APO Frac versus Camera lens quality question

[Tommohawk]

… I mean its a question about the relative quality of those items, not that its a quality question.

I've been using a Tamron SP300 F2.8 camera lens for some time not, initially with  a DSLR and now with an ASI1600. It fits together beautifully mechanically and I've been generally pleased with  the results, -  but not 100%.

Three real issues I guess - chromatic aberration, weight and star size. Focusing is usually considered a problem but I do fine with a dedicated Bahtinov and a steady hand.

I cant really use it at F2.8, so usually I go for F4, which means I'm carting around a much  bigger lump than I need to. 2.4kg from memory.

There is a fair bit of CA but I've never used a dedicated refractor to compare it to. Stars always seem a bit bloaty despite reasonable guiding, but again not sure what to expect. 

So the question is - would a budget refractor like the SW Evostar DS PRO be significantly better, or would I have to go a long way upmarket to do any better? I was also looking at the TS Apo 72mm photoline F6 although again that's only a doublet. Have to bear in mind that the TS is dearer and the flattener/reducer is 235Euros - or about 5000 GBP after Brexit.

So that do folk think? I'd really appreciate some advice as I off on hols end of the month and might splash out if I think I could do better. I've posted a typical sub from this lens below as a bench mark - its a RAW CR2 saved as Jpeg, no processing, taken with  the Canon 550D - I didn't have the ASI at that time.

Thanks in advance! 

[vlaiv]

I think decent refractor will beat lens in terms of sharpness and star shapes. CA is non issue for most imaging refractors, except for old faster doublets. Newer ed doublets with good glass elements work fine at small apertures in even F/6 configurations (TS72 F/6 for example)

There are two things that lens excel at - FOV and speed. But this comes at a cost - to get speeds like F/2-F/4 star shapes will suffer - we are leaving realm of diffraction limited optics - similarly to SCT Hyperstar systems - very fast but not diffraction limited.

Lens will always be limited in aperture (unless you go for really bulky and expensive ones) as well. Any 70-80mm refractor will have higher light grasp than most lens. Samyang F/2 comes close to this aperture - it's 135mm at F/2 so max aperture will be 67.5mm. Maybe 200mm F/2.8 lens can beat it with ~71.5mm aperture - so order of 72mm but less than 80mm.

How to deal with small FOV of refractor, as these are usually in 300-400mm FL (small wide field refractors 72-80mm)? Only way to do it is to go for mosaics - shoot multiple panels and stitch them up. Bin your subs so you don't end up with enormous pixel count - or leave at native resolution for large prints - but if you bin them (even x4 for for big panels), you will end up with greater SNR.

[Davey-T]

Discussed this subject with a well known imager and his take on it was that camera lenses aren't built with astronomy in mind and a decent  refractor would win every time.

I think my WOZ61 does a better job than my Canon 300 f/4 on astro' subjects but need to take some images of the same thing on the same evening to compare properly.

Dave

[fwm891]

A telescope is computed to operate at infinity only - therefore in general that's it at it's best . Camera lenses are computed to work over a range and therefore not at best at any distance. Go with a scope everytime

[Tommohawk]

20 minutes ago, Davey-T said:

Discussed this subject with a well known imager and his take on it was that camera lenses aren't built with astronomy in mind and a decent  refractor would win every time.

I think my WOZ61 does a better job than my Canon 300 f/4 on astro' subjects but need to take some images of the same thing on the same evening to compare properly.

Dave

Hi Dave!

Again I'm sure you are correct that the frac has to win, but it would be interesting to compare images side by side. In fact if I go ahead with a frac I'll do exactly that and post the results here for comparison.

14 minutes ago, fwm891 said:

A telescope is computed to operate at infinity only - therefore in general that's it at it's best . Camera lenses are computed to work over a range and therefore not at best at any distance. Go with a scope everytime

Yep that must be true - a system which has to cope with huge variations in focal length must suffer compromises in some way. To achieve this the Tamron lens has 10 elements which in itself must cause some reduction in transmission + a degree of internal reflection.

 

[Tommohawk]

53 minutes ago, vlaiv said:

I think decent refractor will beat lens in terms of sharpness and star shapes. CA is non issue for most imaging refractors, except for old faster doublets. Newer ed doublets with good glass elements work fine at small apertures in even F/6 configurations (TS72 F/6 for example)

There are two things that lens excel at - FOV and speed. But this comes at a cost - to get speeds like F/2-F/4 star shapes will suffer - we are leaving realm of diffraction limited optics - similarly to SCT Hyperstar systems - very fast but not diffraction limited.

Lens will always be limited in aperture (unless you go for really bulky and expensive ones) as well. Any 70-80mm refractor will have higher light grasp than most lens. Samyang F/2 comes close to this aperture - it's 135mm at F/2 so max aperture will be 67.5mm. Maybe 200mm F/2.8 lens can beat it with ~71.5mm aperture - so order of 72mm but less than 80mm.

How to deal with small FOV of refractor, as these are usually in 300-400mm FL (small wide field refractors 72-80mm)? Only way to do it is to go for mosaics - shoot multiple panels and stitch them up. Bin your subs so you don't end up with enormous pixel count - or leave at native resolution for large prints - but if you bin them (even x4 for for big panels), you will end up with greater SNR.

Hmm - not sure what happened there - I responded to  this and it got lost - try again!

Thanks for the input! Agree about the speed - camera lenses push it to the limit and it works fine for many things but not great for star shapes. To be fair though the shapes on the above posted image are pretty good I'd say. TBH that was probably done at F4 though.

