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Pryce

High FL telescope for DSO imaging

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Hey guys!  I could use some advice again.

I'm going to start planning for a higher FL telescope for the smaller DSOs out there and I'm looking for advice on what to concider.

 

I only have experience with refractors and newtonians so far and I've got all the(or atleast most of the) equipment needed for imaging with the SW 72ED and the SW 150P. 

But I'm looking to replace the 150P with something bigger. I'm a Skywatcher guy but hey, a good scope is a good scope. So I'm not set on it. 

I've been considering the 250PDS/300PDS but I'd like your input on the other options out there!  

My pricerange is £600-800 including the needed accessories.  I'd LIKE to stick with a newt or frac, but if I can get a good scope with all the accessories I need for that price I'm down to switch it up! 

 

Cheers, Fred

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I'd begin by finding out what sampling rate you'll get at various focal lengths with the camera you intend to use. Because pixels are getting smaller you can reach high resolution with shorter focal lengths these days. Other factors:

- You'll be lucky if your seeing allows you to get down to 1 arcsec per pixel and very lucky if it allows better.

- Your guide RMS in arcseconds needs to be half or less of your imaging sampling rate. (So to image at 1"PP your guide RMS needs to be no higher than 0.5".)

- If going for a 250 or 350 Newt, are you well protected from the wind?

Olly

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4 hours ago, ollypenrice said:

- If going for a 250 or 350 Newt, are you well protected from the wind?

I can be! I usually go to a friends farm and thats in a depression surrounded by trees. So I havent had problems with wind so far. 

 

Can you elaborate on the rest? So far I've been using the "does this work? Nice." Approach

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On 17/09/2020 at 14:16, Pryce said:

I can be! I usually go to a friends farm and thats in a depression surrounded by trees. So I havent had problems with wind so far. 

 

Can you elaborate on the rest? So far I've been using the "does this work? Nice." Approach

Sure. Resolution of detail is generally dominated by the sampling rate of the imaging setup, measured in arcseconds per pixel. There is a calculator here: http://www.12dstring.me.uk/fovcalc.php The resolution goes up as the pixel size goes down or the FL increases, or both. You may also be limited by the optical resolution of the telescope, particularly when using small refractors with small pixel cameras.

There is little point in trying to image below about an arcsecond per pixel because other factors, notably the seeing and the guiding, will blur out the theoretically possible detail available below that. When you try to over-sample in this way you simply reduce the amount of light striking each pixel and slow down the imaging process. Your final image will give you a bigger object but a bigger one containing no detail not found in a properly sampled one.

As pixels get smaller it becomes increasingly possible to over-sample. For example, you could put a Canon 5D Mk4 in a C11 and this would be sampling at 0.39"PP. This would be an utter waste of time because that level of detail cannot be resolved and, at a more realistic focal length, you could capture the same real detail while putting more than twice as much light on each pixel.

My own high res imaging is done at a metre focal length (TEC140 refractor) and at 0.9"PP (Atik 460.) This gives a decent 'real world' result in my view. A 10 inch F5 Newt would give a similar result with slightly larger pixels and would be faster because of its greater aperture.

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Just keep your sampling rate realistic.

Olly

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On 17/09/2020 at 13:16, Pryce said:

I can be! I usually go to a friends farm and thats in a depression surrounded by trees. So I havent had problems with wind so far. 

 

Can you elaborate on the rest? So far I've been using the "does this work? Nice." Approach

Do not underestimate how much of a 'sail' a large newtonian is.  It also requires a very robust mount.  It isn't the overall weight that is the problem but the moment arm of the telescope.  A 300PDS is an 18kg beast with most of the weight at one end.  That puts an awful strain on your mount and is even worse in windy conditions because of how far the non optic end has to extend to balance the mirror.  You also have to think about set up time.  If you want a longer focal length have you considered a 6" - 8" RC or CC (but similar focal length and still both reflectors per se).  They are slower, but as noted above you are limited by seeing most times so you may need to bin images to get the most out of them at these lengths anyway.

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