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DSLR Cameras for Photometry

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ZPA
ZPA's picture
DSLR Cameras for Photometry

Hello all! 

I realize this is a relatively simple question, but for those of you who regularly use DSLR cameras for photometry, which brand or type of camera have you had the most success with? I'm going to try to purchase one soon for all types of photography, but I would also like to try my hand at variable star work with one in the future. 

Thanks in advance! Any suggestions are greatly appreciated. 

ZPA

Bernhard
Bernhard's picture
Hi i use a Canon 200 D,

Hi,

I use a Canon 200 D, because it was in a bundle and the internet forums are saying that canon has a slightly better software support in the world of astronomy (...at that time..but maybe decide that point for yourself).

You need: big pixels, big QE, no removed IR Filter (good for nebulas, bad for photometry)

take a look at:

https://astrophotography.app/EOS.php

https://astrophotography.app/nikon.php

My Canon has:

24 Mega Pixel (less is better! because Muniwin has to convert this to fits format and this is taking time...)
Pixel pitch: 3,7 µm (bigger is better up to 9µm)
Pixel dense  7,3 MP/cm²  (??)
14 Bit, (better is 16 Bit)
Crop Sensor 1.6 times
Sensor Resolution: 6026 x4017
Sensor: 22,3 x 14,9 mm
Save in RAW Format! Shut off all automatic noise, reduction, csensor leaning, whatever! & defokuss

A flip touch screen (important for your nec (-;)
Without Mount: Objektiv 18-55mm you can shoot 10-15s small startrails are no big problem

Mag Error with the Digic 7 Processor: ~ 50 - 100 mmag, with darks + flats + ensemble Photometry: 3-10mmag !!!
Price (2018): < 500€

The first 14 Bit Canon was the EOS 450D. Before of the 450D, there were only 12 Bit if you go for used cameras. So avoid 12 Bit cameras.

 

Ah and i had to update my telesopes OAZ to 2 inch. Otherwise with a 1,25 inch OAZ the DSLR cannot be fixed accurately (-->  problems)

 

Also take a look at: https://astrobackyard.com/astro-photography-tool/

 

wbea

 

 

HBB
HBB's picture
DSLR Camera for Photometry

I use a Canon 40D and a Canon 70D. You can actually use any DSLR for photometry and it is ok to not have one with a lot of settings since a lot of these settings will be turned off for photometry.  As long as it produces a RAW image then it should be ok for photometry. It would be a good idea to take a look at the DSLR photometry manual that is available under the Observing tab---observing manuals on this website.

 

 

 

 

Barbara

DEY_VAR
DSLR Cameras for Photometry

Greetings,

You don't need big pixels--the Pentax's have 3.89 micron pixels. They are both APS-C size sensors.  My experience with full-frame cameras (Canon 6D 6.54 micron pixels) are few telescopes have a flat-field over that sensor size so coma and vignetting becomes a problem even with good flat-field lenses installed.  You don't need big QE.

You do need to run the camera at low ISO.  For the two Pentax cameras ISO 200 gives about 1e/ADU which is good.  You do perhaps need to try to pick a camera with pixel sizes that match you camera lens or telescope you will be using it with.  I use short focal length lenses (300mm) and telescopes (550 to 952 mm) so small pixels are better for sampling the star light with more pixels.  You want 14-bit analog to digital conversion  

Having a flip-out view screen is worth it when focusing, checking tracking, checking for satellite/aircraft trails), etc.  The K-3II & Kp have the same sensor but the Kp has the flip-out view screen which makes life easier and what I currently use.

A worthy add-on is a good intervelometer--I use a wi-fi version.

Jim (DEY)

DEY_VAR
DSLR Cameras for Photometry: addendum

The first sentence got cut-off somehow... my cut-and-paste bad!

The two Pentax cameras I have are the K-3II and Kp.

Jim (DEY)

SFS
Mine's for sale

DEY_VAR is correct:  you do not need big pixels.  In fact, smaller pixels are advantageous, particularly in crowded fields.  The reason for this is because in DSLR photometry, it is necessary to defocus so as to ensure that the light from the target, comp stars and check star are spread over multiple pixels. 

That said, I have been using a Canon D600, aka EOS Rebel 3Ti, an 18 Mpx camera exclusively for DSLR photometry.  The camera body is in like new condition, and is for sale for $165 including shipping to anywhere in the US lower 48.  It comes with everything I received when I bought it, all but the camera body and battery unused.  I also have a copy of Hendon & Kaitchuck,Astronomical Photometry  in excellent condition for $55, again including shipping.  Add $10 to the price for the book and I'll throw in my copy of the AAVSO Viariable Star Atlas, 2nd edition.

These offers expire Friday, May 22, 2020 at midnight GMT.

SFS
Prompted by Heinz-Bernd,

Prompted by Heinz-Bernd, below, here are the shutter data for my camera:

Actuations:  11 236

Rated shtter life:  100 000

% rated life remaining:  88.76%

Bikeman
Bikeman's picture
Just to show this is not a

Just to show this is not a moot point, my own EOS 1100D, used heacvily mostly for photometry, has 80k actuations already :-(. So again, buy Stephens DSLR, not mine :-)

Bernhard
Bernhard's picture
Hi all,

Hi all,

as far as i know is QE: (Quantum Efficiency)

https://en.wikipedia.org/wiki/Quantum_efficiency

how many photons a ccd or cmos sensor can collect over a period of time. So the bigger QE is, the better. You can collect more photons of a faint star, within the same time period as with a sensor which has only 30% QE. It is more a question of pricing. A sensor with 85% QE is more expensive.. (But if i can have a sensor with more QE, i will go for it..)

