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Is it Possible to Use "Web Based" Telescopes?

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Ccolvin968's picture
Is it Possible to Use "Web Based" Telescopes?

Good evening.

I currently use an online telescope service (Slooh) to measure and report positions of Near Earth Objects/Near Earth Asteroids.

I know all of the details about the CCD's and telescopes, but am wondering if it would be at all possible to use these telescopes for Photometry?

Are there people who already do this and if so what is your recommended approach?


Thank you for your time.

Bikeman's picture

Sure! Not to promote but as an example, is used a lot for photometry, I 've done so myself.

Some things to keep in mind:

a) photometric Filters: many (but not all) of the telescopes at have photometric filters, at least B and V. You can do photometry with "pretty-picture" RGB filters as well (see the DSLR observer guide) but it takes a bit more effort.

b) Calibrateted (but otherwise unprocessed!!) FITS files is what you want in the end. will do the calibration of your images for you and you will get FITS files, because as you'll probably know, JPEG files or image files that have extra processing applied to make them look "pretty" (noise reduction, histogram stretching,...) are more-or-less  worthless for photometry

I think any online telescope service that meets these two criteria can be used for photometry.



Ccolvin968's picture


That's good to know.

Slooh telescopes give out FITS files no problem, and there are a few different filters that can be used.

I'm honestly not as into the "pretty pictures" when it comes to this stuff. Regular AP is another hobby of mine, but this would be purely scientific.

I'll give the CCD imaging guide a read and see if I can figure out how to do it from there. Slooh is much cheaper than iTelescope from what I can tell. $25/mo gives you between 25 and 50 minutes per night for an unlimited number of days in the month depending on the telescope you select. 

Appreciate your reply!

Ccolvin968's picture
Upon digging further, Slooh

Upon digging further, Slooh will not work for photometry.

All of their processing options are pre set and they stretch and color merge all the objects.

Looks like I'll have to consider itelescope! 

Thanks again!

Slooh telescopes

I've been using the Slooh telescopes for over 6 months now (I'm DPOA; check out my AAVSO work), and I believe some of them are in fact useful for Photometry.  Their processed PNG images are of course useless for photometry, but if you are an "Astronomer" member you will have access to un-processed FITS images on the day following your exposure. 

I use mainly the Slooh Chile (Widefield "C1") instrument.  All their telescopes have significant limitations though:

1) There are no Photometric filters available.  I request the "generic" exposure option, which provides LRGB filters.  I use only data from the "G" filter, since this filter is very similar to the photometric "V" filter, and report it as "TG" to AAVSO.  I consider this application to be preferable to using an RGB color CMOS sensor (as in a DSLR), since the resolution and sensitivity are better.  "R" and "B" filters could also be used, but are less comparable to the corresponding photometric options. I don't consider the L filter to be useful for photometry at all.

2) Slooh applies a fixed 25 second exposure to all exposures using the R, G, and B filters; this effectively limits the useful magnitude range to between 9 and 13.  It's possible to go deeper with a 25 second exposure, but only a poor SNR will be obtained.  Slooh images are typically binned 3x3 (also not under your control), so they are quite sensitive (but often saturate on magnitudes brighter than 9).

3) Slooh calibrates the FITS files automatically, but full calibration is limited to the Chile "C1" and Canary "T1" instruments: the other instruments lack adequate flat field calibration.  

4) Slooh's FITS headers are incomplete for use with Vphot and possibly other processing programs as well, but Vphot allows you to supply the missing data.  It's much more time consuming than processing data from iTelescope though. Note; iTelescope data is much faster to process in Vphot since it can be transferred automatically.

In comparing my data with that posted on AAVSO by others using high-quality "V" photometric filters and good calibration, I find that my data is generally within 0.1 magnitude and consistent within at least 0.05 magnitude on common LPV's between 9 and 13 magnitude.

I believe that Slooh offers an economical means of doing photometry, and I especially appreciate their exellent sites (as compared to my cloudy, hazy, light-polluted backyard).  I've also used iTelescope with good results, but that's much more expensive than using Slooh.  

I like Slooh; its focus is mainly on the "pretty picture" and educational aspects of astronomy, but it's a good way of promoting Photometry.  It's affordable for those of us with modest resources.  There are many forums that are both serious and educational in nature, and it's my perception that there are many high school age members who are potentially serious astronomers.

