Home › Forums › Photometry › To do or not ?
- This topic has 6 replies, 4 voices, and was last updated 2 years, 9 months ago by Mr Jack Martin.
26 June 2020 at 12:08 pm #574656
I have a 4 inch Bresser refractor attached to my C14.
Is it good enough to do useful Photometry, or do I need a better refractor (flat field) if so which one ?
Jack26 June 2020 at 1:21 pm #582687Dr Andrew SmithParticipant
I don’t see any good reason not to use it. Photometrists often employ defocused stars to improve S/N.
Regards Andrew26 June 2020 at 3:42 pm #582688Andy WilsonKeymaster
You don’t need high end equipment to do photometry so that should be fine.
What really matters is making sure you do flat fields, and that is the same whatever equipment you use.
Andy26 June 2020 at 3:59 pm #582690Dr Paul LeylandParticipant
It’s not the size that is important, it’s how you use it.
You can do accurate photometry with telescopes of almost any aperture. The only thing that increased aperture buys you is a fainter limiting magnitude for a given exposure time.
People do precision photometry with nothing more than a DSLR and a standard lens. It is rare for the aperture of such a system to exceed 50mm, or half that of your telescope.28 June 2020 at 6:13 pm #582698
Thank you all for your responses which filter(s) do you recommend and from where ?
Jack29 June 2020 at 10:33 am #582699Dr Paul LeylandParticipant
There are two approaches, one potentially of zero additional cost.
If you are going to be using a colour camera, a typical DSLR for instance, it already comes with a set of three filters — one each for red, green and blue. None of these are standard photometric filters but it is possible to make usefully accurate estimates of what the intensity would have been in standard bands such as Johnson-V. The process takes far too many words to explain here but comprehensive instructions are available on-line. I don’t have URLs immediately to hand but can dig out the references later if you want them (and if you haven’t found them independently).
If you have a mono camera and/or wish to purchase a filter to begin your research-quality observation program, I would recommend buying a Johnson-V filter. They are available from a number of suppliers but please be careful to buy a Johnson-V photometric filter. Supplier’s product listings can be confusing at times.
In the case of DSLR (and other colour) cameras, always download your images in RAW format. Conversion to JPG or whatever will destroy the accuracy of your data. If you are using a filter as well, post-process the images first to separate out the RGB components and then sum or average (not median) combine them to produce a grey-scale image for subsequent analysis.
If you have a colour camera then I would personally use it as-is to learn the basics of the trade before spending more money. But, then, I’m a cheapskate. You are going to have to educate yourself anyway …29 June 2020 at 8:59 pm #582706
Thanks for your feedback.
I have an Atik 314 mono attached to the 4 inch refractor, so the filter you recommend would be a good option, as I want to obtain research-quality results.
I have a Canon 400D DSLR, but from your description, the first option seems more straightforward.
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