Can you compare a RASA 8 to a Canon 400mm f/2.8L photolens

[festoon]

Found some wonderful images over the past few months of users imaging with the RASA 8 scope. And when paired with an APS-C sensor giving amazng widefield shots. If I ever get the time to set up a perminent imaging set up and observatory, I'll probably end up buying a RASA.

Probably a stupid question, but I was wondering would there any comparison if you had a Canon 400mm f/2.8 lens or a similar lens? Its a fraction of the weight (2.8kg vs 7.7kg) and no central obstruction. The usable diameter of the RASA is still larger with an effective diameter of 180mm compared to 142mm and the RASA is optically faster. Probably the reason this is not compared - The price of the Canon lens is significantly more £12,000 new compared to the RASA at £2000.

However, I've been amazed with what I've seen using the Samyand 135mm f/2 lens and just out of curiosity what are your thoughts on the question?

[newbie alert]

Too much emphasis on f ratio..

[festoon]

9 minutes ago, newbie alert said:

Too much emphasis on f ratio..

Is there an issue with discussing f ratio and imaging?

[newbie alert]

I see it as the 3rd componant... focal length and aperture are the key componants , to make it faster or slower as you put it you have to alter either the aperture or the focal length

Focal length ÷ aperture = f ratio.. you can't change it..

[catburglar]

I think I saw a comparison between one of the canon L series lenses and a WO Redcat- the results were similar....I'd expect a similar situation here...with some important things to note....they are both 400mm FL and  therefore will have the same image scale if used with the same sensor. As you note this means that the RASA will be faster.

But, I would also expect the lens to be slightly less sharp if used at it's maximum  aperture- this is typical of many camera lenses even good ones. So to get comparable sharpness, you'd probably need to stop the lens down, which would make the RASA speed advantage even more significant. In which case the lens will produce diffraction spikes when you stop it down because the aperture blades aren't circular...You can get around this by using a round aperture mask.

So- if you want to operate at 400mm and you want to do it quickly, I think the RASA is a better option than the canon lens, unless you also want something for conventional daytime photography...

 

But I've never used either of these, so I have no practical experience

[festoon]

Thanks @newbie alert
Sure, and in this case what I was wondering is you have two optical systems with the same focal length and a similar aperture.

One is designed and well known as an imaging scope (the RASA). I was wondering if you could get comparable results from the canon?

Edited February 4, 2021 by festoon

[festoon]

@catburglar, yes with my Samyang 135mm I do get sharper images when stopped down slightly, but with the effect of introducing diffraction spikes.

Do you not get a similar effect on the RASA with wires from the camera crossing the optical path?

[Alien 13]

I think the Canon lens would be quite happy wide open and be sharper than the RASA after all it was used in the "dragonfly" telescope project.

Alan

[newbie alert]

3 minutes ago, festoon said:

Thanks @newbie alert
Sure, and in this case what I was wondering is you have two optical systems with the same focal length and a similar aperture.

One is designed and well known as an imaging scope (the RASA). I was wondering if you could get compatible results from the canon?

The canon has a aperture of 200mm?

Also most won't use it at it's full f ratio and stop it down, when you stop a lens down you reduce the aperture..

[festoon]

Thanks @Alien 13...I have never heard of the dragonfly project...what was that?

[Alien 13]

2 minutes ago, festoon said:

Thanks @Alien 13...I have never heard of the dragonfly project...what was that?

Its this one...https://www.dragonflytelescope.org/

Alan

[festoon]

1 minute ago, Alien 13 said:

Its this one...https://www.dragonflytelescope.org/

Alan

Wow that is awesome! That is one amazing imaging rig!

[festoon]

Sorry had to post this picture from the gallery 

 

[Alien 13]

1 minute ago, festoon said:

Sorry had to post this picture from the gallery 

 

You beat me to it, its very impressive and not cheap...

Alan

[festoon]

Phew! Yes just think how much that rig cost to build.

What a fantastic project to work on!

