We are excited to announce the launch of our new forums! You can access it forums.aavso.org. For questions, please see our blog post. The forums at aavso.org/forum have become read-only.
you get a field of view of 25.5x17.2 arcmins, with 0.70arcsec pixels. If you put a 0.63 reducer into the system, you get a focal length of 1260mm and a field of view of 40.5x27.3arcmin with a 1.11arcsec pixel.
A general rule of thumb is that you would like 2-3 pixels per fwhm star images. Most Continental U.S. sites have seeing around 2-3 arcsec. If you use a typical 2.5arcsec value, then you would have about 3 pixels with the native mode telescope and 2 pixels per fwhm with the focal reducer.
So then you need to look at the advantages and disadvantages. The Meade 0.63x focal reducer is not meant to be used with the "coma free" recent Meade telescopes, so I'm assuming that you are using a non-ACF model. The focal reducer adds additional optics to the system, which can limit the wavelength range (though the LX90 has a glass front corrector plate that already limits the wavelength use somewhat), reduce the throughput by a few percent and also give one more place where ghost images/reflections can take place. At the same time, the focal reducer also works as a field flattener, which improves the star image quality across the flat CCD. The 8" telescope will have more comparison stars available using the focal reducer with its larger FOV, especially for bright targets. The vignetting will be more noticeable because you are using a bigger FOV, but the vignetting can be corrected with good flats.
I've seen good photometry with both setups, so I think you will have a nice setup either way.
thanks for the info. i have installed the Celestron f/6.3 focal reducer/Corrector................the images are very good even the S/N ratio. Regarding vignetting the stars at edge of the images without flats are pretty much good with round stars. i will do some more tests as at the moment we have full moon thus limits me from obtaining good images.
i fully agree, i apply flats, darks and Bias all the time, i try to obtain the best results out of the image. i am really happy with this setup, at the moment we have full moon and at a 35 deg distance away from the moon i can still have good images for photometry.
Yes! You'll need to make new flats of course and there may be more vignetting in the corners of the field.
Do you need the larger FOV to get comp stars?
Peter
tnx for the info
tnx for the info
The LX90 8" f/10 system has a focal length of 2000mm, and the ST10 has 2184x1472 6.8micron pixels (17.9mm diagonal). In native mode, using
http://celestialwonders.com/tools/imageScaleCalc.html
you get a field of view of 25.5x17.2 arcmins, with 0.70arcsec pixels. If you put a 0.63 reducer into the system, you get a focal length of 1260mm and a field of view of 40.5x27.3arcmin with a 1.11arcsec pixel.
A general rule of thumb is that you would like 2-3 pixels per fwhm star images. Most Continental U.S. sites have seeing around 2-3 arcsec. If you use a typical 2.5arcsec value, then you would have about 3 pixels with the native mode telescope and 2 pixels per fwhm with the focal reducer.
So then you need to look at the advantages and disadvantages. The Meade 0.63x focal reducer is not meant to be used with the "coma free" recent Meade telescopes, so I'm assuming that you are using a non-ACF model. The focal reducer adds additional optics to the system, which can limit the wavelength range (though the LX90 has a glass front corrector plate that already limits the wavelength use somewhat), reduce the throughput by a few percent and also give one more place where ghost images/reflections can take place. At the same time, the focal reducer also works as a field flattener, which improves the star image quality across the flat CCD. The 8" telescope will have more comparison stars available using the focal reducer with its larger FOV, especially for bright targets. The vignetting will be more noticeable because you are using a bigger FOV, but the vignetting can be corrected with good flats.
I've seen good photometry with both setups, so I think you will have a nice setup either way.
Arne
thanks for the…
Dear Arne,
thanks for the info. i have installed the Celestron f/6.3 focal reducer/Corrector................the images are very good even the S/N ratio. Regarding vignetting the stars at edge of the images without flats are pretty much good with round stars. i will do some more tests as at the moment we have full moon thus limits me from obtaining good images.
once again thanks for your reply.
regards
Martin Mifsud
"...the stars at edge of the images without flats are pretty much good with round stars."
Martin,
This has also been my experience with SCT's and the 0.63X FR, but you still need to apply flats to use the images for photometry.
Phil
i fully agree, i…
Hi Phil,
i fully agree, i apply flats, darks and Bias all the time, i try to obtain the best results out of the image. i am really happy with this setup, at the moment we have full moon and at a 35 deg distance away from the moon i can still have good images for photometry.
ragards
Martin