Telephoto Lens VS Apo Telescope

[ReZ]

Hey Guys,
I'm not a newbie to astronomy (or reading on SGL ), but am a Newbie to AP (and writing on SGL ).
I have a Sky watcher 8 inch dob, and had SW 120 and 102 achromatic in the past. I have a pretty solid Visual background (messier, top 111 NGC and so on), and did some decent job on photographing Sun, Moon and planets with the dob. Now I want to do more serious things in AP field.
I have to mention I live in Iran. AP gear can be found here, but it is usually twice or even thrice the price of online markets. For example a SW ProED 80ED has an online price tag of $650 while here is being sold $1300 (Online markets does not deliver in Iran). Fortunately lenses are far better priced and found here, a little higher than their real price. While searching online for a rig, i encountered another alternative, using a telephoto lens instead of a refractor, and found a particular one, AF-S NIKKOR 200-500mm f/5.6E ED VR (http://www.nikonusa.com/en/nikon-products/product/camera-lenses/af-s-nikkor-200-500mm-f%2f5.6e-ed-vr.html).

I have read a couple of reviews (mostly daytime reviews) for this glass, all saying that it can't beat prime supertele lenses but is far better than similar junk-brand lenses (Tamron and Sigma 150-600mm f/5-6.3). It has a little CA which can be removed in post. 
It is a buy once and buy good scenario for me, the optic I buy is going to be mine indefinitely because there is no AP gear buyers here. (Having a hard time selling my dob, let alone AP gears.)

Now with the given situation, my question is: Can I buy a telephoto lens for large deep sky objects, instead of an apo telescope, without sacrificing quality?
( I will buy a longer FL telescope (say a RC or SCT) in future, but now i have to start with shorter Focal Lengths. )

Lens ads for me:
- Can do wildlife in daytime (another fav genre, but AP has priority)
- Has a wide Focal Length range, so i can do composition shots of deep sky and landscape
- Has a fixed focal Ratio (junk brands FR is not fixed)
- More Portability
- Can sell it (with a very low price, say a quarter)
- Focus is easier
- 95 mm aperture (instead of 80mm on apo)

Lens cons for me:
- if stepped down 2 or 3 steps, star diffraction takes place
- Stuck with Nikon system
- Cant use filters
- Little CA (I don't know if it is going to be a problem, It has 3 ED out of 19)
- Price is higher than the apo (around $150 which is not a problem)

Thanks in Advance, and sorry for bad English, ReZ.

[Astroboy239]

Well I'm wondering about it too, I've ordered a star adventurer to use with my cam so even I thought what's the advantage/disadvantage of a telephoto lense over a refractor. Well but I think a refractor is better as the focus of the lense might change. 

Varad 

[lrt75914]

I don't know if this factors into your decision making at all but as far as I can remember variable focal-length lenses tend to use upwards of 10 optical elements in their design. This may reduce the light gathering capability of your variable zoom lens, when compared to a telescope with similar aperture and focal length.

[ReZ]

1 hour ago, Astroboy239 said:

Well but I think a refractor is better as the focus of the lense might change.

Thanks for Quick replies. yes that  can be an issue, especially when the target is overhead.

[ReZ]

1 hour ago, lrt75914 said:

I don't know if this factors into your decision making at all but as far as I can remember variable focal-length lenses tend to use upwards of 10 optical elements in their design. This may reduce the light gathering capability of your variable zoom lens, when compared to a telescope with similar aperture and focal length.

Thanks for your reply. Yes that will reduce the amount of light being gathered. but i think 1 more minute for each sub will compensate the less light, right?

[StuartJPP]

I wouldn't particularly buy a telephoto lens to use solely as a refractor however if you are going to use the lens for daytime photography as well then it can make sense.

Although I have got a Canon 300mm f/4 L IS lens I don't use it much for astrophotography, that is almost exclusively reserved for my Canon 500mm f/4 L IS lens which is much better optically.

I think the 500mm compares pretty well with some scopes, it isn't a substitute for a premium scope but performs reasonably well and it is pretty fast at f/4, however it's cost is pretty eye-watering, luckily I use it mostly for wildlife photography already.

I don't know how well something like the new Nikon 200-500 f/5.6 will work, it should be decent but I haven't seen any reports, Also not sure if there is any creep in the zoom.

