Super telephoto (2.8/300) vs small APO thoughts?

[GTom]

Some really high end DSLR optics, such as Nikon 2.8/300 (various editions, even the newest VR version) are becoming affordable as the photography market moves on to mirrorless, with new lens designs.

I was wondering how such a beast even stopped down to e.g. 90mm aperture would compare to a 70-80mm triplet+reducer combo?

 

CaNikon high end glass tends to be well corrected especially if using a small sensor, the only major drawbacks I see is poor astro-ergonomy and very likely poor thermals.

[Elp]

It'll likely be decent but just because some lenses make for fantastic daytime lenses doesn't necessarily mean they work for astro. I've used at least a dozen lenses and pretty much all have star elongation/CA/coma in some instances. Adjusting BF distances can be tricky, you pretty much don't have that option when using camera bodies which is their optimum usage scenario and they still distort.

The best bar none for speed, size, price is the SY135, it beats most telescopes too. A telescope however will be better corrected out of the box, especially with the correct field flattener and backfocus distance set, and fine focusing will be much much easier.

[vlaiv]

Try using such lens to view the moon or planets and you will immediately understand the difference vs 80mm scope

[Fraunhoffer]

I dont much imaging but when I do Ive been using an old  Sony camera with old Canon lenses via an adapter.

* As previous post - I was quite surprised to see the amount of coma in the lenses. Its not much, but I was surprised to see any. I think the extreme contrast of the dark sky and pinpoint stars makes this much more noticeable than a daytime photo.
* Its not easy to figure out how to use readily available round filters with the camera lens adapter combo. Much easier with a small telescope.
* I've never been sure if the amount of glass in the camera lenses reduces the amount of light a bit.
* Mounting a big telephoto (that you would normally support by hand) can be tricky. Mine were quite heavy to support just via the camera body screw mounts. Its not do bad if the lens has a foot with a thread somewhere near the CoG (which usually means you can rotate the camera).  Trying to use rings can be pain without fouling or squeezing the focus or aperture ring. Telescope mounting is much more straightforward.
*  Whether its worth buying an expensive one specifically to do this - Im not sure, unless you also have another use for it / birds / ships etc. Or, already have it.

Hope that helps.



 

Edited Friday at 09:43 by Fraunhoffer

[Tony Acorn]

42 minutes ago, Elp said:

It'll likely be decent but just because some lenses make for fantastic daytime lenses doesn't necessarily mean they work for astro. I've used at least a dozen lenses and pretty much all have star elongation/CA/coma in some instances. Adjusting BF distances can be tricky, you pretty much don't have that option when using camera bodies which is their optimum usage scenario and they still distort.

The best bar none for speed, size, price is the SY135, it beats most telescopes too. A telescope however will be better corrected out of the box, especially with the correct field flattener and backfocus distance set, and fine focusing will be much much easier.

Thanks Elp,

If imaging against a narrower background landscape, I'm thinking that perhaps the Samyang 85mm or 135mm lenses are probably a good choice to sit on my Nikon Z6ii. I'm more than happy with my Samyang 14mm f/2.8 MF for wider field astro work but am considering how best to shoot Comet C/2023 A3 against some coastal chalk stacks later next month. This month's Lunar Eclipse will be best shot at 70mm from my chosen location to capture the event with an old water tower in the foreground. Unfortunately my 70mm lens only opens to f/4.

[Elp]

I've found the (Pentax) Asahi Takumars m42s quite excellent for 50 year old lenses, the 135 and 200mm in particular are pretty much flat across an APSC frame (even modern lenses struggle to do such a simple thing). They normally have to be stopped down to F5.6 or so, and they were designed to focus red and green at different points so a standard lens will have red star halos at infinity focus, with a quick adjustment of the focus barrel hard stop, you can then focus past this point and into the green, then back off until red and green are not seen.

I ultimately sold on the 200mm as I replaced it with a 180mm other lens, still have the 50mm (this comas/astigmatises at f1.4, again needs to be stopped down), still have the 135mm (excellent) but generally use the SY135 because it's so good.

I know the Canon 200mm USM L2 is supposed to be good also, beyond that I haven't really looked as my Z61 telescope I can start using at 234mm and beyond.

[GTom]

4 hours ago, vlaiv said:

Try using such lens to view the moon or planets and you will immediately understand the difference vs 80mm scope

Viewing is a different story. A flatfield astrograph can be (very likely will be) a crap planetary scope. Yet it (should...) selivers stellar DSO results, what it is designed for. A 2.8/300 telephoto is not far from these, depending on coma/astigmatism results.

