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Photometry work with a C9.25

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#1 agmakr

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Posted 12 July 2013 - 04:47 PM

Hello from Greece,

i am trying to find my way into photometry work (mainly binary systems and maybe exoplanet transits) and i am thinking of buying a Celestron C9.25 for that purpose.

I already own an NEQ6 mount (hypertuned) and i have previous experience in CCD imaging (deepsky stuff).

Would you suggest buying a C9.25 for photometry? Should i get the HD version or just the good old one?
I understand that there are some issues that come with this telescope as i have owned and used a C11 in the past.

Here are some questions that come to mind:

1. How could i solve the problem of mirror shift or is it not a problem for photometry?

2. Are 9.25 inches adequate for exoplanet transits, imaging through a 100.000 population city with light pollution?

3. How could i automate focusing with that OTA? With my current refractor, i use MaximDL+Focusmax and a stepper motor from rigelsys, and i would like to have the whole set up automated to image through the whole night (with CCDAutopilot for example).
Can this be achieved with the C9.25, without having to touch the scope to add bahtinov mask etc?

4. What CCD camera should i buy (used or new)?
I am thinking about the Atik 314L or the Atik 420M. Which would be more suitable?
Are there any other suggestions that i could easily find through either Astromart or Optcorp?

For starting i will get an Astrodon Photometric V filter and later add the other filters (already own SX motorised filter wheel).
I also own MaximDL,the Sky6 and AIP4WIN.

I would really appreciate some help on these subjects.

Thanks in advance for your time,
Angelos

edit: Forgot to mention that i will autoguide with an OAG and QHY5 or DSI I camera.

#2 btieman

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Posted 12 July 2013 - 11:54 PM

Angelos welcome!

I've been measuring exoplanet transits for a number of years now. As with most things, now that we've learned more about what we're looking the techniques have gotten easier and we know more about what is important and what isn't :)

I'm not much of a blogger--too many interesting things to do in a day!--but I did blog some useful resources here:

http://exoterraviews...-resources.html

Don't bother with any of the rest of my useless, drivel, but if you think you want to try exoplanet transits, that link has some information on some great resources!

I have a CPC1100 (pre-Edge). For reference, you can see some of what I've done with it here:

http://var2.astro.cz...n&submit=sho...

Note that my first dozen or so exoplanet transits were measured in alt/az mode!!

I'll try and answer some of your questions from my perspective...

1) You're planning on using an OAG--don't worry about mirror flop. Aperture photometry is pretty forgiving. The stars don't need to be round--you're not taking pretty pictures. At worse you lose a frame here or there if the image shifted a lot. The biggest problem is the software may lose track and you may need to break your runs up into batches to process everything, but that's a fault of poor software.

2) I don't know what your skies are like, but I live in an urban area of ~2 million. Again, aperture photometry is very resilient. I measured one transit with the full moon less that 20 degrees away. Large gradients and not the best results I've ever managed, but the transit was still pretty clear. There will be limits on how dim a star you can measure is though.

3) Don't bother with auto focusing. Often a soft focus is preferred. Kepler was designed to have a soft focus. I have a temperature compensated focuser and have found that to be plenty sufficient. I even have measured transits with no compensation and lots of focus drift--I'm talking donuts!

4) A temperature regulated CCD is best--preferably monochrome. If it has an anti-blooming gate, it's best to measure your linear response range and stick to exposures within that range. Most modern CCDs are linear over a large range.

Filters are not necessary. They do help with color bias due to extinction but you get better S/N without one. I've used V and R but most are C. Bruce Gary has done some studies and suggests a blue blocking...even had a blue blocking filter designed specifically for exoplanet transits. For the cost, start with what you have...even if it's nothing!