Not an easy decision I'm afraid.

You wouldn't want to be stopping down an f/5.6 lens much, 3 stops as you mention would make it an f/16! 

 

[DaveS]

What telescope mount have you got? Unlike visual astronomy, when you come to astrophotography the mount is the most important part of the system. i know this sounds crazy at first, but if your mount cannot hold your target steady through a long exposure then all your efforts will be in vain. By "steady" we are talking about no more than 1, or perhaps 2 arc-seconds in any direction, through a 5 or 10 minute exposure.

[RikM]

Here is a quicky from last night taken with a zoom lens. Okay, so I pointed it in the wrong direction but it shows you can use one. This was a 70-200 f/2.8 stopped down to f/4. 

[ollypenrice]

2 hours ago, RikM said:

Here is a quicky from last night taken with a zoom lens. Okay, so I pointed it in the wrong direction but it shows you can use one. This was a 70-200 f/2.8 stopped down to f/4. 

The star image quality is exceptionally good, here, and better than what I have normally seen. Great stuff. The answer I post below needs to be seen in the light of this excellent image which argues against what I'm about to say! 

In a nutshell my lens versus scope response would be to say that zooms have a lot of elements and that this is not a problem in terms of light through-put so much as in terms of scatter/dispersion/aberration. Nothing of which I'm aware presents an optical challenge that is as severe as a star. A lens which seems great from edge to edge in daytime photography can produce a royal mess on stars. Lens designers have more factors to consider than scope designers, the obvious one being depth of field. A scope works at infinity and the user wants everything in focus. The daytime photographer frequently does not.

If you go for a lens you can stop it down from the front for 'no spikes.'.

Olly

[Alien 13]

I can only speak Canon but the short zooms 70-200 are very good for astro the f 2.8 might even be better than the prime version, I never worry about the number of elements in a lens a decent APO might have 6 or more if you include the FF what does matter is the quality of the glass.

Alan

[happy-kat]

You already have a ST 102 why not just use it. Yes it will show some CA but it can be processed out, if it gives a start to try before making decisions to buy why not use what you have.

Edit: reading again I guess 'had' is past tense so no longer have the 102 to try with.

[ReZ]

4 hours ago, ReZ said:

Thanks for your reply. Yes that will reduce the amount of light being gathered. but i think 1 more minute for each sub will compensate the less light, right?

Thanks for your time. Those glass are superb but Expensive as hell :)) . I didn't mean a full 3 stops. Blame the newbie. i meant to max to f/8.

[ReZ]

3 hours ago, DaveS said:

What telescope mount have you got? Unlike visual astronomy, when you come to astrophotography the mount is the most important part of the system. i know this sounds crazy at first, but if your mount cannot hold your target steady through a long exposure then all your efforts will be in vain. By "steady" we are talking about no more than 1, or perhaps 2 arc-seconds in any direction, through a 5 or 10 minute exposure.

I plan on buying a NEQ6. That should be sufficient even when I upgrade my rig.

[ReZ]

3 hours ago, RikM said:

Here is a quicky from last night taken with a zoom lens. Okay, so I pointed it in the wrong direction but it shows you can use one. This was a 70-200 f/2.8 stopped down to f/4. 

That is a superb Picture there. well-done. Of course your lens is quiet good for the purpose, f/2.8 down to 4 makes either canon or nikon lenses a great piece of glass.

[ReZ]

52 minutes ago, ollypenrice said:

The star image quality is exceptionally good, here, and better than what I have normally seen. Great stuff. The answer I post below needs to be seen in the light of this excellent image which argues against what I'm about to say! 

In a nutshell my lens versus scope response would be to say that zooms have a lot of elements and that this is not a problem in terms of light through-put so much as in terms of scatter/dispersion/aberration. Nothing of which I'm aware presents an optical challenge that is as severe as a star. A lens which seems great from edge to edge in daytime photography can produce a royal mess on stars. Lens designers have more factors to consider than scope designers, the obvious one being depth of field. A scope works at infinity and the user wants everything in focus. The daytime photographer frequently does not.

If you go for a lens you can stop it down from the front for 'no spikes.'.

Olly

Thanks for your cm Olly, I had the step-down rings in mind, this is a way cheaper solution (due to 95 mm filter size of the lens).