Edited Friday at 13:48 by GTom

[vlaiv]

2 minutes ago, GTom said:

Viewing is a different story. A flatfield astrograph can be (very likely will be) a crap planetary scope. Yet it (should...) selivers stellar DSO results, what it is designed for. A 2.8/300 telephoto is not far from these, depending on coma/astigmatism results.

My point is that any lens is far away from being diffraction limited optics and that any scope will outperform any lens on that front. Sure you can use lens to create wide field shots - but any image produced with the telescope will simply be sharper.

 

left M31 taken with Samyang 85mm lens and right M31 taken with 80mm scope ...

[GTom]

That one is simple, primarily an image scale problem. It doesn't make sense comparing a 85mm FL system with 9"/pixel vs a telescope with approx 2"/pixel (80/400 with 533 sensor).

The 2.8/300s are interesting exactly because their focal lengths are near the typical "travel apos" AND in contrary to a 1.4/85, lots of them are great even wide open.

[vlaiv]

4 minutes ago, GTom said:

The 2.8/300s are interesting exactly because their focal lengths are near the typical "travel apos" AND in contrary to a 1.4/85, lots of them are great even wide open.

It's still a photo lens:

MTF of Nikon

MTF of Samyang

Note that both show 30 line pairs per mm - that being equivalent of 16.666um pixel size and both are below 1 in contrast

[GTom]

1 is the theoretical max, its a 0 to 1 scale. I was wondering how a typical triplet+0.8x reducer system would fare in an mtf test?

Here I found some lab data referred amongst loads of daylight pictures. It says:

"I benchmarked the 300mm f/2.8 using the 45.6MP( 8256 x 5504) D850 and Imatest software. It delivers strong results on the demanding high-resolution sensor. At f/2.8 the combination resolves 3,779 lines on Imatest's sharpness test. The evaluation is center-weighted, but that doesn't matter—results at the extreme edges of the frame are as good as in the center. We classify the performance in the very good range, with 2,750 lines as the lowest acceptable score."

I am not all pessimistic trying such optics with a crop sensor astrocamera. Main attraction is a maybe 95mm useful aperture (after masking a good 10mm off to keep the diffraction issues at bay). That's significantly more light gathering than the actually heavier gt71+reducer combo.

Edited Friday at 16:27 by GTom

[vlaiv]

14 minutes ago, GTom said:

1 is the theoretical max, its a 0 to 1 scale. I was wondering how a typical triplet+0.8x reducer system would fare in an mtf test?

In my experience, 80mm triplet + 0.8 reducer is usually capable of say ~4um pixel size in normal seeing - or about 2"/px sampling resolution - and that is mostly due to atmosphere.

Here is crop of a single M31 sub at full resolution:

I would say that is at least x2-x3 sharper than what a lens can produce. If you note 4 stars in line that are nicely separated in top left quadrant of the image - here they are in lens image:

Btw, here is what M31 looks with Nikon 300mm F/2.8:

https://www.cloudynights.com/topic/743968-using-a-nikon-300mm-f28-prime-as-a-refractor-for-dso-imaging/

So those stars do get separated - but barely.

 

[GTom]

I might be looking at the wrong thing, but on the bottom image I see darker, more pronounced splits between the 4 stars.

I need to see more of these from both sides 🤩. What I do know is that the unmasked, wide open 2.8/300 produces terrible star shapes because of all kind of diffractions along the path. Sadly none of the astrobin authors I found cared to cut a simple aperture mask.

[Elp]

If I'm looking at the right area, and given more time and better NR, I think my SY135 can resolve those stars (135mm FL and just less than the full 60mm aperture as it's at F2.8). This was just 2 hours messing around with my 485mc when I first got it:

[GTom]

9 hours ago, Fraunhoffer said:

*  Whether its worth buying an expensive one specifically to do this - Im not sure, unless you also have another use for it / birds / ships etc. Or, already have it.

Yupp, that's exactly the case. I am also doing nature, ships, planes etc daylight photography: there even a manual focus camera lens is far more productive if the target is moving.

[Elp]

A bit slower but there are the interesting form factor Askar SQA55 (there's also an 85 but no published info as of yet), which are petzval and have dual focusing rings to get fine focus, can be used as a general purpose lens too. It's too close in FL to my current equipment though, but may be a decent Redcat alternative if user reviews prove productive (also the Redcat is supposed to be good).

