Fishing around for the "right" focal length

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dghent
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Fishing around for the "right" focal length

Post by dghent »

Apologies in advance if I'm asking an oft-repeated question and I've missed something in my trawling around for info on the subject. I've been out of the solar game for a good while due to other obligations. But with the sun awakening again, I'm starting to get the shakes and hear that siren song.

I recently lost the telescope I used to use for solar viewing (in a flood. it was on the floor and is unsalvageable) so I'm hunting around for something to replace it. It was a large 152/900mm achromat and, while it was alright, it wasn't great for all the obvious reasons and I now want something smaller that I can toss onto my Rainbow Astro RST-135 and head outside. I'm thinking something 120mm and down, f7-ish or so.

I have a Quantum PE+4x PowerMate that I upgraded from an old Del Woods-era University model and a few flavors of Quarks that have been collected over the years, so the blocker apertures vary. I'll be using it for outreach (once COVID allows) but mainly for imaging (ASI174M or ASI1600M.) Full disc is neat but I can't say that I'm drawn to it or that being able to see the whole disc when under visual use is an imperative.

Cheap doublets that have focal ratios >= f/7 and have decent focusers seem to be a unicorn, and the closest I've been able to figure is either the SkyWatcher EvoStar 80ED (f/7.5) or the Astro-Tech AT102ED (f/7). The problem is that I'm trying to figure out what my actual FOV will be at a given focal length with the blockers factored in. I can go tool around with the FOV tool on astronomy.tools, but I know what that shows assumes a fully clear aperture.

Anyone care to tender their thoughts or advice? Thanks in advance.


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MalVeauX
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Re: Fishing around for the "right" focal length

Post by MalVeauX »

Hi,

Depends on your seeing more so than anything if you care to image.

The AT102ED (102mm F7, ED frac) is a great option for this, good focuser.

Another great one is the T.S. 102mm F11 ED with great focuser too (and can be modular for binos natively), for a great price and is really excellent in all ways.

The final effective focal length depends on what you use with the scope, so its variable. For FOV, well, you can control it with focal reducers and larger sensors for imaging, and for visual, use the longest focal length you can (like a 32mm plossl).

Very best,


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