American Association of Variable Star Observers (AAVSO)
Sat, 02/18/2023 - 16:38

I've only been doing CCD photometry for around 10 months so I'm still on the learning curve.  Based on my reading, I've always thought that guiding was not that critical for differential photometry.  If the stars are slightly out of round, it doesn't matter as long as all the light fits into a reasonably sized aperture.

However, I was just looking at Dennis Conti's "A Practical Guide to Exoplanet Observing" ( and he seems to indicate guiding is critical for the accurate results needed to measure a transit.

Given that we strive for accurate measurements in all aspects of photometry, maybe I should be guiding as well.  Most of my exposures are less than 1 minute and I haven't thought this was necessary, but maybe I'm wrong.   What do people think?

American Association of Variable Star Observers (AAVSO)
Guiding or not

     I presume the idea behind rather precise guiding for exoplanet transits is to reduce to some extent whatever variations there are in some fixed set of pixels from flat-fielding errors and other electronic funnies.  Sort of like imitating a single-channel photometer.  You _would_ have to make sure the little batch of pixels are all well-behaved, not just for the target but also all the comps.  Seems like you'd have to have the detector very well characterized at the pixel level (bad pixel map, really good flats).

     Then the guiding itself would also have to be really good.  Not only the software doing the centroiding of one (or two?) guide star(s), but the mechanical feedback loop would also have to be tight, no slop in the mechanical parts of the telescope.  And how often to make corrections -- once per second (following the image wander?), every 10 seconds or something else?  Inevitably the seeing and/or focus will change during a long run on an exoplanet event, so the batch of pixels involved will vary somewhat.  Finally, the differential refraction, also unavoidable over a long run, will shift the stars at the few-pixel level across the field, even if the guiding on a single guide star is perfect.  

     I was peripherally involved with this paper including a transit of WASP-148:

See Figure 2.  The Lowell 42-inch Hall telescope was run by Tom Polakis.  The left-most plot is from mainly 30-second exposures with no guiding on the telescope.  In my reduction (not used in the paper, but done for quality control), the five comps had rms scatter of about 3 mmag over the full night (no 'detrending' etc), about as good as one can expect especially since the night was not photometric.  Notice in the same plot that three Unistellar telescopes combined did not do very well on this event.


American Association of Variable Star Observers (AAVSO)

Guiding is a good idea, but as Skiff points out, what does "guiding" mean? 

If you have made good darks and flats, bias and thermal noise and pixel-to-pixel variations are largely cancelled. Other techniques such as slight defocus can also reduce the effects of pixel-level variations. If you guide to pixel-level accuracy, you can probably reduce those random errors a little more. Depending on the aperture of your telescope, short exposure times display noise from scintillation. Large apertures suffer far less from scintillation than small apertures.



American Association of Variable Star Observers (AAVSO)
Guiding for Exoplanets


The Manual prepared by the Exoplanet Section folks takes a bit of a different view on guiding. It ranks the best approach to guiding as an ‘On Axis Guider’ followed in descending order of acceptability by ‘Off Axis Guiding’, then Guiding with ‘separate guidescope’, and finally, by ‘no guiding’ at all. 

It’s fascinating, and refreshing, that folks at the pinnacle of the AAVSO’s photometric expertise, might still differ in their assessment of the best practices in acquiring data suitable for scientific photometric purposes. It does leave those of us, me anyway, nearer to the bottom of the expertise parade, with some uncertainty in choosing our level of investments in pursuit of good data. 

After investing in quality mounts and optics, do we carry on by investing in relatively expensive On Axis Guiding equipment or go with more conventional ( on no ) guiding gear and focus instead, on upgrading our preparation of calibration frames - flat frames in particular. Which approach will work best to minimize the systematics associated with our Target and comparison Stars wandering far and wide across our sensors? Has anyone studied the outcomes of different gear and approaches? Ultimately, we all want to actually provide the pros with high quality data - not just think that we have. 



American Association of Variable Star Observers (AAVSO)



I SUSPECT the exoplanet manual suggests guiding because they are on an object for hours and to minimize errors (and see smaller dimmings!) they want to keep the object on as small a group of pixels as possible.

