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What to do in this situation?HELP ME!!!

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mbennett's picture
What to do in this situation?HELP ME!!!

i have just started to get into photometry and i dont actually have a full fledged DSLR however i have a DSC(a sony mvc-cd350) with a exposure of 2-3 seconds through a meade etx 70-at telescope(at 14X) can get me down to 7th magnitude easily. Yes ik my set up probely isnt the greatest but im in a extreme budget(my parents dont wanna help :/) anyway i imaged a double star just for fun and decided to practice calibrating magnitude constants and getting my margine of error.There was about 4 stars in the feild of veiw 3 of them i was pretty accurate(according to skytools 3 starter addition) but the star in question was dimmer than it should be. This star is a suspected variable with little magnitude diffrence(again according to skytools) but it was a whole magnitude dimmer. outside its range and the only reason i question it is because the other stars were pretty accurate(+/- 0.003 a magnitude measurement but then agan this star suprised me...i dont know what to tell me.


onj's picture
More information needed

Hello Matt,

Could you give the position (Right Ascension and Declination)  of the star you have a query about. It would be helpul if you could try to get the star designation or catalogue number (available from SkyTools).

Also it would be great if you could upload your image.


rmu's picture
What to do

Matt, regardless of what will happen with that suspected variable star, I recommend you to do visual observation of variable stars; your etx-70 would be a good one for that task, you could observe mira variables with a limit magnitude of 10. Goto mount will help you to find the stars quickly.

In the time you can save money for a refrigerated, basic monochrome ccd camera, or a DSLR, you could make lots of visual measures and get experienced in variables.

Here in the AAVSO you can download free the visual observing manual and generate charts.



Does your camera have RAW mode?

Hi Matt,

I did a quick search on your camera. According to, the Sony MVC-CD350 does not have a RAW mode. The RAW image format is essential for accurate photometry, and if your camera saved the images in JPEG or TIFF format, I regret to say that the photometry from your camera isn't reliable--certainly not on a millimagnitude level. TIFF and especially JPEG perform a lot of behind-the-scenes manipulation to the sensor's raw output, seriously compromising the photometric integrity of the images.

I know that you're on a tight budget, but if you're willing to look at used DSLRs on eBay, there are a lot of good deals on older DSLRs which work well for astronomy (e.g., the Canon 10D).


Best Regards,


mbennett's picture
yah it has a tiff format

yah it has a tiff format which i was told was good and when i calibrated it on a few stars it seemed accurate that said im probley am just gonna stick with visual only because of my complications haha. anybody have good short variable stars that i can greate a light curve wit out waiting forever?Also some stars that will be especially helpful for the aavso?

MDAV's picture
I would suggest you simply

I would suggest you simply try the tutorials on tha AAVSO Site.

Decades ago I cut my teeth (or eyes rather)  on Algol, Bet Lyr, Del Cep, Eta Aql, etc. All relatively short periods.

For a star of current interest GK Per went into outburst last month. This one was several magnitudes fainter and shortlived compared to the last one three years ago. More like the one in 2008. Currently it appears to be sitting back at minimum around mag 13.

TCB168's picture
Mag calculation error

I'm not farmilliar with the camera you mentioned but I assume you are taking unfiltered images. If you were using the printed magnitude for a star from the skytools catalogue, this may be completely different to an unfiltered magnitude estimate from your camera. The colour of a star will vastly affect a magnitude extimate from an unfiltered image.

If you are using a colour camera and you can extract just the green channel then this can be used as an approximation to a V magnitude. Not perfect but certainly much better than an unfiltered extimate.




Roger Pieri
Vignetting, TIFF

Hi Matt,

If I understand well you used the camera in afocal mode, the lens being coupled to the eyepiece of the ETX 70 ? This is a very difficult configuration for photometry due to the vignetting. Depending the way the coupling is done the vignetting could be very strong or even very irregular. The rule is to have the exit pupil of the eyepiece exactly superposed to the input pupil of the camera lens on the 3 axis X,Y,Z. IF Z (the axis of the optics) is not ok the vignetting could be very strong, either way: dark center or dark periphery of the image. Any X,Y missalignement will make it strongly assymetrical. Very difficult to control and get stable.

In addition the pupil of zoom lenses (or wide angle lenses) is not always accessible, far back inside. The front lens is not the aperture / pupil but a field lens. The best is to set the zoom at its longest focal length to make the Z superposition possible.

To check the vignetting you should make a "flat". That means take an image of a well uniformly illuminated target like a fine texture, non-glossy photopaper. Then check the pixel values accross the image. It should be unifrom within few %.

Next is TIFF. TIFF is a container, the coding of image inside such container could be of many types. Not sure its a RAW image data. I have not been able to find the info for this camera. 

The issue is that all image coding to be reproduced by any kind of display (monitors, projectors...) need a special transformation to some standard like the common sRGB. Such transformation from the sensor RAW data (proportional to photon count) to a standard color space is very non-linear then the luminance info is coded into something near a logarithm (a 0.45 gamma, The BT709 standard) As Colin said this is this kind of transformation that makes image data improper for photometry. 

Any image under JPEG, BMP, TIFF, PNG... that is able to show on a monitor... is sRGB transformed (or similar standard) before to be processed to the specific file format, possibly with compression. The compression itself is not much an issue. TIFF can support true RAW images data on 16 bits but it's only one possibility within number of others. If your image directly shows up using a regular image viewer (Windows one, any...) and the grey scale and color are ok that means it's not a RAW. 

The ETX 70 would be an interesting base for photometry if directly coupled (without lens and eyepiece) at the prime focus to a DSLR. I use a similar configuration. You should be able to reach mag 12~13 with 30 s exposure (maybe 60 ? ). The tracking accuracy of the ETX is not very good but probably enough for photometry (we are not looking for pretty images ! ) Anyhow I understand your problem but in some future you could be able to afford a second hand CMOS DSLR ? (there are good ones at very low cost, try to avoid CCD ones) 

It's also true you should use the green channel output only, as Terry said. It's not too far Johnson V. A very good Vj can be achieved using a classical transformation (to see the Citizen Sky site and tutorials, just under "about us")

Clear Skies ! 


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