Hi all. I just recently discovered the AAVSO and I'd like to get started with observing. I've read through the manuals on visual and CCD observing. I think to keep things simple I should start out by doing some visual observations.
The first problem I ran into is which stars to observe. I found the ten star tutorial, but it appears none of them are well above the horizon tonight over southern California. Can anyone recommend some targets of low magnitude?
Then, do I need a photometry filter to get started? I have a Celestron Nexstar 8SE, a ZWO ASI120MC CCD cam, and for filters I have a Lumicon UHC and OIII filter. I'm not sure I want to drop a couple hundred bucks just yet on a specialized filter until I get my feet wet. What do you folks recommend?
Also, how much does light pollution matter? I'm in a suburban area so the light pollution is pretty bad, although not terrible.
Finally, once I've found the target, and I pull up my star chart, how do I actually make a visual observation? I know I'm supposed to compare it to the nearby stars, but it seems strange that I would be able to get it down to 0.1 magnitude. What are some tips to up my accuracy? And is there a graph for each of these stars that I could look at to see what the magnitude SHOULD be tonight?
Thanks!
[quote=Mostlyemptyspace]
Hi all. I just recently discovered the AAVSO and I'd like to get started with observing. I've read through the manuals on visual and CCD observing. I think to keep things simple I should start out by doing some visual observations.
The first problem I ran into is which stars to observe. I found the ten star tutorial, but it appears none of them are well above the horizon tonight over southern California. Can anyone recommend some targets of low magnitude?
Then, do I need a photometry filter to get started? I have a Celestron Nexstar 8SE, a ZWO ASI120MC CCD cam, and for filters I have a Lumicon UHC and OIII filter. I'm not sure I want to drop a couple hundred bucks just yet on a specialized filter until I get my feet wet. What do you folks recommend?
Also, how much does light pollution matter? I'm in a suburban area so the light pollution is pretty bad, although not terrible.
Finally, once I've found the target, and I pull up my star chart, how do I actually make a visual observation? I know I'm supposed to compare it to the nearby stars, but it seems strange that I would be able to get it down to 0.1 magnitude. What are some tips to up my accuracy? And is there a graph for each of these stars that I could look at to see what the magnitude SHOULD be tonight?
[/quote]
Nobody answered yet, so let me just make some very important points:
1. DO NOT CHECK IN DB FOR VARIABLES BRIGHTNESS FIRST!!! This is a serious cause of observer bias. Just do your best to estimate the star, enter it into the database, THEN check it compared to other observations. If it turns out to be a mistaken ID or other serious error, that observation can always be deleted.
2. If light pollution is a problem, CCD gives better results than visual. But, its a LOT more difficult to make good CCD observations if you have no experience doing them.
3. At least get a standard Johnson V filter for your CCD. You can also get them in sets with B, V, R, I is good, but cots more.
4. Making visual estimates to 0.1 or 0.2 mag is not that hard, if the variable has good comparison stars within a few tenths magnitude of it, and spaced a little brighter and fainter, and located in the same field of view. You should start with some variables that have such good chart available. Also, you can defocus the stars to a small disk, that helps visually compare brightness a bit easier than sharp points.
This will hopefully help you get going on the right track. Keep posting here for further help.
Have Fun!
Mike LMK
Hey Mike. Thanks for the reply. These are great points. So which stars would you recommend I target first if I'm in San Diego, CA and I have a view of the sky probably above 30 degrees above the horizon? I have houses around my back yard, so I gotta slew up a ways to see the sky.
[quote=Mostlyemptyspace]
Hey Mike. Thanks for the reply. These are great points. So which stars would you recommend I target first if I'm in San Diego, CA and I have a view of the sky probably above 30 degrees above the horizon? I have houses around my back yard, so I gotta slew up a ways to see the sky.
[/quote]
Hi Mostly,
Well horizon blocked isn't a problem if you are in light pollution (LP), the higher up you look, the better it will work visually. I can suggest one star right away - T CRB its a pretty bright recurrent nova, in the 10 magnitude range, so should be easily estimated with any scope even under poor conditions. It also has a nice easy to find set of comparison stars surrounding it, of the proper sequence in brightness steps, to allow pretty accurate estimation. Its going to be passing very high overhead in the late evenings now.
SS CYG is the classic dwarf nova type that has regular outbursts from 12th mag to 7-8th magnitude. Its easy to located in a triangle of stars and has a good comparison sequence as well. It gets higher up later at night now, but will be a good summer object in the evenings.
R VIR is a nice fairly shorter period Mira type with a quite noticeable change in brightness over just a week or so.
Try these!
Mike
Hi !
First the ZWO is not a CCD sensor camera but a CMOS one and a COLOR version. It is originaly designed for planetary imaging but it could certainly be used for photometry. It uses a very good CMOS sensor having a very high QE and very low fixed pattern noise.