 

Pixel size:

Hmm... afaik, the bigger the pixel is, the more photons it can collect until it gets saturated.

Of course i can do photometry wit my 3,7 microns pixel size. But a bigger pixel upt o 9 microns can collect more photons. Maybe one has to make a compromise, if observation is going to crowded fields (which i do not observe yet..)

Also the dynamic range is important. A high dynamic range, can collect photons of all compare and check stars if one observes a star with big magnitudes.

Example: variable is 7 mag vis, compare or check stars are 12 mag vis (an extreme example (-;) So now you are collecting photons. If 7th mag vis variable star has e.g. 75% collected photons of its max ADU (14 BIT = 16 381 ADU) lets say ~12 000 ADU, your check + compare stars maybe still have less photons collected. While the 7 th mag star is beeing saturated, you will still collect photons of the compare stars. And this is leading to false results.

Therefore for photometry, a bigger pixel size is better. (As far is i know..)

But there are more than one practical exercises, where different things, are resulting in different settings.

regrards wbea

 

SFS
As I said, with a DSLR it is

As I said, with a DSLR it is necessary to defocus (unless the Beyer array has been removed), and even then some sort of binning is generally necessary to ensure that the Airy discs of the target, comparison star(s) and check star are all included.  So if as is quite often the case there is a field star within some 10s of arc seconds of any of the stars, chances are much better of it being inadvertently included in a measurement aperture if the pixels are big than if they are small.

It took me a while to realize this, because it was not taught to me.  One of the traps one falls into with less than rigorous training is the illusion that things are simpler than they are, that what you don't know won't hurt you.  (No political context intended.)

Bikeman
Bikeman's picture
In addition to the many

In addition to the many important things mentioned already (so I won't repeat them) concerning the sensor, there are some useful points to keep in mind concerning the rest of the DSLR package:

 

Firmware compatible with photometry (true raw images): If you Google "star eater problem Sony" you'll read all about a feature in some of Sony's DSLR firmware revisions that will perform some pre-processing even on RAW (!) images, without the user being able to switch that off. For photometry and even astrophotography in general this was a real problem. You don't want to have a firmware that does this.

Computer interface options: Well, actually the great advantage of a DSLR for photometry, especially for beginners, is that you do NOT need to hook it up to a computer (or external power, external storage  etc). However, if you anyway have a computer hooked up to your telescope hardware already (for guiding or mount control etc), you might want to use something like the AstroPhotographyTool (APT)  [1], which is great for many things like focusing, image sequencing, storage, polar alignment .... you name it...via a USB connection to your DSLR. But the number of supported features differs with the DSLR model and maybe you want to make sure you pick a model that is well supported. See the link below for a feature/compatibility matrix.

 

Shutter wear: the mechanical shutters of DSLRs have a certain live expectancy and repairs are expensive to the point of being prohibitive (depending on model), so when buying a used DSLR you'll want to make sure the shutter count isn't excessively high (e.g. you shouldn't buy my DSLR, I guess) Google for shutter live expectancy for the model you have in mind buying used and ways to display the shutter actuation count.

Flange distance and back focus/vintage lens compatibility: Different DSLR systems have different distances between the mount plate and the focus plane, so if your telescope setup has little back focus left, you want to be sure you actually can get into focus at all with a given DSLR [2]. An interesting aspect of this is the ability to adapt used, vintage telephoto lenses (which are often quite cheap relative to their light collectiing capability), to your modern DSLR for wide field photometry. That only works if the flange distance for the vintage lens is not smaller than that for the DSLR (you just insert a cheap adapter ring to bridge the difference). E.g. Canon DSLRs or ThreeThirds DSLRs can be adapted to work with old Pentax-K telephoto lenses easily this way, but Nikon's flange distance is too big to make the adpater ring solution work for Pentax-K lenses, as an example.

 

[1] https://astrophotography.app/

[2] https://en.wikipedia.org/wiki/Flange_focal_distance

 

GaryH
For some reason I believe we

For some reason I believe we need an image scale of at least 2 arcsec/pixel. Perhaps something I read or was taught, don't recall. The image sacale is a function of focal length, pixel size, etc. So something to consider when deciding on what camera to buy is the optical system (telescope or telephoto lens)you want to use for photometry. I use a free program which can be downloaded off the internet called Ron Wodaski's CCD Calculator. This program will calculate the image scale and field of view for any camera and telescope system. There are some brands already installed. If you do not find the camera/telescope there it allows you to enter them (pixel size array size, telescope ot telephoto focal length etc.). This does not require a picture ahead of time thus allowing you to do an analysis before buying the camera to see which will give the requires image scale and desirable field of view. I found the calculated field of view helpful when getting a star chart from the AAVSO Variable Star Plotter.

I use a Canon XSi and it has worked great for many, many years for photometry and sports photography. Canon includes the software to operate the camera. Do not know if Nikon does but in the past it was an expensive option. My camera is old and thus does not have wifi so I have attached a wifi to it and used it with an IPAD for Live View. Some of the newer Canon cameras have wifi. But check to see if allows remote shutter, ISO and file download to the laptop. My camera view screen is too small to see what is in the field. Also cannot see the star either. Now gone to a hardwire from the camera to a laptop. I like this because during setup an image can be viewed ( I use AIP4WIN or IRIS) to see what the FWHM and ADU are.. If the they are not acceptable the focus can be adjusted between pictures then you are ready to take the science images. The problem with using the IPAD is the FWHM could not be measued until the shooting session is over. On my scope just a fraction of a millimeter of fucus adjustment can make the out of focus star to have the wrong FWHM.  Another avantage of being able to see the image on the computer screen is checking with a star chart to see if the variable is in the FOV.

You'll have lots to learn but will be fun.

Good luck,

Gary

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