Refractor vs Astrophotography Refractor?

[Guest]

Can I ask those far more knowledgeable than me, what are the major differences between a refractor designed for visual astronomy and one designed for astrophotography?

Edited April 15, 2020 by Guest
Mistake...

[Peter Drew]

You have answered the main question already.  Which one is most suitable depends on the prime use envisaged for the instrument.      😀

[Guest]

2 hours ago, Peter Drew said:

You have answered the main question already.  Which one is most suitable depends on the prime use envisaged for the instrument.      😀

Okay, let me phrase the question a different way then...   What makes one refractor good for visual observing and another good for astrophotography?

Edited April 15, 2020 by Guest
Mistake...

[andrew s]

For visual you don't  need a large flat field but for imaging you do. Getting a large flat field requires more complex optics. In general visual reflectors have a higher focal ratio than imaging.

Regards Andrew 

PS I was assuming deep sky imaging not solar system.

Edited April 15, 2020 by andrew s

[Ricochet]

I think the main difference is that an "astrophotography" newt will be designed with the focal point further out from the tube than a visual one. This necessitates a shorter distance between the primary and secondary mirrors and as a result the secondary will be larger. There may also be a further adjustment to fully illuminate a certain sensor size. 

[Nova2000]

Astrophotography reflectors have faster f ratios and a bigger secondary mirror as far as I know . The tube are shorter so that you can mount instruments and get them focused as when the tube is shorter the focus moves further out .

[Guest]

My bad, I meant refractors.😏

[Dr Strange]

4 hours ago, merlin100 said:

Okay, let me phrase the question a different way then...   What makes one refractor good for visual observing and another good for astrophotography?

Visually if you are not bothered by CA (Chromatic Aberration aka the blue and/or yellow fringe around bright objects) then an Achromatic refractor works fine. And there are slower (higher f/stop) Achro's out there that minimize the CA. The Skywatcher Evostar 120 comes to mind as an example. Also more aperture is better for visual so a bigger heavier refractor that has more aperture is better for visual. For AP the quality of the glass, the mating and figuring of that glass, the focusing of the three bands of light (red, green, blue) on a single focal point (Apochromatic or Apo) as opposed to what you get with an Achro where only two of the three are focused on one point the flatness of the field, and the quality of mechanical parts are important. Aperture is not necessarily important. More aperture means more weight which means a bigger more expensive mount. 

There are good compromises that work well for AP and visual with some constraints. The visual end will not be as good on small targets like planetary nebulae or globular clusters but they are still visible and enjoyable to look at. On the AP end it means a bigger mount to hold the scope and still do well for AP. For example my Takahashi TSA-120 does well for both.

[Peter Drew]

@merlin100.  I didn't mean to appear flippant but the truth of the matter is that telescopes designed specifically for AP are better for AP than telescopes designed specifically for visual and vice versa.  Every compromise in between is the dilemma faced by most potential purchasers.    😀

[Chris]

Imaging refractors tend to be shorter, optically 'faster' and very well corrected for chromatic aberration. Plus they're designed to be used with a field flattener and to have a larger circle of illumination to avoid both 'warped' stars and vignetting.

Visual telescopes tend to be longer and slower as to be kinder on eye piece edge performance, and are probably more likely to be achromatic rather apochromatic as chromatic aberration isn't quite so exaggerated visually as it is by a camera sensor. 

Of course there is absolutely no rule which says you can't image with a visual orientated reafractor and vice versa, only if you choose the right tool for the job it of course makes things easier and the results are likely to be better. 

[Dr Strange]

As an addition to my post I would add the caveat that it will also depend on your visual observing interests. For example I do not image with my TSA-120. Getting it to the point where I can is very cost prohibitive. Takahashi has a bunch of adapters you need that are almost at the level of a sexual fetish and very costly to get the scope there. In addition the focal reducer for the scope is over $1,000 US. 

I use a Tele Vue NP101is. The adapters and extenders I needed cost me $140 US as opposed to over $500 US for the Tak ones and I don't need the $1,100 focal reducer. In effect to get the TSA to where I wanted it I would be spending over half the cost of a used NP101is in the US which is about $2,800. The NP101is was the better choice. No fiddling with adapters and extenders. No fiddling with focal reducers.  The NP101is is a Petzval design at a significantly lower cost than the Takahashi option which in the same aperture is the FSQ-106 which costs $6,000 US and goes for about $3,500-$4,000 US used. It is f/5.3 has a smaller image circle than the FSQ BUT that isn't important to me since I do primarily NB instead of large sensor  LRGB work and when I want to do really big things I am fine doing a mosaic. And lastly it is lighter than the FSQ which is easier on the mount.

I digress. My apologies.

The NP101is is an amazing wide field visual scope as well. I can get the entire Veil complex in it using a 31 Nagler EP. And with a OIII filter that view is amazing. It also does remarkably well on open clusters. Really framing things nicely as well as being able to take in even the largest of them. So if your visual thing is wide field then an AP oriented scope will work well for you. 

Edited April 15, 2020 by Dr Strange

[Guest]

Thanks for everyone's input, that helps to explain it better.