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Help choosing a triplet refractor for imaging


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I'm starting to look towards moving from a reflector + DSLR combo for imaging, to a triplet frac + CCD (eventually NB) imaging.

Although my pockets aren't deep, I reckon I could stretch to about £900 for the OTA, then in a year or so, look at CCD + NB filters.

I'd love to buy new, but will happily go used as long as condition is good (which most will be).

I've looked at the William Optics 81GT, the SW Equinox 80ED and a handful of others.

Given the budget and the fact it will sit on a tripod mounted AZ-EQ6 until I move house and assemble something more permanent (in a year or 2), can anyone give good reason for 1 scope over another?

At the moment I'm thinking WO 81GT.

I know it's an easy question to ask "which is best", so what I'm looking for is more like reasons not to get a certain scope.

I'm looking at mainly galaxy and nebula imaging.

i already have a canon 70-200 f2.8 L + 2x extender so would I be wasting money on a triplet frac when the L lens should be "close" to good enough for now?

Thanks.

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Just bought an ES 80mm carbon fibre triplet with field flattener for £756 from telescope house as a bundle with the 3" field flattener / reducer and diagonal. . Lots of other retailers are doing the same deal.

The OTA is well made and light and sits well on a Vixen Sphinx mount with an old ETX60 OTA as a guidescope. Works very well visually but I'm having some issues with the field flattener; the images at its native f/6 are rather better corrected than with the flattener in place working at f/4.2 (using an 1100D). Also, you have to buy a specially thin Canon to T-thread adapter for the system to focus at infinity for which they sting you another £34. A standard adapter won't focus at all at infinity; you can't rack in far enough.

There is nowhere in the field flattener where you can add a screw-in 48mm filter. I'm using a astronomik CLS in the camera which works fine, but it might be extra cost if you don't already have one.

The travel case is an excellent piece of engineering if that's relevant.​

The Crayford focuser holds the camera and heavy field flattener without issues pointing at the zenith.

You're stuck with a Meade finder shoe. Fortunately I had an old Meade finder; none comes as standard. Not a dealbreaker by any means but you don't want to go drilling into the OTA to mount up something more common or more useful.

In retrospect, I rather wish I'd waited for a s/h GT81 to come up. The WO product seems better thought through from the AP point of view.

I'd waited for weeks for one to come up on ABS; 2 appeared the week after I'd got the ES scope!

I did consider the new Star71 but thought it too much of a one-trick pony although it does that AP prime focus trick very prettily indeed.

Hope this helps...

RL

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'Galaxy and nebula' is pretty well impossible from one scope once you've done M33, M31 and M101. The galaxies very soon call out for much longer focal lengths.

Only a bit over budget* is the very remarkable WO Star 71 which is in a class of its own from what I've seen. It can actually cover a full frame chip which is pretty remarkable because the Takahashi FSQ85 cannot do so.  Buy from a place with good returns policy because QC may be slightly questionable, but that applies to many manufacturers. This scope is a quadruplet so needs no custom bits to get the chip distance right and it is very fast. Colour control, based on Yves' images with a Nikon astro DSLR, is very good as well.

Olly

*Over budget recommendations are a fine SGL tradition and I would hate to buck the trend.  :evil:

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One thing I haven't been able to translate from daytime photography to astrophotography is aperture vs focal ratio vs price argument.

For example which is generally a better option:

80mm frac at say f5 or 100mm frac at f6?

Let's say these cost the same, which path would I need to look down to get the best the budget will allow? Aperture or focal ratio? Assume the focal length is the same at say 800mm.

I also understand the need to choose an imaging circle to fit the sensor, which at the moment is an aps-c, but may be a really good full frame ccd later.

I'm thinking I may need to up my budget in order to prevent a 'buy cheap, buy twice' later on.

I only want to buy an OTA once and really nail this before I go down the ccd NB route later.

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Ps. When I say galaxy and nebula I know they're all different sizes and am happy to attempt mosaics!

Ideally, an 800mm+ focal length fast scope is what I'm after, assuming aperture isn't a major factor.

Oh and a budget is a starting point, not a limit! Ha.

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Ps. When I say galaxy and nebula I know they're all different sizes and am happy to attempt mosaics!

Ideally, an 800mm+ focal length fast scope is what I'm after, assuming aperture isn't a major factor.

Oh and a budget is a starting point, not a limit! Ha.

800mm FL in a refractor is going to be hard to find at a fast F ratio. In general, the bigger refractors get the slower they get for a comparable degree of colour correction. Colour correcting large refractor apertures becomes very difficult and very, very expensive. This is why you can find lots of great small refractors at around F5 but you cannot find the big ones at those F ratios. The TEC140 is F7. The SW ED120 is F7.5. Even if it existed an F5 triplet with a FL of 800mm would have an aperture of 160mm. To the best of my knowledge no such instrument exists and, if it did, it would literally cost a sum that would buy you a Porsche. You could get close to these numbers with a Tak 150 and reducer, but... £££ ouch!

I would also be wary of mixing focal reducers with larger refractors of other makes. Some combinations work, many do not, or don't work on some targets because of reflections.

For an over-view of F ratio, focal length and aperture I posted some simple graphics I designed recently to try to explain the situation in AP. It's here.

http://stargazerslounge.com/topic/253704-care-to-check-my-f-ratio-visual-aids/

Olly

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Hmm, so a long FL refractor to be fast enough (f4 or f5) has to have a large aperture and large price tag?

So for the £1-1.5k range, I'd be looking towards a 100/120mm 600-700FL f6ish?

I have read about the WO star71 as being superb and am happy to get a smaller aperture for faster optics if this is the way to go. Or maybe the GT81.

I would ideally like 800mm FL but as I probably can't afford it, I suppose 600mm or so is the target.

I tried my Canon 70-200mm 2.8 L + 2x extender (400mm f5.6) last night on M31 - too many moving parts for this to be pursued seriously and very hard to focus correctly, it's so tight to get it nailed.

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Right, I've put together a spreadsheet containing refractors up to £2k new, and ordered by focal length, split into doublets and triplets / quads.
 
I've added information where I can find it, prices are not lowest, but are ball park based on what I came across on various sites.
 
Not sure how to upload a spreadsheet to here, but as I have found out, following Olly's advice on refractor FL vs price, if I want to achieve a fast FR at long length, for a reasonably affordable amount, then I have to look at the APM 107 or WO FLT 98.

Problem is, these don't have built in flatteners, hence more cost.
 
But then there's the focuser too, there's no clear cut winning advice for R&P vs Crayford. I like the idea of R&P personally, as these are mechanically held, not held by friction.
 
If I could afford it, a Tak 106 would be great, but I can't.
 
My budget needs a good talking to!!
  
I certainly cannot decide.

If anyone wants a copy, please let me know and I'll email it to you.

Unless someone can show me how to upload a file, as the 'files' page doesn't show an upload button?
 

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