Hello,
I have a HEQ5 mount and I would like to create a setup for variable stars observations. (I'm particularly interested in transient phenomena like flare stars and novae). I have done tests using my DSLR and 50mm lens, and using the Citizen Sky method I was able to reproduce the magnitude of catalog stars to accuracy of about 0.05 mag. I also own a 200/1000 Newtonian tube, which is however difficult to guide on HEQ5 mount and its relatively narrow field of view makes finding good calibration stars difficult. Right now my detector is Nikon D90 DSLR, however once I get some skill in photometry my plan is to get a low-end Atik CCD and a Johnson V filter.
What would be your recommendation for optical tube for this setup? I was thinking about Skywather Equinox 66/400 or WO Zenithstar 72mm -- both of these telescopes would give me quite large field of view and enough backfocus to install a filter wheel. They would be so light that they would require no guiding. However, using a refractor for photometry sounds risky to me, espectially using B or U filters. The other option would be using a super-fast Newtonian (like 150/600), but that would come with neccesity of coma corrector and rather troublesome alignment.
I'm open to any suggestions.
Clear skies!
g.
I used a Sirius mount with an Astrotech F4 Newtonian for almost 2 years with good success. I did not use a field flattener. Since I was using a small chipped SBIG CCD, ST-7E, There was not much edge distortion. Figuring out how to balance the tube is the big issue but can be solved.
No problem if you use a refractor as long as you are using a filter. I wrote a paper several years ago about using the green color filter built into a camera chip (so you don't need a V) to do precision photometry, This is now incorporated into the DSLR photometry manual so I understand. Spectroscopy with a refractor is much more suspect than photometry in this regard. Since you are only using one small window of wavelengths at a time it does not impact the CA issue built in to using refractors.
When I first got into photometry I used an F/8 Celestron achromatic refractor to do good photometry. As long as you stuck to filters there were no issues but when I just wanted to take pretty pictures you had a white blob of CA around brighter stars. The photometry was very good however. The quality of optics and mount are more important than the type of scope!
Thank you for a very detailed answer. I'm glad to hear that small refracting telescopes are okay for CCD photometry. They're so light and handy!
Didn't you come across any trouble with collimation of such a fast Newtonian telescope? I have trouble getting round stars even in my 8" F5 GSO Newtonian (however I think that may be due to pinched secondary, since I collimated many scopes in my life with much more satisfactory results).
Now I'm almost decided to get a second-hand Atik 16HR or Meade DSI III camera. To be honest, that's maximum for my financial abilities (still a student). I would be happy to get Atik because their software seems to be much more customizable and the camera also has peltier cooling. Both cameras have ICX285AL 2/3" chip (10mm x 8mm). What would be optimal focal length for such CCD sensor? 1000mm in my current Newtownian sounds way too much to me. The FOV is going to be very narrow. I was thinking about something around 500mm.
Hi Dominik,
Your D90 is already a very good camera for photometry ! It provides a much larger FOV than the CCD you are considering and its CMOS sensor is one of the best. Even if such CCD have a cooling they have very high dark current compared to recent CMOS and the result would be overall not much different. The CMOS of the D90 has 14 true bits of dynamics and a much higher electron capacity. Depending the case, I use a Newton 8" F4 on a Celestron AVX mount (costs a little lower than the HEQ5), I don't see any tracking issue for photometry (the single image exposure time is never very long in photometry or we get to saturate). The only issue was the coma due to F4 at only 800 mm focal length, a TV Paracorr fully eliminated it, now stars images are perfect on the full FOV of an APS-C sensor (Canon EOS M). This provides a 1.5 x 1 degrees of FOV that is usually ok for mag 8 and higher. For brighter stars I use a 200 mm F4 lens (an old Nikon F that is usable on Canon and cost only 100 € at second hand sale) It provides 6 x 4 degrees and it's ok down to mag 3.
Clear Skies !
Roger
Aren't NEF files
Hello Roger!
Aren't NEF files from D90 compressed? I browsed EXIF info very carefully and it clearly said the file had lossy compression. I think I read somewhere that signal is saved in logarithmic spacings which compresses it to 9 bits. I must take some shots and plot histograms to find it out myself. Luckily I'm going for astro-holiday tomorrow so I have a of time to play with my cams! Time to go back to notes from my photometry classes :)
Of course I'll continue using my current setup! I want to finish programming my automatic pipeline for data reduction. :)
Looking for a cheap CCD, I found Mammuth Lyuba 419. It has rather small 1/2" chip and poor resolution, but because of its large 8,3um pixel it has stunning 72000e- potential well with readout noise at about 15e-. For comparison, most Atik cams have shallower wells (about 20000e-) but also lower noise (5e-). I also like that Mammoth being one of the cheapest cams on the market has a fixed-point temperature stabilization which makes calibration much easier.
Clear skies!
g.
Hi Dominik,
RAW image files of DSLR are compressed but using a lossless technique. Usually the extracted data are just the ADU generated by the analog to digital converter of the sensor. The response after RAW decoding is perfectly linear in case of CMOS sensors, and your D90 provides true 14 bits linear data.
CCD sensors could have some non-linearity at the top of the range but it's depending of the anti blooming being used, it's an analog issue, nothing to do with the lossless compression or any digital processing.
Clear Skies !
Roger
I'll have a look at my RAW frames. I also found interesting tutorials how to calculate readout noise and true gain of the camera detector. I can't wait to do it on D90 :) http://spiff.rit.edu/classes/phys445/lectures/gain/gain.html
Returning to the main topic -- I just ordered small TS 65Q quadruplet 60/420 with in-built field flattener. I hope it's going to make a good lens for my portable setup :)