[GTom]

12 hours ago, Elp said:

A bit slower but there are the interesting form factor Askar SQA55 (there's also an 85 but no published info as of yet), which are petzval and have dual focusing rings to get fine focus, can be used as a general purpose lens too. It's too close in FL to my current equipment though, but may be a decent Redcat alternative if user reviews prove productive (also the Redcat is supposed to be good).

55mm vs ~95mm is quite a difference, especially if your time is limited. The scope is listed around £700, that buys you e.g. a clean AF-S Nikkor f2.8/300. That counts for daylight use as well. I would love a 100/400 flat astrograph but those are definitely not in this price and weight range.

[Elp]

To be honest, even after using 6 inch aperture I find you still have to image for a few hours to get a decent image, f ratio is quite the myth, you really need aperture if you're stuck for time.

Edited 18 hours ago by Elp

[PhilB61]

I started my DSO imaging using DSLR lenses, I tried a Nikon 70/200 f2.8 af-s, Nikon 200/500 f4.5 af-s and Sigma 400 telephoto. All suffered from poor star shapes/ colours and difficult focusing, too much backlash in manual focus mechanisms etc, although the two Nikon's are great daytime lenses. I then picked up a Skywatcher 72ed at a fraction of the cost of the Nikon's and the difference in image quality and ease of use was immediate and massive, I would never go back to using camera lenses other than for very wide nightscape for which I use a Tokina 11-16 f2.8.

I know you are considering Tele rather than zoom lenses so maybe you will have a better experience than I did?

[Elp]

Not all lenses are built equal though. Only certain lenses perform for astro imaging. Read enough forum posts and the same models keep cropping up.

[Captain Scarlet]

Being entirely visual, I've only ever taken one deep sky photograph, which was of M31. That was in 2017, using my Canon 300/2.8 on my AZ-EQ6. In other words a 107mm fast refractor. I took nine 50 second subs and combined them in PI I think, using only its most basic integration tool. The little line of four stars that @vlaiv mentions seem reasonably sharp to me? I've attached the small cropped jpg and the much larger uncropped tif.

Magnus

 

integration_ABE_2.tif

Edited 15 hours ago by Captain Scarlet

[vlaiv]

11 minutes ago, Captain Scarlet said:

The little line of four stars that @vlaiv mentions seem reasonably sharp to me? I've attached the small cropped jpg and the much larger uncropped tif.

I'm talking about:

1. resolving of those stars

2. gaps between stars

We have seen that better lens have the ability to resolve those stars and most images show them as 4 stars - but look at 80mm refractor enlarged image:

each star is roughly 1/3-1/4 of gaps between the stars, but if we do the same with your image:

each star is roughly the distance of that gap.

Since the sharpness / resolution of the lens is directly related to star size - we can see that 80mm refractor produces much tighter stars / sharper image.

Now, you don't need much sharpness for wide field work - but if you want a bit more sharpness in your image - then refractor is way to go.

 

[GTom]

...btw that's a fantastic result after just EIGHT minutes and not overly sophisticated post processing. Under our Scottish "you got 30 minutes clear sky every 2 months" climate that's the way to go...

3 hours ago, Captain Scarlet said:

Being entirely visual, I've only ever taken one deep sky photograph, which was of M31. That was in 2017, using my Canon 300/2.8 on my AZ-EQ6. In other words a 107mm fast refractor. I took nine 50 second subs and combined them in PI I think, using only its most basic integration tool. The little line of four stars that @vlaiv mentions seem reasonably sharp to me? I've attached the small cropped jpg and the much larger uncropped tif.

Magnus

 

integration_ABE_2.tif 114.27 MB · 1 download

 

[GTom]

4 hours ago, Elp said:

Not all lenses are built equal though. Only certain lenses perform for astro imaging. Read enough forum posts and the same models keep cropping up.

100% valid statement. There are definitely good camera lenses out there that do great under the stars. However, major testing sites hardly ever test astronomically relevant parameters: randomly picking a "good lens" doesn't work. The ones @PhilB61 mentioned are not amongst the top ones. The sy135, canon 2.8/200 masked to f/4 are great though, heard good things about the old 4.5/300 and 2.8/180 nikkprs too. First of, I wouldn't even start with a zoom lens...

Edited 10 hours ago by GTom

[Elp]

25 minutes ago, GTom said:

First of, I wouldn't even start with a zoom lens...

I'd never entertain it either, all mine are primes that I've kept for AP purposes. I recently acquired a Sigma Art 18-35, considered to be one of the best lenses for apsc daytime, it's absolute pants for AP. Their fixed 40 though is one of the best as are other primes in the range.

Edited 10 hours ago by Elp