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The price of obtaining sharp stars in a refractor


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Although this question involves 'video' astronomy, I'm posting it here because I'd really like to hear the views of imagers.

I mainly do electronically-assisted observing with a small-sensor camera (the Lodestar X2) using an 8" f4 Newt. I normally capture subs of 30-60s and live stack as many as I need to obtain a decent image that I can manipulate 'live' in a limited way. 

Now I'd like to acquire a shorter focal length scope for other objects (open clusters, bright and dark nebulae) which will also double up as a travel scope, and I'm looking at fast refractors in the 70-80mm aperture range. Here I find a bewildering array of options: achromats, ED doublets, some of which claim APO status, triplets, with/without various types of correctors. I already own an old Stellavue Nighthawk, which is an 80mm f6 achromat, and while it fine for visual use, I really really dislike the non-sharp, bloated stars it produces when I'm doing electronically-assisted observing.

So my question is: how far up the quality ladder do I have to go to obtain sharp stars (given the seeing)? One rider is that I won't have access to most post-processing steps, including any that deal with stellar imperfections ("tighten my stars!"). How important is post-processing anyway to achieve sharp stars? 

Part of me feels that scopes like the WO 71 Star would be wasted on electronically-assisted observing with small sensors, but I wonder if that is the only way I'm going to get sharp stars like those I see in my Newt. Or will a good-quality ED doublet be sufficient? Also, I realise that the Lodestar will be badly undersampled for the FL I'm looking at and maybe part of the answer is to use a sensitive, low read noise camera with smaller pixels like the ASI 224? 

Any opinions gratefully received!

Martin

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There's no doubt that more expensive apos generally produce smaller stars, mainly because they are better corrected into the blue, which is where most of the bloat comes from. It's also hard to know the true star sizes because imagers spend hours doing all sorts of things to reduce star size. A short book could be written on all the techniques...

Perhaps the ideal weapon in your situation would be a Hyperstar for short FL.

I've seen some relatively unprocessed results from the Star 71 and the stars were very small. I was impressed.

Olly

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Thanks Olly -- much obliged. EAA is a little strange because (leaving aside some of the great results with high ISO DLSRs) we typically don't use much of the available light cone so outer-field aberrations are not so bothersome, but I personally want the best quality for the centre ground. On this basis I'm wondering if what I need is a non-flattened/non-reduced but well-colour-corrected triplet f/6 APO (something like the TS Photoline 80mm) rather than plunging in with a system that is already reduced/flattened like the Star 71. That way I can experiment with increasingly brutal focal reduction (I'm thinking x0.6 Lightwave) which might not be too detrimental for the small sensor.

I'd like to read up on the methods used to reduce star size, even if I can't use them, mainly because I want to understand the relative contributions of good raw data and post-processing. I think over the coming years we're going to see more and more traditional post-processing being brought into live imaging too...

Martin

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Go with apo, TS photoline 80 was just a scope that was on my mind when I read your post. If using ED and particularly if using aggressive focal reduction you will most certainly need yellow #8 / fringe killer type of filter to control violet/blue part of spectrum (and even then, I'm not sure how well it would work). Using this technique coupled with aperture reduction I managed to take some decent shots even with F/5 achromat using ASI224's big brother - ASI185, but it was way too slow (due to aperture mask) even for astrophoto let alone EAA. Setup that you already use (8" F/4 + lodestar) will be hard to beat on speed using scopes with speed greater than F/4 and pixels as small as 3.75um - you would like to aim in ballpark of 300mm FL. I think ASI cameras coupled with short FL would have an edge in following scenario: using short exposures (order of second rather than minute) and stacking enough of them in realtime - no real benefit (and even less SNR for given integration time compared to long exposure stack) apart from having your target appear faster on screen (maybe a good thing when you do public display or have some friends over to do EAA together).

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Thanks Vlaiv. I agree that my current narrowish-field setup (8" F/4) is really well-suited to the Lodestar, definitely in some kind of sweet spot, and I love it. I have some trips coming up where I hope to take in some super-dark skies and that is in part motivating me to put together a refractor-based lightweight travel EAA setup. I reckon I can just about do it in 7-8 kg including tripod, tracking mount, scope and camera (well, 50g for the Lodestar barely counts), which is equivalent to one and a half of the counterweights I use with the Newt....

I'm holding back on the ASIs at the moment until the kind of integrated software that makes EAA so pleasurable with the Lodestar appears, but low read noise cameras must be the future for EAA.

Martin

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I don't know what kind of workflow do you use for EAA, but I did some experiments and was pleased. My setup and workflow was following: 8" F/6 originally dob mounted newton that I put on HEQ5 and used QHY5L-IIc with GSO focal reducer at that time (now I'm planing to use ASI185 in such combination when I get the chance). Focal reducer gave me focal ratio in the ballpark of F/3.8 or so (not full reduction of x0.5 due to closer FR placement). I used SharpCap live stack feature with exposure times in 1s-4s range. Had no problem looking at the bright object like M81/M82 and M42 even in heavy LP, under a minute of stack. Flame was also good target, but Horse Head was barely detectable. After that I did some thinking about what could be really good setup for EAA. And this is my conclusion so far (I was aiming for cheap as chips option as well - if there is such a thing in astronomy :D ):

GSO 150mm F/4 newton, ASI178 camera, either a guider/guide cam, or software feature to track target based on frames (if frames are taken under few seconds of exposure) and live stack with alignment. Should be a lightweight setup depending on mount used (don't know if EQ3 type would do it, but I'm certain that EQ5 would).

This camera has really good resolution, and at 2.4um pixels at 600mm FL gives around ~0.83"/pixel that should be ok for down to 2" seeing. Better seeing than that would miss out on resolution. Other benefit of this sensor is that it has fairly big sensor resolution of 3096x2080 pixels which can be used to give you range of "magnification" - highest being native, lower mags achievable by binning (hardware or software - bonus being somewhat better SNR). So you can go down x3 for screen res of 1032x693 (I thought of 3 distinct magnifications - 1x1, 2x2 and 3x3 bin).

So I think that EAA is possible with ASI range at the moment using SharpCap, some guiding app and EQMod + Stellarium kind of software combo.

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