Now, most of what I said !*IS*! poor technique and will not give the best results. I am NOT advocating being lazy about your technique lest I be ridiculed out of cloudy nights ;) But all of it is possible and I have measured multiple transits that way! The thing to remember is, for most of us, this is a hobby. We have an interest in science and want to push the limits of what we can do, but our lively hood is not on the line if we aren't the best scientists in existence (For the record, I put 20 years into working at a national laboratory as an engineer working directly with prominent scientists...I do understand what real science is about) We can afford to play and see what happens and discover things people never thought possible (no one thought measuring a transit in alt/az mode was possible...many still don't believe me)

For !*BEST*! results, whatever increases your precision is beneficial. You want hours of collected data with as little changing as possible--detector temperature, air mass, position of photons on the chip, readout noise, color changes, etc, etc, etc...but there is a cost/benefit to achieving higher precision! My approach was to start with what I had and fix the problems rather than spend tons on what I *thought* I needed. Guiding was my single biggest gain. While I could see a transit in alt/az, guiding helped ease the processing, lets me sleep at night, and had the biggest positive effect on precision. I can guide to within a few pixels over hours now...still not paramount quality, but I haven't bought a paramount either :(

I *thought* auto-focusing would be helpful, and bought an electronic focuser for that reason. Turns out focusing had little to now gain but a fairly large expense. I do like the temperature compensation, but it hasn't really helped my precision much and I've totally given up on bothering to take time out of a run to try and refocus!

So, start with what you have...even a 4" refractor will get you a number of targets. Analyse the data, then fix the problems. Post back here if you're not sure what "the problem" is.

And good luck!!! I enjoyed it for several years...until I bought a spectrograph :)

#3 Hubert

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Posted 13 July 2013 - 09:15 AM

C9.25 is a perfect telescope for photometry. Be sure to use a focalreducer so you can work at f6.3. Best is to use a NAGB camera. They are more sensitive then the AGB versions.

You don't need a V-filter right away. I would suggest that you do timings of HADS or RR Lyrae stars. Eclipsing binaries are also observed without a filter. Timing the maximum is the goal.

Success!

#4 agmakr

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Posted 13 July 2013 - 03:40 PM

Hello Brian and Hubert, and thanks for your detailed answers.

So, if i understood correctly, guiding with an OAG will minimize the problem of mirror flop.
Also, focus changes through the night will not be a real problem in differential photometry.

Maybe then i won't install an electronic focuser right away.
It is just that i really enjoy the convenience of autofocusing my refractor with focusmax.

I intend to start imaging some short period variables with a 65mm refractor so that i check some techniques and get to know the methods required.
I also have to study Bruce Gary's excellent book.

Thanks again for your time,
Angelos

PS. Nice blog Brian, with a wealth of information!

#5 agmakr

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Posted 13 July 2013 - 03:59 PM

Another question:

If i use my Atik 460M with the C9.25:

at f/6.3 the image scale is 0.59 ''/px and a FOV of 21.8' x 27.2'.

At f/3.3 the image scale is 1.13''/px and a FOV of 41.6' x 52' .

Which option is more beneficial for photometry?
(also i don't know which option will require stranger adapters/spacers)

#6 btieman

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Posted 14 July 2013 - 05:56 PM

Angelos,

Guiding with the OAG will not cure mirror flop, but, as long as the flop isn't huge, the guider should catch up with it. You'll probably have some focus shift along with the flop but that shouldn't be a problem.

If you have an electronic focuser, may as well install it. I have an electric focuser on mine. One the cover is pulled off, I can do everything from my kitchen--great in the winter! I just don't bother trying to get precise focus. Eyeball tight stars is about the best I ever do for photometry runs...I purposely do worse if the target is bright enough. If your focuser has temperature compensation, that's probably a better option than refocusing during a run. I routinely use my temperature compensation.

How are planning on processing the data? You mentioned binary systems (I assume eclipsing) and possibly exoplanets. That means you'll likely end up with a few hundred frames of data in a time series that you'll need to process to create a light curve out of the individual photometric measurements. This is where mirror flop and focus changes can cause problems. A large mirror flop may throw off the algorithm that's tracking stars--most don't track the target star by plate registration but instead re-centroid within a small search radius. A large mirror flop can move the star far enough that it will not be found. When this happens, you can restart the processing on the frame after the flop. In practice, it rarely happens that my mirror flops that far between any two frames.

