Is the popularity of imaging making 'ordinary' observing worse?

[CumbrianGadgey]

This seems like an odd thing to ask.

I'm not having a go at people who are into imaging...I've even got thoughts about trying it myself one day, funds permitting. No, I'm aiming it at the telescope manufacturers themselves, if I'm right about some conclusions I've drawn from research into selecting eyepieces for different scopes. You see, when I did this, I was drawn further into the design of the scopes themselves.

What I gather is this. The telescope focus to the focal length specified but it does not focus everything to a point. It focuses only the centre of the image to that point. The rest is spread out giving a circular image that is called the linear field. Our eyes cannot use this (but a camera can) and so we use eyepieces to take a portion of it and make the rays parallel again to present to our eye lens. Our Eyepieces approach the linear field until they are the eyepiece focal length away from it. At this point, the EP is not necessarily taking light from the entire linear field and takes less at higher magnifications. It is therefore not using all of the light the scope gathers until it encompasses the whole field....which is why higher magnifications get dimmer.

So what has that got to do with imaging? Well, since our pupils only dilate to 7 or eight mm at best, it is pointless making the linear field much bigger than this for observing via an EP and the eye. It used to be the case that telescopes were designed to give a 10 to 14 mm linear field to give a margin of error and use the brightest central part of the image for the eyepieces to use. Then came cameras and imaging.

Now most manufacturers seem to be offering faster scopes that are better for imaging but not only that, they are typically designing for a 28mm linear field that cam fill the camera with light. Even my Skywatcher Explorer 130 seems to have a linear field of 20mm, when I worked it out.

If my reasoning is correct, then our eyes and EPs can not take all of the lovely light that the scope has worked so hard to produce, because they are looking at a small circle in the centre of a whopping 28mm one. Not only that but for any given geometry of Newtonian telescope, producing a 20 or even 28mm linear field requires a bigger secondary mirror and cuts out even more light. Maybe this explains the comments of some 'veteran' astronomers who are stating that secondary mirrors are bigger than they used to be!

What do you think? If this is right, then some old scopes of the same basic spec (aperture and focal length) should be able to provide brighter images through an EP.

[Davey-T]

A problem I found recently, with refractors at least, was that in the effort to promote scopes as "astrographs" they failed to do either, "affordable" triplets meant that 100mm was f/7, not really fast enough for imaging but some were unable to reach focus

with some 2" diagonal and eyepiece combinations due to trying to make them work with cameras so to use them for imaging you still had to spend another couple of hundred pounds on a flattener/reducer.

Almost had to but an FSQ 106

Dave

[DrRobin]

Hi,

As you correctly point out the Mk1 eyeball is a sensor area and a lens combined (the lens is not detachable) and therefore we need an eyepiece which focuses at the same point as where the main telescope focal plane is, otherwise we wouldn't see anything.  You could get rid of the eyepiece and put a ground glass screen at the focal point of the main scope, then you would see an image on the screen, but it would be low in contrast and detail, much better to deliver the image directly.

A telescope object lens (or primary mirror) collects much more light than your eyeball due to its larger area and on axis focuses all this light to the centre of the eyepiece.  As you move off axis at the front of the scope the object lens still collects the same amount of light (less a little due to the smaller area that it presents to the target) and focuses this to one side of the axis in the eyepiece. 

As you move further off axis the image in the eyepiece moves further off axis and dims, this is called the Field of View (FoV).  So what you are saying really is that modern scopes are optimised to give a wider field of view (more off axis) than an eyeball can accommodate?  This is true to an extent, but only an extent and the scope is still providing you with lots of light over the FoV you can see with the eyeball.  Put a low powered eyepiece in and you can then enjoy a wide FoV, you might have to move your eye around the image to take it all in, but that is a imitation of the eye, not the scope.

Okay so why does an astrograph optimise the light over a large area, well it comes back to the gradual dimming of the light as you move off axis.  You don't notice it with the Mk1 eyeball and the brain compensates, but a camera chip is not that clever and therefore if you buy a true astrograph (and you pay a lot of money for one) then it flattens the field, in other words it tries to make the brightness even across the whole of the field.  Astrographs usually correct for distortion in the image as well, again the brain sorts these out when you look with the eye.

Most of the main suppliers manufacture a scope for visual and visual/imaging and then they often manufacture a similar scope or field flattener just for imaging, avoid the second category and you won't be paying for something that you get no benefit from.

Hope this answers your questions?

Robin

[crashtestdummy]

Personally seeing people's images doesn't do anything for me at all.I really don't find imagining,sitting around for hours to be remotely enticing or fun.I think realistically people see a picture of m31 and then train a telescope on it and lits are dissapointend by what they get but is it anything worse than a picture of someone at the creare at cricket compared to watching it in th stands?

[NGC 1502]

Well I wouldn't let it worry you.

In my local club we have a complete mix of people.

Visual,  imagers, armchair, obsessed, casual, high tech, low tech, beginner, advanced...............

We all get on fine 

Regards, Ed.

[Paul M]

I really enjoy seeing people's images here on SGL and admire their skill.<br />

I don't do imaging (yet) but my visual observing is my own personal pleasure. <br />

Visual observers can make great achievements too . But unless they do a sketch their is no proof! <br />

<br />

Sent from my HTC One using Tapatalk<br />

<br />

[Zakalwe]

I am probably missing something, but where's the problem over and above Newtonians? Refractors are getting a larger, flatter field as manufacturing techniques are improving. The field is greater than the eye can see, but thats not an issue is it?

For Newts then there is a difference that the eye, possibly, can see, as secondaries get bigger. Is it actually visible in real use though?

The other thing to be cognisant of is that overall the scopes are getting better. Focusers, construction, mirror/lens quality is all improving.

Its a bit like saying that modern cars are faster than can be used on the road. After all, who needs a car that can go at 150MPH when the speed limit is 70MPH? By limiting the discussion to that small area you are ignoring all the other improvements- MPG, reliability, comfort, safety and so on. now, you might be in a sector of people that will never, ever use the speed capability of the vehicle. But you will take advantage of all the other improvements. Similarly, if you are visual only, then you will never take advantage of wider flat field, but you will take advantage of the other improvements (cost, availability, improved focusers and so on).

Ed's point is really well made. The usual suspects complaining that imaging isn't "real" is about as monotonous as saying that GOTO is a cop-out. I am in the other camp....if all I was "allowed" to do is to put an EP in and be visual only, then I'd go and get a far more interesting hobby than peering at murky grey blobs in the cold.

[rowan46]

the reasoning behind faster visual scopes it seems to me is not to make them more suitable for imaging (although it is a side effect) but to make the larger appartures that people want in a manageble size just imagine an f7 12" dob or worse f10