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agmakr
member
Reged: 12/14/07
Posts: 24
Loc: Athens, Greece
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Greetings from Athens,Greece!
I have been reading about Variable Stars lately and i would really like to get involved in CCD Photometry.My equipment is a C11/NEQ6, so the next step in approaching my goal is getting a CCD camera suitable for Photometry.
However, my budget is tight rigt now and i have been thinking about a DSI Pro.I think of it as a cheap CCD that will introduce me to the charming world of Photometry.
I have no previous experience in CCDs yet so there are some issues that i need help:
1. Will i need guiding with this setup,or just a good polar alignment?
2. Is the Meade f/3.3 CCD Focal Reducer compatible with my scope?What adapters are necessary to get my setup ready for imaging?
3. Are there any other cheap CCDs that i could use for Photometry?
4. Along with the camera i will buy a "V" filter to start with.Is there a specific brand that that i should look at?
5. What magnitude range of Variable stars will i be able to hunt?
Thanks for your time,
Angelos
PS. If this is not the right topic for this thread, please move it to the right one.
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walt r
Post Laureate
Reged: 02/13/07
Posts: 3458
Loc: Doylestown, PA
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This is the best place on CN for your questions.
The DSI pro is a very usable camera for starting in photometry. There is some here that started with one and hope he will get you pointers. Here is an article on DSI Pro mods for photometry. The mods are not necessary but do help. More on DSI photometry.
The Meade .33x reducer may be ok, I'll leave this for some else to answer.
Guiding would help when you are trying for long exposures but with a good polar alignment you can get good photometry results without guiding and even if the stars smear a little.
With a C11 and reducer stars down to Mag 13-15 are possible and as bright as Mag 7-8 as a guess.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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groz
Pooh-Bah
Reged: 03/14/07
Posts: 1071
Loc: Duncan, BC
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Quote:
1. Will i need guiding with this setup,or just a good polar alignment?
2. Is the Meade f/3.3 CCD Focal Reducer compatible with my scope?What adapters are necessary to get my setup ready for imaging?
3. Are there any other cheap CCDs that i could use for Photometry?
4. Along with the camera i will buy a "V" filter to start with.Is there a specific brand that that i should look at?
5. What magnitude range of Variable stars will i be able to hunt?
The dsi-pro will do the job, depending on what you have defined for 'the job'. If you are planning to track variables with relatively large magnitude changes over time, it's a great camera for that task. In warmer weather, mine starts to show an excessive amount of hot pixels, but I think I have a very poor example of the dsi-pro as well. In the winter, mine is pretty good.
Questions in order.
1 - You can do well without guiding. Exposures for photometry tend to be short compared to those used for pretty picture imaging, and having pinpoint round stars is not quite as critical. Heck, Brian was doing very good using his CPC11 in alt/az configuration.
2 - The 3.3 reducer will work, but, in the longer term you will likely be better served by a 0.63 reducer than the 0.33 reducer. I use the 0.63 reducer on my C8.
3 - If you watch the online used market carefully, there is occaisionally a ccd older model showing up cheap, which is a great ccd for photometry. As an example, there was recently a starlight xpress mx716 on the astrobuysell website that was listed for $350. Essentially the same chip size as the dsi-pro, but a cooled camera, older model, with usb 1.1 connection. Possibly a bit of driver hassle getting it going, but, once going, a camera in a league up from the dsi-pro.
4 - V filter is the place to start. Again, they can be spendy, but they dont have to be. Watch astromart and occaisionally one pops up there. I found a vendor on ebay selling filters, has what appears to be good pricing. We kitted chris telescope with a ccd about 2 weeks ago, and I've ordered filters from there for her kit. The LRGB set is listed on ebay, was $160 for the set. Same vendor didn't have the V listed, but, after a couple emails I was quoted $80 for a V, and ordered it along with the LRGB set for chris camera. At 80 bucks, that's about half what other vendors are quoting. I dont have them yet, expect them all to arrive any day now, so I cant speak for the quality of the set just yet.
