18. Guidelines for visiting observers

Some advice from somebody who has a long record of observing, of supporting visiting astronomers, of doing service observations for others, and of seeing programmes fail and time wasted because people didn’t listen or didn’t care.

I have seen it all! At least I hope so. Well, thinking about it twice, I think I haven’t. Most of this stuff is really avoidable, but it happens again and again. If only one programme is saved because you as an observer read this, then it was worth it! All observatories will thank you! And I will thank you too, because I have to answer less questions about data reduction ;-)

18.1. Planning

Do plan your observations well in advance! You will save time at the telescope, be more efficient, and come home with data that can actually be calibrated! You’ll be happy! In addition, you will make your support astronomer and/or the staff executing your observations happy, and you will save the telescope operator lots of nerves.

  • Use the exposure time calculators. Even though some are infamous, they’ll put you in the right ballpark, and some of them actually work pretty well!
  • Make visibility plots of your targets such that you can observe them efficiently at night when they are high on the sky, and not when they are 10 degrees above the horizon making the telescope, the telescope operator and the support astronomer ache. Use Chris Benn’s excellent Staralt tool.
  • Think about how large your dither pattern has to be. Don’t think about NOT dithering; there are hardly any science cases for that. Hmm, thinking about it twice, I think there aren’t any at all!
  • If you need standard stars, select them in advance. Keep in mind that for spectroscopy the spectral type of your standard star can be essential for telluric correction. If you have to dig through long lists at 3 a.m. in the morning to locate a suitable standard next to your target you’re not doing anybody a favour, at least not your science case. Remember to observe standard star fields at significantly different airmasses, otherwise no extinction correction can be performed.
  • Make sure the filters/grisms/prisms/slits you need are actually present at the telescope or in the instrument. If the observatory staff needs longer lead times to set up the instrument, then tell them well in advance. Actually, make sure the instrument you want to use still exists at the telescope! I’m not kidding, I had one observer who wanted several hours of the night to be executed with another instrument, but never mentioned this at any point anywhere before: O: “Could we please switch to XX in Nasmyth focus for the rest of the night? Then I could also do this and that…” SA: “Errrh, XX was decommissioned two years ago and replaced by YY, which is only operated at Cassegrain focus and back on sky in three months. May I suggest a service proposal?”. Another one applied for an instrument that was actually mounted at a telescope in Australia instead of in Spain; this was flagged in due time though before entering the time allocation process.
  • Check when your observations are taking place, and plan your travel in advance. Arrive early! Observatories are usually in remote locations and in case of bad weather they can be difficult to access.
  • Make sure you know the actual overheads of observations!
  • Have a backup plan if transparency or seeing are not as good as you may want.
  • Read the instrument manual and the observatory’s guidelines for observers in advance. If possible, familiarise yourself with data reduction as this can reveal better observing strategies.
  • Check the moon distance, and the lunar illumination. You can’t do any deep observations in blue filters in bright moonlight.

18.2. Substitute observers

If you cannot observe yourself and you have to send somebody else, make sure this person knows how to observe, and arrange with staff at the telescope to ensure proper support.

Ideally, make sure that your observer has a scientific interest that the data taken is being useful. If you send unmotivated observers, they might go to bed at 4 a.m. wasting the several good hours before sunrise (which the observatory staff will then likely use for their own purposes). Or they observe an entire night with a totally defocused telescope, which HAS happened on a 2.5m telescope where the observers are by themselves, unsupervised by a telescope operator or support astronomer.

18.3. Dithering

When planning your observations, make sure that you dither your exposures (apply small telescope offsets after each image). If you observe with a multi-chip camera, make sure that the dither pattern is wide enough to yield a sufficient overlap between neighbouring CCDs, and does not just barely cover the gaps between the CCDs. This helps astrometry and photometry enormously. THELI was designed to handle data with large dither patterns, so make use of it. Inhomogeneous effective exposure times in the coadded image can properly be taken into account using weight maps.

Remember: No dithering,

  • no defringing
  • no superflatting
  • no background modelling
  • no correction for chip defects
  • no removal of certain instrumental signatures
  • no good signal-to-noise
  • no good astrometric distortion correction
  • no filled inter-chip gaps
  • no good science. At least not as good as it could be.

18.4. Background models

If you observe empty fields and plan to superflat them or subtract a background model, then make sure that the dither pattern you apply is larger than the largest object in the field of view. Should this be unfeasible because you observed an extended target, then calculate the superflat from different pointings obtained in the same filter in the same night. If you know that you have to superflat your data or to create suitable background models, then there is no way around blank field observations.

If you observed very extended targets (comparable to or larger than your dither pattern), then you cannot calculate a superflat or sky background model from these data. In this case you must observe a neighbouring blank SKY field Take 5-10 well-dithered images, exposure times can be shorter than for the main target as long as the background is recorded sufficiently well. Near-IR observers (hopefully) know all of this, but optical observers often don’t. The overhead for these blank fields can be significant, but it’s better to have somewhat shallower but well-calibrated data than a worthless pile of pixels.

THELI has full built-in support for SKY fields which are reduced automatically with the main stream of data and readily applied.

18.5. Extended low surface brightness objects

If you are observing extended very low surface brightness objects, visible or invisible in single exposures, then you must take great care in all background modelling and sky-subtraction steps. Choose a very wide dither pattern that is at least twice as large as an optimistic estimate of the extent of your target. If this is unfeasible, use a sufficiently large number of blank field exposures (at least 5-10).