Similarly, you set up an aperture, inner and outer annalus to compute photometry. This is usually done on a sample frame. If the star is defocused, you can easily pick an appropriate aperture to still select all the light. If focus changes significantly, some of the light may leak out of the aperture. The fun thing is, the photometry still works! All stars will defocus the same. Not a requirement, but you should be picking comp stars similar in magnitude so you should leak similar amounts of flux from each star and you get a result! The bad news is, the S/N will have dropped and so your precision will suffer. Incidentally this is true for tracking errors that cause dumbbell stars and such as well. Keeping a similar focus (temperature compensation is generally good enough) frame to frame helps to maintain your precision :)

As far as plate scale and photometry, there are trade-offs. The thing to remember is you're not taking pretty pictures, you're counting flux. The shape/size of the flux doesn't matter except for certain physical constraints of our detectors. I like the smaller plate scale to increase FOV--you want comp stars in the same frame and the more, the merrier! The downside is tighter stars mean the take up less pixels and each pixel can only hold so many photons. S/N goes up with fewer pixels due to less read noise, but you also need to measure enough photons to have a high resolution in flux to see small changes in flux. The other trade off is time resolution. Larger f/ ratios mean longer exposures to good S/N. There are ways to compensate...stars too tight, defocus, go to higher f/ ratio. f/ ratio too high for good time resolution, bin the frame. You don't always have the knob you want when you need it, but after you've done a few runs, you'll start to get the feel of what the trade-offs are for the knobs you do have. As a point of reference, WASP has found numerous exoplanets with a 200mm f/1.8 lens.

Myself, I prefer the wider FOV. I typically run my CPC1100 at f/7 for photometry. That's about the widest FOV I can get before I start having issues with my optical train.

Again, my advice is to jump in and try something! The AAVSO has a variable star index:

http://en.wikipedia....umulation_diode

You can search for short period variables or differing magnitude changes to try. Some are short enough you can get several oscillations during a single evening...which is quite fun!

#7 agmakr

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Posted 15 July 2013 - 05:53 AM

Hello again Brian,

wow that's a lot of info!

I am in the process of reading Bruce Gary's book and
info on the net about the methods used in photometry work.

I am planning to measure some short period variables like
V0523 Cas or UX Uma and see in practice what is needed.
However, these first tries will be with a 3'' refractor.

After i order the C9.25, i will try different combinations with the reducers and see what suits me better.

Thanks again for your time and effort,
Evangelos

#8 Hubert

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Posted 15 July 2013 - 07:50 AM

Hello Angelos,

I see that you are from Greece. I'm a member of the Belgium group of variable star observers and we are working around photometry of High Amplitude Delta Scuti stars. Most observers are from Belgium but we also have some foreigners who are observing our program. One of the is Anthony Ayiomamitis from Athens, Greece. Maybe that you can contact him? He is doing photometry not only for us but also for other organizations like the AAVSO.

You can find his website here:

http://www.perseus.gr/

Look at his photometry section. Who knows, maybe you two live in a close range.

#9 agmakr

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Posted 25 August 2013 - 04:17 AM

Hello again,

i am considering buying a C8 instead of C9.25 for variable stars and exoplanet transits.
Most likely, either of these will be used with an Atik 314L.

The downside is that i don't know yet the exact type of variable projects i want to dive in,
so i don't know if the C8 will be a regretting choice later, compared to the C9.25 .

Would there be huge differences for photometry work?
Again, i am in mediocre light polluted enviroment of 100.000 people.

#10 Hubert

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Posted 25 August 2013 - 08:28 AM

With the C9.25 your FOV will be smaller but you can go somewhat fainter but there's no big difference.

#11 agmakr

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Posted 06 September 2013 - 02:36 PM

Hello again,

i have received my C9.25 and i'm currently waiting for an Atik 314L mono.
I am in the process of deciding which focal reducer to buy and use with the above set up.

Does anyone have experience with the Meade f/6.3 and the Antares f/6.3?
Which would be a better option with the C9.25?

Kind regards,
Evangelos






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