5 - A better question is, what magnitude of stars do you want to hunt ? Dimmer stars are actually easier, the hard part with the real bright ones is setting up so the bright star and an equally bright reference star land in the frame. For some of the brighter stars, this just isn't possible with a small chip ccd setup. With my C8 I've done some good light curves on stars in the mag 13.8 range, using 3 minute exposures and a V filter.
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brianb11213
Carpal Tunnel
Reged: 02/25/09
Posts: 2076
Loc: 55.215N 6.554W
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Quote:
the hard part with the real bright ones is setting up so the bright star and an equally bright reference star land in the frame. For some of the brighter stars, this just isn't possible with a small chip ccd setup.
Sorry but I don't see the necessity of this. It's just convenient ... but surely if you take two frames with the same exposure, calibrate them seperately but using the same setup and apply the appropriate correction for differential atmospheric absorbtion, you should be able to get delta mag between stars in different frames.
The critical problem here is determining the differential absorbtion ... but if the altitude of two stars is similar, the derived delta mag ought to be reasonably reliable.
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gavinm
scholastic sledgehammer
Reged: 08/26/05
Posts: 804
Loc: Auckland New Zealand
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V filters (especially cheap ones) can have a problem with surface crystallisation due to humidity. They require constant cleaning. So much so that I just threw out a cheapo V that I was given by Arne Hendon (sorry Arne). The other filters dont suffer from this. My advice is to spend as little as possible (or find a free one) or be prepared to spend a lot more and get a coated filter.
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Gavin
Mt Albert Grammar School Observatory
Auckland, New Zealand
http://www.mags.school.nz/astronomy/index.html
12" LX200R F6.8 AP
SBIG ST7-XME + CFW10
Moonlite SCT focuser w/ temp
Skywatcher Equinox ED80 Pro (ADM dovetail)
+ other stuff
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groz
Pooh-Bah
Reged: 03/14/07
Posts: 1071
Loc: Duncan, BC
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Quote:
Sorry but I don't see the necessity of this. It's just convenient ... but surely if you take two frames with the same exposure, calibrate them seperately but using the same setup and apply the appropriate correction for differential atmospheric absorbtion, you should be able to get delta mag between stars in different frames.
That would work, if you can take them _at the same time_, but, you cant. If you want to do differential photometry, they must be in the same frame, so they are exposed to the same conditions, ie wispy clouds drifting thru during exposure, etc.
You _can_ work with separate exposures, but, to do that, you may want to go read up a bit on 'all sky photometry'. it's math that makes the differential stuff look like trivial grade 1 arithmetic.
If you are just starting out, you want to be doing differential photometry, and that by definition means, you need the target and reference stars in the same frame. This is not a 'nice to have' thing, it's the definition for differential photometry.
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groz
Pooh-Bah
Reged: 03/14/07
Posts: 1071
Loc: Duncan, BC
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Quote:
V filters (especially cheap ones) can have a problem with surface crystallisation due to humidity. They require constant cleaning. So much so that I just threw out a cheapo V
Hmm, ya learn something every day. We ordered the 'less expensive' filters because, setting up for a second telescope was starting to get spendy, another camera, another wheel, and a batch of filters. May turn out that it cost more in the long run to save in the short term, but, only time will tell.
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brianb11213
Carpal Tunnel
Reged: 02/25/09
Posts: 2076
Loc: 55.215N 6.554W
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Quote:
If you want to do differential photometry, they must be in the same frame, so they are exposed to the same conditions, ie wispy clouds drifting thru during exposure, etc.
You can't guarantee that stars in the same frame will be affected exactly the same by wispy clouds ... it's perfectly possible for one star to be affected by a localised patch of e.g. contrail and another only a short distance away not to be, or to be affected at a different time. I don't see that taking two frames in the same part of the sky only a short time apart is a particular problem. In any case you can only get a reliable estimate of your errors by taking a reasonable number of frames and using some sort of statistical technique (simple averaging, finding the median or something more sophisticated like kappa sigma deselection of the outliers).