Filter: Never use an additional V filter with such color camera, it's like a DSLR, what you would get is a combination of both filter response curves not a Johnson response ! You can instead extract a very good V mag using the G channel RAW output. This is what we all do with DSLR, have a look at the DSLR tutorials under "Citizen Sky" (the AAVSO should soon issue a "DSLR manual" that's also usable for such camera). The only issue is it's a very small sensor and you could have a problem to find the proper comparison and check stars in the narrow field of view provided by such small sensor combined with the Nexstar 8SE. It would probably be better to use it with a photo lens.
Anyhow you should check the linearity of the FILE FORMAT you use as the output of that specific camera. The CMOS sensor itself is VERY linear and has a much larger electron capacity than a CCD and by the way an high saturation level. But the camera INTERNAL PROCESSING could apply the standard "gamma" signal transformation. It's a near log representation of the image data that is the usual imaging standard but improper to photometry. A RAW output format (like the SER) shall be used. The doc I have access to is unclear on that point.
Clear Skies !
Roger
[quote=Roger Pieri]
Hi !
First the ZWO is not a CCD sensor camera but a CMOS one and a COLOR version. It is originaly designed for planetary imaging but it could certainly be used for photometry. It uses a very good CMOS sensor having a very high QE and very low fixed pattern noise.
Filter: Never use an additional V filter with such color camera, it's like a DSLR, what you would get is a combination of both filter response curves not a Johnson response ! You can instead extract a very good V mag using the G channel RAW output. This is what we all do with DSLR, have a look at the DSLR tutorials under "Citizen Sky"
[/quote]
Hi Roger, Thanks for that clarification. Shows my quite limited CCD knowledge. I am 95% Visual observer and a little experience now with the BSM system (SBIG ST8). Obviously, even DSLR type photometry involves quite a bit more technical work than visual does!
I normally recommend beginner observers start out visually, but this fellow lives in a pretty light polluted area, which swings the "best results" balance more toward digital vs. visual.
I guess it is up to our friend, whether he feels like digital photometry and all its details is what he is interested in, compared to the relative ease of visual photometry.
Mike LMK
Ok cool. I also have a dslr I can use, so I'll try that instead. Thanks for letting me know I can just use the green channel.
So with the DSLR do I still need to take dark flat and bias frames? How many of each? And what camera settings and exposure times should I use?
Hi Mostly !
If you try DSLR photometry I think the best is to start with a photo lens, a 100 mm F2.8~4 or a 200 mm F4 are best for it, able of very accurate photometry of bright stars down to mag 8 and even mag 14 when used on equatorial with tracking. But you could also start with a smaller aperture. It could be used on simple photo tripode in case of bright star.
Using a telescope with larger aperture is right only in case of faint stars. In the recent Nova Delphini campagne many guys have been in trouble using telescopes, either saturating or using far too short exposure. The total exposure time shall be long enough to eliminate the effect of the scintillation, one minute total is usually needed for each point.
As an example, for eps AUR or Nova Delphini (mag 3~4) I used a 450D (APS-C) and an old 200 mm F4 lens, ISO 100 (max electron capacity of the sensor to avoid saturation and maximize the SNR), 5 series of 10 images 8~10 seconds each. A defocus is also applied to avoid saturation. Each series is stacked or averaged. The five results provide an accurate mean and the standard deviation to check its accuracy. You could find processing tutorials at Citizen Sky.
Flat is mandatory to take care of the vignetting of the lens (that's strong in photo lens), take about 20, but a good flat can be used for number of observations (dust is no issue if you work at F4 or lower F). In the case of short exposure, like the exemple hereabove, darks are not needed with recent CMOS sensors. For long exposure and/or high ambiant temperature you need it, the number needed is very depending of the shot condition: defocus, no tracking, tracking, guiding... start with 10 before and 10 after your series. If you make such classical darks during the observation you don't need bias, they are included in the dark. But there is a technique that avoids to lose observation time: making a strong master-dark at any time and rescalling it to the measure of the dark impulses in the sky images (available in IRIS).
You will find a detailled reporting and analysis of a DSLR testing campaign in:
http://www.aavso.org/ejaavso402815
You could be interested in color correction in:
http://www.aavso.org/ejaavso402834
The hereabove test campaign has been made under the well light polluted sky of Dijon in France ! The visual limit mag is 3.5, at best at zenith. Little stars to see near the horizon !
I attach a revised IRIS tutorial.
Clear Skies !