Ccolvin968's picture

Which processing option for Slooh is the G only?

For asteroid astrometry we use either the Luminence 20 or 50 depending on the target. I'm already an "astronomer" on Slooh, so like I noted above in my initial post, I have access to the FITS.

I just want to be sure I'm understanding you correctly as well... We can only relaibly and effectively perform photometry on stars between mag 9 and 13?

What is the missing data from the FITS files?

Thanks a bunch for your reply!

Using Slooh

I use the "generic" processing option for my photometry LRGB missions, but any others that get you a color image will work: the processing options mainly guide Slooh's automatic processing system, which is not employed for FITS files.  NOTE: luminance-only missions are mainly used for dim or fast-moving options, and will not get you any RGB exposures.  When you download your FITS files, you will see the "g" in the file name: that is the Green frame.

Aside from the Luminance 20 and 50 options, you will have no control over the exposure: as far as I have been able to determine, the G frame is always 25 seconds at 3x3 binning.  Depending upon the spectral characteristics of the star (and check star, and comp stars) you are interested in, the camera sensor will generally saturate or become non-linear at magnitudes between 8 and 9 and below.  Again depending upon spectral characteristics, the SNR will drop below 50 around magnitude 13.  This is for the Chile C1 instrument which I use most frequently; the others may be somewhat more or less sensitive.  You can of course report low-SNR observations or stack many images if you need to measure a dim star; I have reported magnitudes of 15 or more on a few occasions for some of the LPVs.  VPhot allows you to stack as many images as you specify, but it will often not automatically identify a star that has an SNR below 3, and the uncertainty can become quite large at low SNRs.

VPhot requires the following data to be in the FITS header: Object name, RA, DEC, and Filter;  Other than the filter (which is given as GREEN if you're using that frame, but which VPhot does not accept) Slooh does not provide this information, so VPhot will prompt you to provide that information manually as part of the upload process.  As I mentioned before, I direct VPhot to process the Green frame as "V", and then report it to AAVSO as "TG" (tricolor green, which VPhot also does not accept, but AAVSO does).  This can be a bit time-consuming, but you don't have to use VPhot if you have a more suitable process.

As you know Slooh allows only 5 reservations at a time, but on some nights I've been able to get well over 30 missions when not too many other users are active on a particular telescope, by staying up all night and adding reservations 5 at a time as the previous ones have completed.

Bikeman's picture

I had not tried Slooh before, but from what I read here, if you want to spend money on the order of 2US$ per minute of exposure time, might be the better choice for photometry.

You can easily see that the services at were actually designed with (also) photometry in mind:

  • AAVSO members can actually have their images automatically forwarded to VPHOT for processing (!), although I haven't tried this yet. I prefer my own processing
  • There are options to intentionally defocus if you find the focus is too sharp and you are struggling with saturation issues (usually not needed because the atmosphere will blur the PSF enough for you , but the fact that the option is there shows that those folks care for "our" needs)
  • Photometric filters are installed on many of the more interesting telescopes

I really don't want to start a sales-rant for here but I think it might be worth explaining some aspects that (at least to me initially) were not obvious about


  • Monthly fee: Actually there is no such thing as a "membership fee" as opposed to usage fee. All your payments are usage fees payed in advance, and the different "membership plans" differ in the amount of advance payment you commit to pay every month. The higher your monthly advance payment, the lower are the rates per minute exposure time.
  • Here's an example. I pay 40US$ per month (actually every 28 days), so my usage rate to use (say)  this telecope in Australia currently is 127US$/hour so about 2US$ per minute of exposure time  
  • If I don't want to observe 40$ worth of exposure in a billing period, the remaining balance can be used in the following months (no expiration limit), so no single $ of your monthly payment gets wasted if you don't observe for some time
  • If you want to make observations that will cost more than your current balance (the monthly fee for that month and whatever you carried over from previous months), you can of course buy more exposure time on demand
  • Important: you only pay for the exposure time, not the reservation time! Because of "overhead" tasks like slewing, filter changes, focusing, dithering etc etc, you will usually have to reserve a time slot that is about 1.5 x your exposure time. For exposures that are at least 60s longs, only the exposure time is charged, not the time for all those overhead tasks.
  • Exception: For exposures less than 60s long, they will be charged as if they had been 60s, but with some capping so that the charged time can still never exceed the reservation time for short exposures (planetary imaging, fast cadence time-series,...)
  • There are discounts when the moon is up, depending on it's phase
  • If you are unsatisfied with an exposure for a good reason (clouds, tracking problems), it's super-easy to claim a refund.