Absolute "all sky" photometry is of course a completely different kettle of fish ... the math looks complex (nevertheless a trivial problem to a modern computer) but really it's the necessity to have reliable sky conditions that makes this out of reach of most of us.
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brianb11213
Carpal Tunnel
Reged: 02/25/09
Posts: 2076
Loc: 55.215N 6.554W
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Quote:
V filter is the place to start.
This is a very complex area. V filtering is only valid if the filter is used with the exact detector it was designed to work with. Using a "generic" V filter (probably designed for use with a photoelectric CdS detector, not modern CCD cameras) is highly unlikely to give a result closer to true "V" passband measurement than using the green filter from a colour seperation set ... which is likely to have a much higher transmission, allowing fainter stars to be measured.
Many variable star programs (e.g. monitoring dwarf novae) can be done perfectly adequately with no attempt at controlling the passband of the detector.
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btieman
sage
Reged: 07/24/08
Posts: 481
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Quote:
Quote:
V filter is the place to start.
This is a very complex area. V filtering is only valid if the filter is used with the exact detector it was designed to work with.
Huh? The whole point of using a *standard* filter is to remove the color response bias from different detectors. With a known pass band hitting my sensor I can compare me erading to the readings of others independant of the response of my CCD. The comparison between V and C will adjust for sensitivity and gain effects--but I need a band pass to filter out color sensitivity differences.
You do not *need* to filter for a number of observing programs. If you're going to report your observations to somewhere like AAVSO, the use of at least V is encouraged.
Brian
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CPC 1100
Orion 120st
Milburn Wedge
QSI 520i
Meade DSI Pro
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gavinm
scholastic sledgehammer
Reged: 08/26/05
Posts: 804
Loc: Auckland New Zealand
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Angelos, here's the website of a mate of mine here in NZ. She does stunning photometry, most of which is used by international collaborations. All of her data is UN-filtered. A lot can be done without a filter..
http://www.farmcoveobs.co.nz/
However, more can be done WITH filters, but decide what sort of photometry you might like to get into because that will partially determine the filter you use.. I currently have 5 different photometry filters. (only one more gap left - narrow band might have to wait..)
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Gavin
Mt Albert Grammar School Observatory
Auckland, New Zealand
http://www.mags.school.nz/astronomy/index.html
12" LX200R F6.8 AP
SBIG ST7-XME + CFW10
Moonlite SCT focuser w/ temp
Skywatcher Equinox ED80 Pro (ADM dovetail)
+ other stuff
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brianb11213
Carpal Tunnel
Reged: 02/25/09
Posts: 2076
Loc: 55.215N 6.554W
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Quote:
Huh? The whole point of using a *standard* filter is to remove the color response bias from different detectors.
No, it's to standardise the bandpass.
Use a filter with a specific bandpass with a detector which does NOT have the bandpass that the filter was designed to work with, and you will have a non standard bandpass.
To do V filtering PROPERLY you also need to measure at least one of B-V and V-R as well, and calibrate your measurements by subtracting a part of the colour index, the exact amount being determined by observing "standard" stars. There is a further correction dor differential atmospheric absorbtion varying with colour index. Once you get to this level, it doesn't actually matter that your filters & detector are non standard - all of them are!
If you want to do time series differential photometry, precise bandpass filtering is a refinement that can easily be done without. If you want to do low resolution photometry (~0.05 mags) for comparison with visual estimates, again an exact bandpass match can easily be done without, your results should be much better than the huge scatter given by different observers working visually - with unstandardized spectral respose of their "sensors".
To anyone starting out, getting a V filter is an avaoidable expense & complication. Just use a green colour seperation filter - if you can stand the poor transmission, an ordinary Wratten #58 works well .... but make sure that the extended infra red response typical of CCD sensors is suppressed.
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agmakr
member
Reged: 12/14/07
Posts: 24
Loc: Athens, Greece
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As i said, i am considering the DSI Pro as an "introductory" camera to CCD Photometry.I will also keep an eye on the Used Market in case something good comes up.
