Fast Refractor for EAA - Suggestions

[Camelopardalis (Dunk)]

Sounds like a beast of a scope

A larger-pixelled camera would help disguise any shortcomings of the colour correction (just like in the good old days), depending on what semi-apo means

At ~900mm native it'd be much more interesting on smaller objects than an 80mm scope, but the reverse would be true for larger objects of course...maybe mount one on top of the other and gouge on your cake

[glend (Glen)]

Which camera would be a good match for that 6" TS scope? Native 900 reduced to 700. Seems out of ASI294 comfort zone. Bigger pixel?

[JA]

Originally Posted by glend

Which camera would be a good match for that 6" TS scope? Native 900 reduced to 700. Seems out of ASI294 comfort zone. Bigger pixel?

The specs for that scope without specifying the image circle say that it supports full-frame, so to use a sensor any smaller than full-frame is potentially throwing away good light. If you really want wide field and the best light collection then at the 900mm native focal length use a full-frame sensor and at the 0.79x reduced focal length of ~700mm, you could consider potentially using an APS-c sized sensor, depending on what size the reduced image circle shrinks to. Depending on the true image circle size (all we know is that it is supposedly greater than or equal to 43.3mm), it may, with the 0.79x reducer fitted, reduce to a minimum of 79% of that (34.2mm), which would clearly be more supportive of APS-c sized sensors, given the potential for vignetting/light fall-off* with the reducer fitted.

As a ballpark calculation using a full-frame sensor it should be possible to achieve the following image scales at the stated focal lengths:
Full-frame sensors at ~900mm = ~1.1 to 1.4 arcseconds/pixel (4.9 to 6 µm pixels)
Full-frame sensors at ~700mm* = ~1.5 to 1.8 arcseconds/pixel( 4.9 to 6 µm pixels)
With APSc sizes the image scale should be smaller still (~33% lower) , given the reduction in the field of view. An APSc sensor would potentially therefore put the image scale below/around 1 arcseconds/pixel, probably on the too low side for all but the very best seeing.

Long story short / minimum effort: You could try a full frame DSLR if you already have one

Best
JA

[Camelopardalis (Dunk)]

I’d speculate that the 294 would be pretty competent well over 1m. The high conversion gain mode, deep wells and good dynamic range should make it pretty versatile. You’d probably want confirmation from someone who owns one that they calibrate correctly. Horrors stories and all (that may be just that).

One potential advantage to the QHY version is the potential to switch it into 47MP mode for shorter focal lengths. Apparently, the quad-Bayer array means that each 4.6um pixel is actually 2x2 2.3um pixels. (Or should that be 4.8 and 2.4?) and can be read as such. A trick the ZWO model doesn’t do.

[rustigsmed (Russell)]

Originally Posted by glend

Haha, not at all Russell, you have read my mind. I have been looking very closely at that scope, in fact i had just mentioned it in a PM to Dunk. It is nice and fast. It will accept the TS 0.79 reducer corrector, dropping it from f5.9 to f4.6 @ 700mm fl. However, it might be too long for good wide field EAA use. And don't point it at anything too bright without a fringe killer on it.

Another possibility is the TS80 6-element Flat field APO at an amazing f4.4. It would seem ideal with great colour and a focal length of just under 400mm.

Great minds Glen, great minds

[glend (Glen)]

Progress is being made, at least in relation to scope choice, and I have decided that I will be adopting a two scope approach to EAA. Conventional wisdom says an effective focal length of 400-600mm at a good speed is ideal for EAA, this is ideal for wide field views. However, it does rather poorly on smaller objects where longer focal length is desired (albeit speed is still important). It was pointed out to me that piggy-backing a smaller fast refractors on the top of a longer scope, has the potential to solve the reach issue. It complicates a single camera choice to some extent, as pixel size has to be kept in mind.
I have now ordered the longer focal length base scope (and reducer), and I will hold off on revealing that until it arrives. I believe I have an ideal candidate for the shorter focal length top mounted scope, and really there are many choices, but there is more work to do there. The invisioned double stack will not trouble my observatory pier mounted CGX.
Where I do have problems is in camera choice, the simple reason being there is very little stock of what I see as preferred cameras available in the near term. That's it for now.

[Camelopardalis (Dunk)]

[glend (Glen)]

An update on the TS152 f5.9 Modified Achro, I had sent a question to the TS product specialist asking about the use of a reducer on that scope. The reply came in today, which was yes, a 0,8x reducer can be used and would not impact the Image circle up to 42mm. They link in a specific adjustable ED reducer corrector for that scope. That would speed that scope up to just under f4.8. However, it does not mention what the SPOT diagram would look like.

https://www.teleskop-express.de/shop...DJUSTABLE.html

[glend (Glen)]

I promised an update on my purchase decision supporting a two scope (long and short fl) strategy for EAA, and here it is:

I decided to go with a Skywatcher ED150DX as the long focal length scope. While native f8, or 1200mm fl, I will be running it initially at f(6,8), or 1020mm fl, with the (0.85x Skywatcher reducer) but anticipate that I can get it down to f(6,3) with a TS reducer, or 948mm fl.
Why this particular scope, simply the Black Friday Special price I was able to get it for was a big factor. The DX, Esprit 3" focuser equipped model, was a big savings over the previous list price for the 2" focuser standard model.
It will be used both visually initially, and EAA eventually (when I sort out my ideal EAA camera), although my Nikon D5300 can shoot video now.

For the short fl scope I am aiming at a 500mm refractor, likely the new Evolux 82mm f6,4 at 530mm native focal length with potential for reduction. The Evolux can easily piggy back on the ED150DX, so they can both live on the CGX in the observatory.
That is all for now.

[Camelopardalis (Dunk)]

Nice one Glen that’s a beast of a scope and should do well.

Look forward to hearing how you get on with it.

[rustigsmed (Russell)]

yes,congrats on the purchase glen, it will actually also be quite good natively if you wanted to view some planets as well, think you have most bases covered

[ChrisV (Chris)]

Sounds interesting Glen. Look forward to seeing some screenshots of your live stacked images (post-processing absolutely forbidden...)

Cheers Chris