Roger (PROC)
Let's start with the simpler to answer, but you have some "homework" to do too so we can support you at best
1 - LIGHT POLLUTION DOES NOT STOP VISUAL ESTIMATIONS / I observe from the edge of a 80k souls city, so limiting magnitude is usually 3.0 for the most part of the year - Alessandria in Italy is wetter and fogger that San Francisco - and I logged more than 12k obs. The amazing part of the game is that as soon as you need a larger scope for fainter variables you work with longer focal lengths and this triggers higher magnifications than a binoculars: and higher magnifications darkens the sky background, Let's say that a 50 mm binoculars allows you sound estimations up to mag 7, a 70 mm up to 7.5 mag, a 90 mm rich field refractor grasps mag 8.5 and an 8" f/6 Newton hits mag 10.
2 - NO FILTERS FOR VISUAL OBSERVATIONS / Light Pollution filters and specific wavelenght narrow band filters such as OIII or HAlpha have not to be used for visual variable star estimations because they alter star "colors" - more properly, they blocks a fair share of light wavelengths. They are designed to increase contrast between faint diffuse lights such as those one by nebulae and galaxies and the sky background, but stars are point light sources and do not benefit so much from this procedure anyway. Moreover variable star and its comparison stars are nearly never exactly matched about wavelength peak emission, so using LPR or OIII filters you will estimated a "modified" variable versus "differently modified" comparison starts - results is a mess.
Try a LPR filter looking at Mars and then Jupiter and Saturn, so you can have an idea about what happens
3 - PRACTISE, PRACTISE, PRACTISE / Only by practise you'll get 0.1 mag estimations, so don't worry. Once you get it, you'll never forget how to do.
End of part one
Let's discuss now both location, instrumentations and star targets.
1. DO YOU KNOW YOUR SKY LIMITING MAGNITUDE ? It's important because this way you will select your stars more efficiently. This should be done naked eye both in moonless nights and in full moon nights, so you can estimate the range of variable brightness you can explore. Observe The Little Bear which from San Diego is always 30 deg above the horizon and check how many of its seven stars you can see, then record the magnitude of the faintest one you can detect - this is training for variable star estimations too. If possible, repeat this check with another constellation you're familiar with.
2. HOW HIGH ARE 30 DEG ABOVE THE HORIZON ? Sometimes it's not easy determine which is the elevation of a building or a tree blocking the view. You can use your hand to do this: if you stretch your arm and aim it to an object you have a "ruler" allowing you measure angles in the sky. A thumb covers 1/2 degree, a fist 5 degree, one palm between 12 to 15 degrees. So if your buildings are two palms high above the horizon you really have a blockage 30 deg high above the horizon. An improved procedures is performed holding a ruler in hand with your arm stretched: align the zero" of the ruler to the base of the bulding and read the "height" of the building on the ruler. If you know the distance from your eye to the ruler (approximately, the length of your stretched arm) you can use trigonometry to calculate the angle of the triangle whose sides are your arm and the ruler.
3. WHAT INSTRUMENT DO YOU PLAN TO USE FOR THE 10-STAR TUTORIAL ? 10 star tutorials are bright and are intended to be estimated by naked eye or binoculars, a C8 is no good at all for this purpose. Ideally you shold have the variable and the comparison stars in the same field of view and considering a 0.50 focal reducer an 8" SCT you have a field of view around 1.5 degree: in the moment you sleeve the telescope from the variable Betelgeuse to the comparison stars Capella, Pollux or Aldebaran you forget the brightness of Betelgeuse and have to restart from scratch. Even from suburban, light polluted skies a 30 mm binoculars is enough to estimate all of the 10-star tutorials.
4. WHAT "ALTERNATE" TARGETS ? You got some good advice about alternate targets in the above post and those are perfect targets for a C8. But if you want ask for variables as bright as those one included in the 10-star tutorial choices have to be different. First of all you have to consider that at the present moment the only stars you really cannot observe are Betelegeuse and Mu Geminorum because they are very close to the horizon and the Sun at twilight: all the other ones can be see at different times during the night. For example Gamma Cas and both Delta Cep and Mu Cep are not higher than 30 deg above the horizon a couple of hours after sunset, but they should be high enogh to be seen before dawn because they travel around the celestial pole. The constellation of Lyra is well suited to be observed around midnight, so you can consider both Beta Lyr and R Lyr among the 10-star tutorial and you can add XY Lyr too. Hercules is there nearby Lyra and Alpha Herculis ( Alp Her ) is a semiregular variable in the 2.8 - 4.0 magnitude range and close to Alpha Herculis you can find Kappa Ophiuci ( Kap Oph ) a GCAS ( or LB ? I alway forget ) mag 3 to mag 5 variable. If you're lucky enogh to have a gap in the building nearby you could even observe Del Sco, a 1.6 to 2.3 GCAS variable and if you can stand close to the dawn you'll have Eta Aql and R Sct rising from the east. Then you have all the stars around the celestial Pole, expecially those one around the Big Dipper, although they are not as bright as all the 10-star tutorial: Z UMa is very easy to locate and TV CVn or Y CVn too. W Boo is just close to Izar in Bootes and in the same field you have R Boo - now it's close to the minimum, so you need the C8 for this one. OP Her is a bright semiregular close to the Dragon's head.