General stellar photometry generally requires far fewer exposures ( a few tens instead of hundreds or thousands) over only a few minutes, so unguided but well polar aligned scopes are sufficient.

I have used at least 6 AAVSONet telescopes to capture 18.8K variable star images over 4 years, all without guiding, and have lost no images due to poor tracking.

Using the ATMoB 17" Planewave for both photometric and just a few exoplanet runs, all without guiding, I have yet to lose an image due to guiding, except when a scope bearing had issues (rodents in them!).I even imaged V1 Andromeda at 20th mag successfully without guiding.

While guiding is undoubtedly a good thing, it seems to me not to be a requirement. As a retired Project Manager, I learned a long time ago that "Perfect is the Enemy of the Good Enough".

Use what ya got, no matter what the manuals state! If you need something else, you'll soon find that out. Otherwise, stay with your present setup.

Just my opinion, and I may be wrong!



American Association of Variable Star Observers (AAVSO)
try it

All good advice here. Seeing exoplanets is relatively easy. I don't guide, but I have a good mount and software that corrects pointing between every exposure. If your mount and drive software will keep the star centered within a few pixels, you won't need to guide. 

I also try to pick comps that are close to the exoplanet on the image, so that exoplanet star and comps are all in the middle of the image. That helps to reduce the jog in the plot when you do a meridian flip. Obviously, take exoplanet data on clear nights. Any cirrus makes a poor data run. I use a V filter to avoid the near-IR clouds. My ST8 can't see mid-IR clouds.

But the thing that worked best for me was to just try it. Start with a star listed at Swarthmore that has 10 to 20 mmag of eclipse depth; easy to see with a little care.

If no transits are handy for you, take data for two or three hours on ANY variable and it's comps and see how well the tracking works with your telescope and software. If tracking is good, you can plot the comps to see how stable they are and get a rough stability baseline for your equipment.

Once you do a few, you will be your own expert on your equipment's capabilities. 

Upcoming transits (


American Association of Variable Star Observers (AAVSO)
Thanks everyone for the…

Thanks everyone for the thoughtful comments.  My take on this is that guiding has a benefit but you probably won't notice it until your technique and the rest of your equipment is up to snuff.

I have a Takahashi 100mm refractor on a Losmandy G8/11 mid-range mount.  I don't have a permanent setup and need to go though the polar alignment routine each time I'm out.  I'm also on the lee side of the Rocky Mountains so seeing is often mediocre.

Right now my standard deviations on a set of "identical" observations are around 0.02 mags.  I'm a long way from the milli-mag observations some folks talk about.

So I will probably hold off on guiding for now unless someone can convince me it will make a significant dent in my errors.


American Association of Variable Star Observers (AAVSO)
Milli-mag transits only


My experience indicates that guiding to about a single pixel does help if you are looking for millimag transits. If they are greater than about 10 millimag, you will see them without guiding and I'm often a little luckier. It is helpful if you keep your flats up to date. I personally take flats every night (morning twilight) and use a running median master flat (e.g., 7 flats per filter per night x 3 nights) for calibration.

For other photometry, I do not guide my Paramount in a fixed roll-off roof observatory. I observe similar precision (std~0.02 mag) for my replicate images. I consider that 'normal' and don't worry about it any more!

HTH, Ken 

American Association of Variable Star Observers (AAVSO)
The benefits of guiding for exoplanet transits

From my experience with exoplanet transit observing, either having a very well polar aligned mount that keeps the target star on the same region or groups of pixels of the image or doing the same with on/off axis guiding, the resultant light curves show less systematics.  The baseline light curve pre ingress and post egress have more of a chance to be "flat" whereas the differential magnitudes are similar from the start of the observation to the end several hours later, assuming the effects of airmass change is not a problem.  Transit light curves with less systematics are easier to use to calculate with more certainty exoplanet transit characteristics sought by ExoClock, Exoplanet Watch, and TESS SG1 follow-up programs, especially for the more challenging shallower transits.  The error of the photometry measurement may or may not benefit from guiding.