[ReZ]

28 minutes ago, happy-kat said:

You already have a ST 102 why not just use it. Yes it will show some CA but it can be processed out, if it gives a start to try before making decisions to buy why not use what you have.

Edit: reading again I guess 'had' is past tense so no longer have the 102 to try with.

Yep I sold that one.

[lrt75914]

57 minutes ago, Alien 13 said:

I can only speak Canon but the short zooms 70-200 are very good for astro the f 2.8 might even be better than the prime version, I never worry about the number of elements in a lens a decent APO might have 6 or more if you include the FF what does matter is the quality of the glass.

Alan

I'm still a novice when it comes to astrophotography so anything I say should be taken with a grain of salt.  My dad has a superb Sigma 105mm Macro lens that we used to capture a pretty nice image of the Orion nebula last year. I just tend to think that above a certain focal length (>300mm) it might be best to use a dedicated telescope since camera lenses are not build with astrophotography in mind. As far as I remember some of the Nikkor zoom lenses feature a 19-22 element optical design, which is a lot of glas the light needs to pass through to reach the camera sensor. Weather this will have a significant impact on the light through-put I cannot say, though. I just thought it might be something worthwhile to consider. 

[RikM]

Mine is the Canon EF 70-200mm f/2.8L IS II USM. 23 elements in 19 groups with 5 UD and 1 fluorite element. In this case, in front of a 7DmkII (4.2"/pixel) sat on an old EQ3-2 with an RA drive. The image was at 200mm 16x 2min subs at ISO640 with a set of bias subtracted flats, no darks.  Pointing slightly to the left of the North America nebula  

I have quite a bit of light pollution and no LP filter on this camera. I wonder if any diffraction effects / scatter may be lost in my sky glow?  Every time I've taken this lens out to a dark site I've been clouded out.

[wxsatuser]

6 hours ago, RikM said:

Mine is the Canon EF 70-200mm f/2.8L IS II USM. 23 elements in 19 groups with 5 UD and 1 fluorite element. In this case, in front of a 7DmkII (4.2"/pixel) sat on an old EQ3-2 with an RA drive. The image was at 200mm 16x 2min subs at ISO640 with a set of bias subtracted flats, no darks.  Pointing slightly to the left of the North America nebula  

I have quite a bit of light pollution and no LP filter on this camera. I wonder if any diffraction effects / scatter may be lost in my sky glow?  Every time I've taken this lens out to a dark site I've been clouded out.

I have the same lens, recommended by Tony Hallas.
It has an impressive set of figures on Photozone but mine has slightly different stars on each side of the frame.

If your lucky you may hit a good one but be prepared to hit the odd bad one.
BTW it's a fantastic lens but as Olly points out, stars will always find out the not so good ones.

Out of all the lenses I have, one of the best is the Canon 200mm f2.8 L II.
I have two of these and they are both equal and can just be used at f2.8, it's not perfect, darn pixel peepers.

[Knight of Clear Skies]

Personally I'd steer clear of the 200-500mm Nikon zoom for AP unless you can find good images from someone already using it (searching on astrobin might be worth a try). Optical design is a compromise and most zoom lenses aren't designed to produce a flat field at infinity. Cheap prime lenses are usually good for AP, I'd recommend them as a starting point. Short focal lengths and fast focal ratios are a good way to get going, everything is more forgiving and and you get to learn from errors more quickly.

Just a thought, but if you're thinking of imaging at up to 500mm focal length have you budgeted for guiding?

[ReZ]

3 hours ago, Knight of Clear Skies said:

Personally I'd steer clear of the 200-500mm Nikon zoom for AP unless you can find good images from someone already using it (searching on astrobin might be worth a try). Optical design is a compromise and most zoom lenses aren't designed to produce a flat field at infinity. Cheap prime lenses are usually good for AP, I'd recommend them as a starting point. Short focal lengths and fast focal ratios are a good way to get going, everything is more forgiving and and you get to learn from errors more quickly.

Just a thought, but if you're thinking of imaging at up to 500mm focal length have you budgeted for guiding?

There's still no pictures of this Lens. but there are some good shots done by Sigma and Tamron which i mentioned above.
Yes I will buy either Syn-guider or Orion starshooter. Personally I prefer mono ccds like (QHY5 or ZWO ASI 120mm) but there's no dealer here.