5 - WHAT CONSTELLATION DO YOU SEE AT A CERTAIN HOUR ? So in order to help you to select suitable targets for visual estimations, as bright as the 10-star tutorial variables, you have to a) decide what binoculars or rich field refractor you want use b) assess what constellations you see above the buildings around you at a certain hour of the night. Once you know your naked eye limiting magnitude, the diameter of binoculars or small telescope, the constellation you can see we can support you about selecting visual alternate targets.
Let's give us some feedback
Giancarlo Gotta
Hey Giancarlo than you for the detailed response. So I went out in my back yard and tried to roughly measure the visible sky. My home is tilted with respect to the coordinates. From my backyard, looking E-SE is my house, so the sky is visible above 45 degrees. To the northeast I have a view above 20 degrees (neighbors fence). To the W-NW it's about 30 degrees (tall trees in the back of my yard). To the W-SW I have a view above 12 degrees (other neighbors fence).
So that means at night I will have a clear view of Polaris, Ursa Major and Minor. I might be able to see Lyra, but Hercules may be blocked by my neighbors house. To the southwest I should have a clear view of Libra and Virgo, but Leo might be blocked.
As for sky limiting magnitude, I would say with the naked eye I could probably see a 3.5 or on a clear night a 4. With the telescope, obviously more.
I don't have binoculars, but I do have an f/6.3 focal reducer. My lowest power eyepiece is a 1.25" 25mm Plossl. I believe that means my TFOV will be 2.24 degrees. Would that work? Alternatively, I have a 6x30 right angle correct image finder. This would give me a 7 degree FOV.
From the stars you listed, it souds like my best bets are Beta Lyr, R Lyr, Z UMa, and perhaps Y CVn. Does that sound about right?
First rule for amateur astronomers is whatever instrumentation you have, it's the best for you. I have been using grandpa 30 mm binoculars for 15 years because he lent me them, you have 30 mm telescope finder and that's fair enough: I never used them for variable star estimation, but I have one spare and I will check holding it by hand as they are a MONOCULAR. I'm curious.
So yes, they are fit for the brighest variable star, but I would add VY UMa to Z UMa because both of them are possibly too faint for 30 mm lenses when they have minimum brightness, so it's either one or the other. TU CVn is fine too. Then add XY Lyr because it's very easy to locate in the same fov of Vega. If you can see nov Lyra then you can see in April Hercules too, so OP Her is good. Given the lower fence obstruction in the W-SW location I would consider Gemini, Cancer, Leo and Virgo when they set, so Mu Gem and then Mekbuda - Zeta Gem and possibly BU Gem are good for the next two weeks and X Cnc and RS Cnc for the next month.
Overhead for the next two months there are both FS Com and W Boo. Cygnus is rising now in the same location previously occupied by Lyra, so P Cyg is there and possibly X Cyg and in August Cepheus with Delta and Mu Cephei, then Cassiopea with V509, Gamma and Rho Cas will be available. If you house does not block the view even R Sct or Y Oph could be suitable targets, but unfortunately it seems you are cut out the more interesting sky section to be observed by lower latitudes such as San Diego one in the Capricorn, Libra, Scorpio, Sagittarius and Hydra.
And then there is the C8 to be used: combined with the finder and the focal reducer it's a terrific instruments for chasing long term variables. They are difficult to locate when faint, but someone is easier than others: R Leo, R Cnc, R Boo, R Cas and T Cas, R And, Chi Cyg, Mira among the other. Then there are recurrent novae such as the previosly quoted T CrB, cataclismic variables such as all the Z And and UG Gem and SS Cyg classes. In order to get the correct chart size by AAVSO please verify your 25 mm Ploss appareent field of view, because the 1.25" verisions have usually an apparent fov around 50 deg and this is not compatible with a 2.3 deg fov and a 1270 mm C8 + focal reducer focal length. Something in the 1.75 deg is more usual.
Now go and estimate. As soon as you spend more time outside you'll record when a specific star or constellation disappears or reappears behind a fence, tree or building or reappear and this will help you to search suitable variables and plan your observations according the period of the year. A free planetarium software such as Stellarium and above all Cartes du Ciel that includes both the GCVS - NSV catalogues and the option to draw the horizon line according azimuth helps a lot.
About DSLR others addressed you, so you're fine. My only suggestion is experimenting with camera on tripod first and try for 200 - 300 mm focals too, under suburban sky this darkens the sky. Piggyback and/or use of devices such as the iOptron or the Polarie will follow and anyway a 70 mm hand held or a 80 mm tripod mounted binoculars is the perfect gap filler between the 30 mm finder and the C8 - and a 50 mm finder would be a finer choice for whatever 8" or larger telescope.
Clear skies.