My one’s longer than your one..... 😉

[johninderby]

Is that single entendre? 😁😁😁

Edited March 31, 2021 by johninderby

[Chris]

36 minutes ago, johninderby said:

Is that single entendre? 😁😁😁

If it isn't, it only JUST qualifies for double 😁 

[JamesF]

48 minutes ago, Lockie said:

If it isn't, it only JUST qualifies for double 😁 

If you want an example of a double entendre, I'll give you one :)

James

[John]

What would be interesting would be to compare the performance of a faster ED doublet with one of these long focus achromats. Something like the Skywatcher ED80 at F/7.5 perhaps ?

Obviously the faster scope could show a wider field of view but could it compete on planetary, lunar and double star observing where the long, slow refractor optics should provide an advantage ?

I'm sure we all have views on what the outcome might be but it would be interesting to do a methodical comparison

 

[Stu]

1 hour ago, John said:

What would be interesting would be to compare the performance of a faster ED doublet with one of these long focus achromats. Something like the Skywatcher ED80 at F/7.5 perhaps ?

Obviously the faster scope could show a wider field of view but could it compete on planetary, lunar and double star observing where the long, slow refractor optics should provide an advantage ?

I'm sure we all have views on what the outcome might be but it would be interesting to do a methodical comparison

 

Very true John. Who is up for lending me an 80ED 😉. Based on the experience I’ve had with these two, presumably the faster scope should show more diffraction that even the F12.5, is that correct? I was surprised by the result as I wasn’t expecting it.

I must say I’m impressed by the colour correction in these scopes. The Telementor for example shows a lot more false colour (from memory). I should compare it with the F12.5 as in theory it should be better due to the smaller aperture and F13.3 focal ratio, but I’m sure it is not.

[John]

3 minutes ago, Stu said:

... Based on the experience I’ve had with these two, presumably the faster scope should show more diffraction that even the F12.5, is that correct? I was surprised by the result as I wasn’t expecting it....

 

My (limited) understanding is that the brighter diffraction ring means that more energy is going there rather than into the airy disk which can be because of modest amount of spherical aberration

There might be other causes as well though.

 

 

[Stu]

2 hours ago, John said:

My (limited) understanding is that the brighter diffraction ring means that more energy is going there rather than into the airy disk which can be because of modest amount of spherical aberration

There might be other causes as well though.

 

 

Ah, so not directly focal length related, that makes sense too. So a well corrected (in SA as well as SA) would likely perform similarly to the F15? I must say the first diffraction ring in the Tak is pretty faint. Might be interesting to compare the F15 to the Tak as well.

[John]

29 minutes ago, Stu said:

Ah, so not directly focal length related, that makes sense too. So a well corrected (in SA as well as SA) would likely perform similarly to the F15? I must say the first diffraction ring in the Tak is pretty faint. Might be interesting to compare the F15 to the Tak as well.

If you could do it, masking the Tak FC 100 down to 80mm would give you an F/9.25 scope to compare with the Scopetech 80's

 

 

 

[Deadlake]

11 hours ago, John said:

My (limited) understanding is that the brighter diffraction ring means that more energy is going there rather than into the airy disk which can be because of modest amount of spherical aberration

There might be other causes as well though.

 

 

The faster scope going to have more field curvature, which will effect the edge of the image. The field curvature can be fixed with a flattener to sync with the EP’s curvature.

I’d argue that most fast scope (F6 and less) have better figured lens (and cost more) so no issue with chromatic shift, and more light into the central airy ring, but spherical abberation hm...

For example John would we see a difference between my LZOS F6 and your F9.2? Difference between a Strehl of 0.98 and 0.99? 😁

[Stu]

50 minutes ago, Deadlake said:

The faster scope going to have more field curvature, which will effect the edge of the image. The field curvature can be fixed with a flattener to sync with the EP’s curvature.

I’d argue that most fast scope (F6 and less) have better figured lens (and cost more) so no issue with chromatic shift, and more light into the central airy ring, but spherical abberation hm...

For example John would we see a difference between my LZOS F6 and your F9.2? Difference between a Strehl of 0.98 and 0.99? 😁

This was an on-axis effect, so not related to field curvature. I would tend to agree that a well corrected faster scope should perform similarly to a slower one with similar strehl ratio. My Tak certainly does not show a bright first diffraction ring despite being significantly faster than the F12.5 for example. I’ll need to do some more testing to see what is what. I’ve actually got a decent artificial star so may be able to test with this down the garden more successfully and in a more repeatable manner.

[Stu]

10 hours ago, John said:

If you could do it, masking the Tak FC 100 down to 80mm would give you an F/9.25 scope to compare with the Scopetech 80's

 

 

 

A cunning plan sir!

[niallk]

On 31/03/2021 at 16:34, Lockie said:

Listening with interest : ) and I'm sorry I just couldn't resist... 

And funnily enough, it's the stubby short ones that draw the gasps of admiration for dobs!! 😂

[John]

1 hour ago, niallk said:

And funnily enough, it's the stubby short ones that draw the gasps of admiration for dobs!! 😂

You could use 80mm F/15 refractors as truss poles for something like that !