Upgrade path for a SW ED80...please educate me on FOV & imaging frame

[oymd]

Hello everyone

I've been using a SW ED80 with the dedicated FR, giving an effective focal ratio of f/6.3

I've struggled with imaging larger objects, like M31 or nebulae like the California or IC 1805. They would just not fit, and my trials with mosaic have been terrible, as I have not learnt plate solving yet, and do not have a guiding set up yet.

I've been considering an upgrade, for a lower f ratio and wider field scope.

I was thinking of the Esprit ED100, at f/5.5, which appears to be a good scope.

I have looked on astronomy tools and I was surprised to see that the FOV of the Esprit is SMALLER than that of the ED80?

How is that possible? Is it because the magnification is more? Shouldn't f/5.5 have a wider view than f/6.3?

Can you please educate me?

Many thanks

[JSeaman]

Hi, the f ratio is the focal length / aperture size. An ED80 has a 600mm focal length (80mm aperture = f/7.5) and the Esprit 100 has a 550mm focal length but 100mm aperture (f/5.5). The focal length can be thought of as the 'zoom' for the field of view and the aperture the light gathering capability. It sounds like you want to reduce your focal length to zoom out or look at alternatives to the ASI294 with a bigger chip size. If you have a look at this link you can see the effect of changing either focal length or camera

https://www.skyatnightmagazine.com/astronomy-field-view-calculator/

For example, an ASI2400 on M31 would give you this:

[vlaiv]

ED80 with reducer has 510mm of FL, while Esprit 100 has 550mm of focal length.

Field of view for given sensor size depends on focal length.

If you want to go wider then ED80 - look into getting something that has ~300-350mm of focal length.

For example, here is custom scope of 320mm FL with ASI294 vs ED80+reducer and ASI294:

You can look up SW72ED with reducer at 375mm or maybe something like this:

https://www.firstlightoptics.com/sharpstar-telescopes/sharpstar-61edph-ii-f5-5-triplet-ed-apo-telescope.html

or maybe

https://www.firstlightoptics.com/william-optics/william-optics-zenithstar-61-ii-apo.html

In the end, you might want to go even wider with something like Samyang 135mm F/2.

Edited January 26, 2021 by vlaiv
One zero too many ...