I thought of buying the Meade .33x ccd reducer because i am a little worried about the narrow field of view and whether i will find comparison stars for my targets.
But i will also consider the 0.63x.
Right now i am more attracted to close binaries eclipsing one another and maybe these will be my first targets.However there is a lot for me to read yet.
Thank you all very much for your answers!
You have been of great help.
I will come up with new questions shortly, that's for sure 
Angelos
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btieman
sage
Reged: 07/24/08
Posts: 481
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Quote:
Quote:
Huh? The whole point of using a *standard* filter is to remove the color response bias from different detectors.
No, it's to standardise the bandpass.
Which, when doing differential photometry, removes the color bias from the detector response.
If I'm trying to do differential photometry where C1 is a blue star and C2 is a red star, I can do that with any single detector because the CCD response curve is the same. Now, I want to compare my observations to yours. They may be very different because my CCD is much more sensitive in blue and yours in red. For me, the blue star is brighter when compared to the red, for you the reverse due to the CCD spectral response. If we both use a V fitler, we've standardized the band pass so both our sensors see the same color stars. The difference in my measurement to yours is greatly reduced as we both now see the same color star. The narrower the bandpass the more accurate the comparison--the BVRI system has pretty wide bandpasses but is still used due to the decades of measurements already on the books.
All sky photometry is another beast all together...
The AAVSO has a "CCD Observing Manual" that's a good reference.
http://www.aavso.org/observing/programs/ccd/manual/
From the section on filters:
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CPC 1100
Orion 120st
Milburn Wedge
QSI 520i
Meade DSI Pro
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btieman
sage
Reged: 07/24/08
Posts: 481
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Quote:
As i said, i am considering the DSI Pro as an "introductory" camera to CCD Photometry.I will also keep an eye on the Used Market in case something good comes up.
I thought of buying the Meade .33x ccd reducer because i am a little worried about the narrow field of view and whether i will find comparison stars for my targets.
But i will also consider the 0.63x.
Right now i am more attracted to close binaries eclipsing one another and maybe these will be my first targets.However there is a lot for me to read yet.
Thank you all very much for your answers!
You have been of great help.
I will come up with new questions shortly, that's for sure
Angelos
Angelos,
You *can* do great photometry without filters--many do. The need for filters depends on what your end goal is. If you wish to submit observations for comparison to someplace like AAVSO, filtered observations are encourage. If you want to measure light curves from short period binaries, you can get very nice light curves unfiltered. What you wil eventually find, however, is that many stars change color over a few hours due to extinction effects from the atmosphere. Red suffers less from atmospheric extinction than blue. What this means is that over the course of several hours a red star may change apparent brightness less than a blue one as measured by the same instrument. Filters lesson this effect by ensuring that the same color arrives at the CCD during the entire observing session. You should still get a clear light curve unfiltered, but a filtered light curve will have higher precision.
If you already own a G filter from an RGB set, you can likely make use of it as BrianB suggests. The Photometric V (green) filter and G filters have similar bandpasses. This is *not* true of R where I have seen very wide differences in bandpass between photometric R and RGB R. If you use a G filter you want to ake sure it blocks IR--many don't and CCD tend to be very sensity to IR.
I have a DSI I and found the f/3.3 reducer a must. I found the f/6.3 less useful with the DSI I. I have a significantly bigger CCD FOV now and the situation is now reversed and I rarely use the f/3.3 anymore.
My interest is exoplanet transits--basically close binaries  Ok, so oneof the pair is tiny compared to the other--and doesn't emit sigificant light of its own. But much of the technique is the same.
Brian
--------------------
CPC 1100
Orion 120st
Milburn Wedge
QSI 520i
Meade DSI Pro
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brianb11213
Carpal Tunnel
Reged: 02/25/09
Posts: 2076
Loc: 55.215N 6.554W
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We're really singing the same song, but:
Quote:
Which, when doing differential photometry, removes the color bias from the detector response.