The way I personally use is that for me it's too expensive to replace my own backyard telescope entirely, but I use the monthly payment as a "piggy bank" to accumulate observing time, and whenever I feel that I really badly want to have some data points for a target but can't do it with my own telescope (too faint, bad weather, wrong hemisphere of the sky...), I use



Ccolvin968's picture
Good to know!

The costs of iTelescope are why I want to see if I can make something work on Slooh. It looks like one of the members above has some success. 

While Slooh would be much more limited in what it's capabilities are in terms of photometry, the cost is significantly cheaper than iTelescope. $25/mo at a "best" of 50 minutes per night assuming only 3 nights per week still comes out to 200 minutes of exposure.

I'll defintely have to give iTelescope at least a try before I say I don't want to use it. 

Thanks so much for your help!

HQA's picture

If you want a cheaper solution, AAVSOnet is available tree to all members.  You have to submit a very brief proposal of saying what you want to observe and why, but once accepted, you get your data about 24hrs after the observations are taken.  Images are either uploaded automatically to VPHOT or left on an ftp site for download.

I think iTelescope is great and does a wonderful service.  However, I'd also like to see people use AAVSOnet more, and maybe contribute that $2/minute to the AAVSOnet Fund to make it even better!


Ccolvin968's picture

So if there was a list of objects that I determined needed an observation I could get it?

Is the proposal a one time deal or once submitted or would I need to make a new request for each object?

Ccolvin968's picture
First Proposal

Just submitted my first proposal! :)


I can only comment about iTelescope facilities. We carried out a three year research project using their wide-field telescopes (former(?) T4 and T5) that had BVRI filters. At the end of the main project about 4 years ago, I have used their 40+-cm-class telescopes from time to time.

While there are many aspects that are convenient (described here, too), there are few issues that can be problematic from high-quality photometry point of view:

  1. Calibration files - their flat frames are done so seldom that they are only useful for pixel-to-pixel variation correction. And some seasons are (very?) dusty at New Mexico...
  2. Stray light (specially in wide-field telescopes) - IMHO a research project with rental telescope should start with estimates of that. We saw as much as 10% (and even more) of it, correcting for stray light is definitely a very problematic area, it would be much better to just avoid it.
  3. Occasional issues with hardware and software combination (wrong filters, tilted camera etc) - but such things can happen everywhere, just keep your eye on the data gathering process (read: analyze your data immediately)

Using their more expensive monthly subscribtions, price per minute is not very bad, fortunately. Especially, if you manage to observe your targets during bright Moon periods, telescope pointing well away from Moon direction.

Best wishes,

myronwasiuta's picture
free online telescopes

I saw this thread and want to let you know of another possibility for your online observing. I am part of a small group of amateur astronomers that opertae the Mark Slade Remote Observatory in Wilderness, Virginia. We offer time on out telescopes for free to individuals and organizations, and are currently organizing as a 501c3 non-profit charitable organization. We realize that there are many online telescope options-many of which can be expensive. Our hope is to provide telescope time for free to memebers of the astronomy community,  eventually with funding from grants and private donations. At this time however, we are providing telescope time for free without these sources. We have two stations-one with an Explore Scientific 165mm APO on G11 Losmandy PMC-Eight Mount. Piggybacked on this is an ES 102mm FCD 100 APO. The 165 has a QHY 163C camera and filter wheel with V-Band.  The 102mm APO has a QHY 174M camera. This station has been and is being used for exoplanet transit work as well. Our other station is a 12-inch Meade LX-200 with a QHY 174GPS and filter wheel with RGB,spectral grating (200lines/mm) . Plans are to get a v band in this filter wheel soon. Piggybacked on the 12-inch is an ES 80mm FCD 100 APO refractor with QHY 178C camera. If you think our telescopes and equipment might serve your needs-please feel free to contact me. We will do what we can to assist you in your observations. I can be reached at, or by cell at 540-388-8209

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