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MTF of a telescope


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"I am definitely confused now, but it is starting feel like the point of this thread is telescope bashing (refractors in particular) and rubbishing people's gear. 

I stopped using cloudy nights because of this type of carry on...pity."

I don't think anyone is bashing refractors. Ultimately the best scope is the one you use 🙂.  From an imaging perspective I've switched from bigger 10" newts and  12" SCTs  to a smaller 8" apo (if you can call an 8 inch refractor small ) and I am more than happy with it. Things like collimation instability, lower quality optics, obstructions, and the endless lost nights of tinkering drove me to the refractive design (actually drove me nuts 😀). Sure smaller aperture means more integration time needed per target ... but that is something I can live with. There are more important benefits that a quality refractor brings to the table. 

Edited by dan_adi
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On 21/02/2021 at 19:18, alex_stars said:

Kind of don't want to let this thread die, so here is the long ago mentioned comparison between a C14 (planetary imaging favourite) and that 7 inch Mak.

  • A C14 is equal to a 248 mm (9.76 inch) refractor and the MTF50 value for both scopes is at 0.91/arcsec or at 1.09 arcsec.

Looking forward to the next round of comments

Pretty much nailed it. I don't have a C14" and a 9.74" APO, but I have a Meade 12" SCT and a 8" APO. In my limited visual experience with the two, the details on Jupiter where kind-off  the same, but sometimes the 8" pulled a little bit ahead (maybe the seeing, maybe the collimation in the SCT, maybe the temperature ... or simply the focus ). Overall the viewing experience was in favor of the refractor but it's subjective to quantify by how much. Also I noticed the background sky was darker in the refractor, while being grey-ish in the SCT.  When observing the Orion nebula the grey-ish background in the SCT made it harder to distinguish the subtle shades in the nebulosity, and I found the image in the refractor better. Again these are not earth shattering differences, but they are there. 

With regard to DSO imaging, the refractor wins.

Interesting thread!

PS: Don't take away from this that I am bashing SCTs. I simply found the refractor to be a better tool, in my case.

Edited by dan_adi
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15 hours ago, dan_adi said:

Also I noticed the background sky was darker in the refractor, while being grey-ish in the SCT.

Given all other observing conditions equal (light pollution and such) isn't this the telltale sign for the refractor having a lot more contrast than the SCT, due to its design. I'd argue it is 👍

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1 minute ago, alex_stars said:

Given all other observing conditions equal (light pollution and such) isn't this the telltale sign for the refractor having a lot more contrast than the SCT, due to its design. I'd argue it is 👍

Most likely this is a valid explanation. It's nice to see the theory go hand in hand with observation. That said, there is no perfect telescope.... refractors come  close, but going up in aperture comes with lots $$$ and becomes impractical for both amateurs and professionals. Also I see very few, if any, comparisons between high quality reflectors and refractors. Ultimately one has to decide what they want to image/observe, and choose the right tool for the job with the best quality they can afford (taking into account the local conditions - light pollution, seeing). 

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1 hour ago, dan_adi said:

Most likely this is a valid explanation. It's nice to see the theory go hand in hand with observation. That said, there is no perfect telescope.... refractors come  close, but going up in aperture comes with lots $$$ and becomes impractical for both amateurs and professionals. Also I see very few, if any, comparisons between high quality reflectors and refractors. Ultimately one has to decide what they want to image/observe, and choose the right tool for the job with the best quality they can afford (taking into account the local conditions - light pollution, seeing). 

This is true, how many AP refractors get reviewed against AP reflectors for example. Maybe a Mewlon versus a TOA would be good, but the marketing blurb always tells us of APO's like view from the Mewlon. :) 

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