Suppose you're using a "standard" (Johnson) V filter with a back illuminated CCD sensor. This will have a relatively higher quantum efficiency at the blue end of the passband than a front illuminated CCD sensor, so the passband of the detected photons will be shifted slightly towards the blue. Same filter, different spectral response ... for super accurate results, calibration is required; in this case you would measure B as well as V, and obtain your calibrated V by subtracting a fraction of the B-V colour index you derived.
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groz
Pooh-Bah
Reged: 03/14/07
Posts: 1071
Loc: Duncan, BC
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I think the discussion on filters is getting way beyond the scope of the original question.
If you want to do photometry, and just gather data for your own use, then you can do whatever you want, with or without filters. If you want to start participating in larger scope organized efforts to monitor various items, there are some campaigns that accept unfiltered data. Most campaigns have filter requirements. Some of those specifically request data with BVRI or a subset thereof. Of those looking for a subset, the majority by far are requesting data taken thru a V filter.
So, the point of filter choice is not about the complex math of various corrections and ccd response characteristics, it's about a far more basic thing. If you want to play in your own sandbox, you can make your own choices. If you want to play in the big sandbox, and submit data into longer term observing campaigns, the V filter opens more doors than all the rest combined, so if you are going to purchase one filter for doing photometry, it's the one that opens the most doors to participating in the big picture.
Now, if the goal is to just have fun, and not submit data into the bigger collections, then eclipsing binaries as mentioned is actually a great place to start. After I got my sxv-h9, the first night out I didn't have any filters yet, but I did shoot this on a fast eclipsing binary. GSC 3074-0114 done with the sxv-h9 attached to the C8 with the 0.63 reducer, no filters. I shot this in July at the Island Star Party held at the Victoria Fish and Game club trap shooting range. I chose this particular one because it would be fairly high in the sky, and with a period of less than 1.5 hours, with a swing of 0.7, I could get two full cycles of the swing in one session. It's good for that desire to find 'instant gratification'.
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jgraham
Postmaster
Reged: 12/02/04
Posts: 6749
Loc: Dayton, Ohio
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Angelos; that sounds like a good approach. I started with an original DSI Pro and Pro II and I'm now using a Pro III. The only filter I use is a V photometric filter, which I consider to be a must-have. Before I started using the V filter my observations were often very close to what was being reported to the AAVSO, but it was sometimes hit or miss depending on the star, since using the V fliter they data looks really good. The only concern I had about your setup was the small field of view, but a f/3.3 focal reducer should help that. You could also piggy-back a smaller scope onto your C11 to get a wider field of view. Some of my early work was done with a DSI Pro on an Orion StarBlast (4.5" f/4 Newtonian) on my LXD75 and that gave a very nice field of view.
Have fun!
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-John
================================================
Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I, II, & III), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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gavinm
scholastic sledgehammer
Reged: 08/26/05
Posts: 804
Loc: Auckland New Zealand
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I know it's slightly off topic, but to step back a bit..
Brianb is correct. The detectors response to a V-filter can be important. Two different CCDs may produce differing fluxes in the V band by as much as 30-40 millimag. For accurate photometry B-V and/or V-R corrections are required.
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Gavin
Mt Albert Grammar School Observatory
Auckland, New Zealand
http://www.mags.school.nz/astronomy/index.html
12" LX200R F6.8 AP
SBIG ST7-XME + CFW10
Moonlite SCT focuser w/ temp
Skywatcher Equinox ED80 Pro (ADM dovetail)
+ other stuff
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phxbird
super member
Reged: 12/24/07
Posts: 116
Loc: New Mexico, USA
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Here is the best resource I know of!
http://www.aavso.org/observing/programs/ccd/manual/
It answers most of your questions on photometry and with the info on the DSI pro that others have given it should get you started!
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Celestron 6" Refractor
Celestron AS-GT Mount
"The Bargain Bucket" 8" Dob
Meade ETX 90 EC
Orion ST 80mm
SAC-7b Imager
SBIG ST-7E Imager
Temple Research Observatory
AAVSO Member
Society of Astronomical Sciences Member
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