Otherwise you cannot calculate a superflat or background model from your data that does not have a low-level imprint of your target. The same holds for the sky subtraction. If you choose to go the standard THELI way and model the sky background for each image, then choose very low detection thresholds and a smoothing scale that is much larger than the extent of your object. If you feel that this is too dangerous, then obtain a constant sky estimate using the methods offered in the sky-subtraction task.

18.6. DO THIS

  • If you know in advance that there aren’t any targets during some hours at night, then let the observatory staff know about it. They can slot in some service proposals and thus reduce the pressure on their queues. Or they’ll use it for their own programmes and who knows, maybe you become co-author and famous in return for sharing your time. Same if observing conditions get unsuitable for your programme. Most likely there are several programmes in the service queue that can make use of bad seeing and/or bad transparency.
  • Show up in time for the introduction at the telescope, even if you are an experienced observer. Things might have changed compared to last time when you visited, and there is almost certainly something which you forgot. A good support support astronomer will also spend some time to discuss and refine your observing strategy.
  • Take the time to fill out the online feedback forms or fault reports. It may seem tedious to you, but any detail you can think of, positive or negative, could be of great help to improve services. If nobody complains about anything, operations will assume that things are alright, and nothing will change. A lot of issues can be resolved the very next day and you have a better working telescope/instrument the following night.
  • Try to sleep! Not at night, but during the day. If you pass out at 2 a.m. then you are not good for anything, not to mention for your science. Not all observatories offer full-time presence of a support astronomer who checks if whatever the tired observer is doing still makes sense (and support astronomers often have other duties at night as well). If you can’t sleep because the shutters in your room are bad or because the air is too dry, ask for a different room or a humidifier. If you suffer from something, then most likely the observatory staff does as well, and thus there are solutions for it (different food, medication, a boxing sack for stress management or physical work-out).
  • If you know you are getting car sick, bring medication. Access to some telescopes is only possible over steep or bad roads with many turns. Same for altitude sickness. Most observatories are between 2000m and 3000m altitude, which is high, but not so high that you wouldn’t get adjusted within a day. Show up a day earlier to allow your body to adjust itself. Observatories at higher mountains will have specific regulations that are enforced. You don’t do anybody a favour if you are sick in bed. Sun-blocker, lipsticks, mosquito repellant, etc are good ideas, and drinking a lot of water, too.
  • Stick to the safety regulations. A broken leg because you didn’t watch your tired steps at night will cause you surprisingly many painful hours before you reach a hospital, and it might not be a hospital you want to be treated in.
  • Telescopes are complex systems and have a small fraction of technical downtime. If that happens to you at night, then relax, because there is nothing you can do (unless it is your visiting instrument). In almost all cases staff has seen that particular failure before and knows how to fix it. It may take some time. Meanwhile, re-evaluate your observing strategy so that you can resume observations, possibly of a new target, when the telescope becomes available again.
  • Over-calibrate your data, don’t under-calibrate it. Take plenty of biases and flats and darks, you never know what they might be good for until you start reducing the data. If you observed extended targets and there is some dead time until the next object is high enough, observe some blank fields or standards.


Any of this will most likely make you end up on some black list. Not that I know of any, but memories are long. The following is no nonsense I’m making up but some of it actually happens quite regularly, and it is by far not a complete list.

  • Do not call the support astronomer four hours before (past) sunset asking (complaining) when (why) your service data will be (hasn’t been) taken, if you were supposed to actually show up yourself and observe yourself, instead of sitting 5000 kms away at home in your comfty warm office/beach/pool. If your support astronomer is really kind he’ll do the observations for you, but most likely your night will be used for something else.
  • Don’t tell observatory staff at dinner time that instead of what you told them earlier in the afternoon you actually do need a lengthy reconfiguration of the instrument. Chances are that staff qualified for doing instrument modifications has left the mountain already. Read the planning section above.
  • Don’t freak out while taking sky flatfields in 4 four different polarisation angles at the same time. The twilight sky is highly polarised, and the fact that the polarisation channels have very different counts (or none at all) does NOT mean that the instrument is broken! Actually, it shows that it is working perfectly fine!
  • If your boss sent you to do some observations for a long-term project or survey, and almost all scripts are automated, don’t just lean back with a beer and watch movies. Do check the data the telescope returns. And do not write a lengthy fault report the next day complaining that the image quality is crap, because what happened in reality was that you never bothered to focus the telescope. Actually, your images were so badly defocussed that the black shadow of the secondary showed up, and you didn’t recognise it all night long. That makes other people who could have used that dark time with 0.5 arcsec seeing angry. I mean really, really angry. And your boss too, by the way. Staff works hard to make everything work perfectly right for you. Don’t flush their efforts down the drain pipe. A night at a medium-class telescope is easily worth EUR 10,000, and with 8m class telescopes we speak EUR 100,000.
  • Do not show up two hours after sunset and then complain why no flat fields were taken for you if this was actually your responsibility. The support astronomer might have taken some for you, but he or she does not necessarily know all the filters you need or the illumination level you want.
  • Do not show up at the telescope at night without a proper plan what to observe first. It’s generally a very bad idea to have the support astronomer figure out from your proposal which targets are visible at the moment and should be observed first/next/etc. It’s your time that is ticking away, and people might have other important things to do, for example health-checking that other instrument you are going to use later at night.
  • Don’t complain about bad reflections in your data when your target is just 10 degrees away from the full moon. Certainly not all time allocation committes will give you the nights you want, but please try and make the best out of it. Maybe that target can be replaced by another one?
  • Don’t attempt to observe a target at -50 degrees declination at an observatory at +30 degrees latitude. The Earth is spherical and not everything is visible from everywhere.

Please bear with me, but I just had to write this down. In this sense… happy observing everybody! :-)