAVSO has a lot of tutorials on photometry.

 

If you are interested in cataclysmic variables (say like DQ Her), you need to make measurements with a noise floor of about 1 milli-mag. This requires about 25 seconds integration with a 10" which are repeated on a 30 second cadence.

[Xilman]

Visual observations: you need no more equipment than you already have.  There are thousands of VS within range of your kit.

 

Electronic observations: a camera and an equatorial platform so your scope can follow the sky. A photometric V filter is very desirable.

 

Both: you need enthusiasm and good finder/comparison charts. Each of BAA-VSS and AAVSO can provide the latter but only you the former.

 

Oh yes: whatever you do, practice will be needed before you can reach your true potential.

[CrossoverManiac]

What equipment would I need to do photometry that would actually be scientifically useful with a 10" Dob?

If you want to put that 10" Dob to use doing photometry with a CMOS or CCD, you will need a way to do track your target and to autoguide.

 

1. *If* you have a solid 10-inch OTA and not the truss kind, you can get mounting rings (with an inner diameter equal to your scope's outer diameter) and a beefy mount.  I tried that with my SkyWatcher traditional Don and EQ6-R and...it was a flop.  The mount could not track properly.  Now some people did get this combination to work after perfectly balancing everything but I would advise going with an the next level with an EQ-8 if you go this route.
2. Buy an equatorial platform with dual axis motors AND autoguiding capability.  Tom Osypowski makes one but it has to make for your latitude.  Wolf Creek Observatory was able to track a target for one minute per subframe.  That should be long enough to get decent results with stacking 

Edited by CrossoverManiac, 07 February 2025 - 11:44 AM.

[Xilman]

 

If you want to put that 10" Dob to use doing photometry with a CMOS or CCD, you will need a way to do track your target and to autoguide.

1. Buy an equatorial platform with dual axis motors AND autoguiding capability.  Tom Osypowski makes one but it has to make for your latitude.  Wolf Creek Observatory was able to track a target for one minute per subframe.  That should be long enough to get decent results with stacking 

 

I confirm 1 minute is easily adequate. I quite often use 30s subs, especially on brighter targets

[CrossoverManiac]

I confirm 1 minute is easily adequate. I quite often use 30s subs, especially on brighter targets

Keep us all up to day on your project.  I would like to know how the platform works out for you if you go that route.

[Xilman]

Keep us all up to day on your project.  I would like to know how the platform works out for you if you go that route.

I fear I may have inadvertently misled you.  My scope is on an equatorial fork mount.

 

Nonetheless short subs are taken and co-added. There are several clear benefits and one disadvantage. The disadvantage is that each sub carries its own noise in addition to the signal and noise from stars and sky. The benefits include easy removal of subs spoiled by satellite trails, cosmic ray hits, sporadic guiding errors from gusts of wind, and intermittent passing cloud. Further, by doing rough and ready real time stacking one can monitor the SNR and so move on to another target once the SNR is high enough. All these optimize the use of telescope time and improve productivity. I recommend the technique to photometrists who are capable of using it.

 

Paul

[CrossoverManiac]

Just out of curiosity, what telescope, folk mount, and equatorial wedge are you using?

Edited by CrossoverManiac, 09 February 2025 - 10:35 PM.

[Xilman]

Just out of curiosity, what telescope, folk mount, and equatorial wedge are you using?

 

The telescope is a 400mm Dilworth made by Larissa Precision Optics. The mount was designed and largely manufactured by the previous owner of Tacande observatory. An image appears here: http://www.astropalm.../equipment.html

 

The grey bits you can see are cast aluminium and castings were from a wooden former made by Joan.

Edited by Xilman, 10 February 2025 - 03:25 AM.