[festoon]

There’s even a photo of the setup similar to what I was thinking of in this post

 

 

[Nik271]

We need to find a lucky person who owns both the RASA 8 and the Canon 400 f2.8 and ask nicely for comparison shoot out 

The main reason the Canon is so expensive is the extra electronics and lens elements for autofocus and image stabilization. I just checked it up: 17 lens elements inside. Wow!

 

[festoon]

4 minutes ago, Nik271 said:

We need to find a lucky person who owns both the RASA 8 and the Canon 400 f2.8 and ask nicely for comparison shoot out 

The main reason the Canon is so expensive is the extra electronics and lens elements for autofocus and image stabilization. I just checked it up: 17 lens elements inside. Wow!

 

Yes the technology of the lens itself is pretty amazing. It would be cool if Canon sold 'for astronomy' rejects with non functional IS or focus

[Alien 13]

4 hours ago, Nik271 said:

We need to find a lucky person who owns both the RASA 8 and the Canon 400 f2.8 and ask nicely for comparison shoot out 

The main reason the Canon is so expensive is the extra electronics and lens elements for autofocus and image stabilization. I just checked it up: 17 lens elements inside. Wow!

 

If you looked at a top end 140mm refractor it would have 5 or 6 elements minimum and that would only get you to f/7 ish so 17 is reasonable I think.

Alan

[festoon]

Had to check it out myself, and learned that there are 3 versions of the 400mm f2.8 lens, the latest being the lightest, with the first Canon lens designs featuring new glass materials. "This glass has a comparatively higher refractive index than general low-dispersion glass, and has a low specific gravity. By using the new glass material in the first, large-diameter lens element [which cannot be reduced in size due to the focal length and max aperture requirements], the weight is reduced and spherical and chromatic aberration are suppressed."

[Captain Scarlet]

“The price of the Canon lens is significantly more £12,000 new compared to the RASA at £2000.”

While that’s true for the very latest model of the Canon 400/2.8, you should be able to pick up a top-condition 400/2.8 L IS USM for around £3k I reckon, perhaps cheaper. All the versions of this lens-spec have had outstanding image-quality I understand. What’s changed has been that it’s been made lighter and lighter.

I know because I own one, and it produces amazing images. But mine is gen 2 or 3 I think, the first IS version, and it’s heavy! Around 4.5kg. I totally love it.

Magnus

[festoon]

10 minutes ago, Captain Magenta said:

I know because I own one, and it produces amazing images. But mine is gen 2 or 3 I think, the first IS version, and it’s heavy! Around 4.5kg. I totally love it.

Fantastic to hear that someone is using this lens for imaging. And it’s really great information regards the versions. At 4.5kg it’s still significantly lighter than a RASA8. Do you have any images that you please can share?

Now I’ll have to start looking out for these second hand  

[festoon]

Another +1 for the canon lens is that you can use a filter wheel with it.

[Captain Scarlet]

Ah I should have added I’m not an astro-imager (yet ... it’s only a matter of time), just visual for photography. For terrestrial use and bird-photography in particular it’s been amazing for me. Though I did stick it on a skytee2 and photograph a full moon once and stacked a few tens of frames with good results. 

[tooth_dr]

8 hours ago, festoon said:

Another +1 for the canon lens is that you can use a filter wheel with it.

You can but it’s tight with 44mm back spacing to the lens collar.  Taking 17.5mm back focus for the average camera leaves 26.5mm to fit filter wheel and adapters.  Atik EFW is 22mm so this just leaves 4.5mm for adapters.  Off the shelf Canon-M48 adapters measure 10mm.  This means you end up too far away and can’t reach focus at infinity.  I have made up a custom M54-Nikon lens adapter that has a length of about 7mm.  This works because the Nikon lens back focus is 46.5mm.  I think that you could use a EFW with this lens but it wound require a thin EFW (I think QHY make one at 17mm) and a custom adapter.

Edited February 9, 2021 by tooth_dr