To completely body objects in your discipline of view, you’ll must know your telescope’s focal size — and a little bit of math.
My first picture of the Lagoon Nebula (M8) was successfully zoomed in up to now that the article regarded nothing like a lagoon. It was taken with a Nikon D3100 on my Celestron 8-inch SCT, with an publicity time of 30 seconds at ISO 3200. Credit score: Molly Wakeling
On the primary evening out with my brand-new Celestron NexStar 8SE again in July 2015, I knew little in regards to the relative sizes of astronomical objects. I used to be stunned by my first view of Saturn — a powerful sight by way of an 8-inch Schmidt-Cassegrain telescope (SCT) — and I used to be wanting to see extra.
I knew the view by way of the scope wouldn’t be like photographs I had seen on-line, however after I slewed to the Lagoon Nebula (M8), all I may see had been stars, with no trace of nebulosity. On my third evening out, with a newly acquired T-adapter to attach my DSLR to the telescope, I took a 30-second publicity, and will lastly see the colourful nebula! However it stuffed the complete body and didn’t look something like photos I’d seen. “Why?” I puzzled.
Focal size and discipline of view
One essential quantity related to each telescope is its focal size, or the gap the sunshine travels after passing by way of the target to your eyepiece or digicam. This determine is often given in millimeters (mm). For instance, my 8-inch SCT has a focal size of two,032 mm. Think about it like an infinite 2,000mm digicam lens!
This quantity can be expressed by way of focal ratio, or f-number. In contrast to digicam lenses, telescopes haven’t any iris to scale back the efficient aperture, so the focal ratio is just the focal size divided by the aperture. For my 8-inch, whose aperture in metric models is 203 mm, the focal ratio is f/10. One other instance is my 106mm f/5 refractor, whose focal size is 106 × 5 = 530 mm.
Folks at stargazing occasions typically ask me what my magnification is. However when framing a specific object in your eyepiece or picture, the extra helpful worth is the sphere of view (FOV), or how massive an space of sky might be seen along with your telescope and a given digicam or eyepiece.
The short and soiled calculation for a digicam is FOV = 3,436 × (sensor dimension) ÷ (focal size), the place the sensor dimension is the size or width of your digicam sensor in millimeters, and the focal size can also be in millimeters. The result’s the FOV in arcminutes; divide by 60 to get levels. For a Nikon DSLR with an APS-C (crop) sensor, with dimensions of 23.6 mm by 15.7 mm, the FOV of my 8-inch SCT is then 39.9′ by 26.5′, or 0.67° by 0.44°. For an eyepiece, FOV = (eyepiece obvious FOV) × (eyepiece focal size) ÷ (telescope focal size).
There are additionally on-line calculators; my favourite is https://astronomy.tools/calculators/field_of_view.
Selecting your goal
Now that you’ve calculated your FOV, you could find out which targets will probably be too large, too small, or match excellent. You will discover the sizes of objects in apps like SkySafari or on-line; the Lagoon Nebula, as an illustration, is 90′ by 40′. With an FOV of 39.9′ by 26.5′, no surprise I couldn’t see a lot of it in my picture — the FOV is much too small! It might be a a lot better goal for my 530mm-focal-length refractor.
Alternatively, there are various galaxies and planetary nebulae that seem tiny in my refractor however are good for my Schmidt-Cassegrain; for instance, the peculiar galaxy Arp 273 is a mere 1.4′ by 0.3′ in dimension, however exhibits up properly in my Celestron 9.25 Edge HD with a focal size of two,345 mm.
Now that you’re extra conscious of the consequences of various focal lengths, you may plan out photographs in your mixture of telescope and digicam or eyepiece. An excellent useful resource for locating targets of a sure dimension (amongst many different standards) is the Imm Compendium, a powerful and detailed spreadsheet from astrophotographer Gary Imm, which might be discovered at https://tinyurl.com/imm-compendium.
Pleased searching!