The Tamron lens has a 300mm FL, so a TS 72mm at F6 with a reducer will be similar and I can live with that. I came to  the same conclusing re mosaics - I've done a couple of small mosaics and they work well if time consuming.

One good thing about the Tamron is its solid as a rock, and build quality is excellent. By contrast I've read some comments about fracs from big suppliers having pinched and dodgy optics - that would definitely be a backward step.

The other thing is that the Tamron connection to the ZWO camera and filter wheel is a breeze, where I've read that some fracs need a bizarre and unspecified collection of spacers to  get the sensor distance correct and achieve focus - could do without that!

[Alien 13]

I think it depends on the lens, there is no scope equivalent to the Samyang 135 mm f/2 and the Canon 400 mm f/2.8 was chosen over a scope for use in the dragonfly project. The big advantages of a lens are it ability to cover a full frame chip, its speed and its inbuilt focus motor.

The downside is that for good CA performance and star shapes is the cost, for example the 400 mm canon will set you back £12K which is far more than any premium refractor of similar focal length.

Alan

[Tommohawk]

6 minutes ago, Alien 13 said:

I think it depends on the lens, there is no scope equivalent to the Samyang 135 mm f/2 and the Canon 400 mm f/2.8 was chosen over a scope for use in the dragonfly project. The big advantages of a lens are it ability to cover a full frame chip, its speed and its inbuilt focus motor.

The downside is that for good CA performance and star shapes is the cost, for example the 400 mm canon will set you back £12K which is far more than any premium refractor of similar focal length.

Alan

Good points - for the money, youd expect the scope to outperform the equivalent cost camera lens. 

Nobody has commented specifically on the test pic I posted above. Maybe its unfair to ask, and I'm not expecting a guarantee, but - should I expect the Evostar DS PRO to do better?

[Davey-T]

Did you take that in the UK Tom ? If so it's a bit of a low altitude to test things at.

Dave

[Tommohawk]

Just now, Davey-T said:

Did you take that in the UK Tom ? If so it's a bit of a low altitude to test things at.

Dave

That's a good observation! No, I took it in Spain so about 15 degrees higher than here. Still lowish though I suppose. That's where I'm off to in a few weeks, and hoping to do some more imaging from same location. Last time I has the DSLR running at >30deg C ambient! - this time I have the cooled ZWO, but I don't think I'll repeat the same target. 

[petevasey]

I have a TS65Quad which does an excellent job.  It's 420 mm focal length and f6.5 so a lot 'slower' than your lens even stopped to f4 .  Useful that the flattener is built in, - just fit and focus the camera.  Beautiful crisp visually as well.   They now do slightly larger models, but all slower than your lens, and the fastest one is pretty pricey.   I've mainly used mine with my QSI683, but here is an image of Comet C/2014 Q2 Lovejoy  from March 2015 with my (modded) Canon 350D which of course has an APS size sensor.  As you can see the stars are good right into the corners.  (The cluster is of course NGC457).  I also have a Canon EF 400 f5.6L lens which also gives excellent results, but can't be used with an off-axis guider.  This is another shot of the comet, this time with my Canon 700D, and the 400L lens fully open.   Not much to choose between the images, the lens is a lot lighter than the telescope, but nowhere near as versatile for astro work.  And cost a fair bit more than the quad.

Cheers,

Peter

Edited January 12, 2019 by petevasey

[Tommohawk]

47 minutes ago, petevasey said:

I have a TS65Quad which does an excellent job.  It's 420 mm focal length and f6.5 so a lot 'slower' than your lens even stopped to f4 .  Useful that the flattener is built in, - just fit and focus the camera.  Beautiful crisp visually as well.   They now do slightly larger models, but all slower than your lens, and the fastest one is pretty pricey.   I've mainly used mine with my QSI683, but here is an image of Comet C/2014 Q2 Lovejoy  from March 2015 with my (modded) Canon 350D which of course has an APS size sensor.  As you can see the stars are good right into the corners.  (The cluster is of course NGC457).  I also have a Canon EF 400 f5.6L lens which also gives excellent results, but can't be used with an off-axis guider.  This is another shot of the comet, this time with my Canon 700D, and the 400L lens fully open.   Not much to choose between the images, the lens is a lot lighter than the telescope, but nowhere near as versatile for astro work.  And cost a fair bit more than the quad.

Cheers,

Peter

Hi Pete. Yes I really like the idea of a built in flattener not least because you know the spacing is good, but also the design ought to  work better as its custom rather than an add on. Some folks seem to  have had to really play around to get the spacings correct.

If money were no object I'd definitely go for the faster TS Quad - the Imaging Star 71  F4.9 is 1299 Euros, which is a bit beyond my budget. This has full DSLR field, where I only need APS-c or similar for the ASI1600.

Those comet pics are fabulous! And as you say the stars look great even with the camera lens. I've found the Tamron lens to be really excellent and fast as anything for camera work - but I find it too heavy for general photography so it doesn't really tick that box very well. I think I paid £300 for it so its a great performer for the price.

[petevasey]

Just came across this topic again, and thought I'd offer this.  Nothing to do with the main discussion, but looking at the original image, I thought it was worth applying the excellent AstroFlat Pro Photoshop plugin filter to see what if anything it would do to the overall pinkish cast.  And this is the result - just one application at the default settings.  Then a medium application  of Hasta la Vista Green nicely clears the greenish areas.

Cheers

Peter

Edited June 11, 2019 by petevasey

[Tommohawk]

Hi Peter and thanks for that. I posted a raw file just to give fair comparison of the Tamron lens capability, but the stacked and processed image was rather better. 

BTW I went for the TS72 in the end and am still evaluating it. There's another thread about that. As predicted the reducer spacing is proving troublesome!