Questions on scopes with regards to imaging

[MonsterMagnet]

I think I know the answers to these questions but I would be very pleased if I could get some views from here to confirm or put me straight:

• What is the advantage of getting something like a 90mm refractor over a 70mm refractor when using a reducer brings the FL of the 90mm to about the same as that of the 70mm?  Is it simply more flexibility in that you can use the 90mm without a reducer at the native FL for more magnified image?
• With a double and a triplet one requires a field flattener, with a quintuplet upwards this isn't the case but if you use a reducer on those scopes then you once again need to use a field flattener?
• A triplet provides better colour than a doublet (all being equal) but does a quintuplet also usually provide that colour or is it usually a doublet with two more corrective lens?

Cheers.

MM

 

Edited February 28 by MonsterMagnet

[Ags]

All a quintuplet is, is a doublet/triplet with a built in field flattener. A triplet is just a doublet with an extra lens for better color correction, but both doublets and triplets have about the same field curvature.

[PhilB61]

A 90mm refractor will collect more light and hence image quicker than a 70mm with the same focal length and possibly resolve slightly more detail.

[900SL]

A 90mm scope will collect more photons compared to a 70mm scope. And it will also be faster with a reducer, which lowers the focal ratio

Then there's pixel scale, (the relationship between focal length and pixel size). For my 3.75 micron pixels, a 90mm aperture f6 is better than a 70mm f6 (undersampled)

And then FOV. Some targets may better suit the 90mm FIV, and some might not

Edited March 2 by 900SL

[GTom]

On 28/02/2024 at 19:28, MonsterMagnet said:

I think I know the answers to these questions but I would be very pleased if I could get some views from here to confirm or put me straight:

• What is the advantage of getting something like a 90mm refractor over a 70mm refractor when using a reducer brings the FL of the 90mm to about the same as that of the 70mm?  Is it simply more flexibility in that you can use the 90mm without a reducer at the native FL for more magnified image?
• With a double and a triplet one requires a field flattener, with a quintuplet upwards this isn't the case but if you use a reducer on those scopes then you once again need to use a field flattener?
• A triplet provides better colour than a doublet (all being equal) but does a quintuplet also usually provide that colour or is it usually a doublet with two more corrective lens?

Cheers.

MM

 

As others also stated, the key advantage is the faster focal ratio, meaning that at the same FL you'll need approx 40% shorter exposures to reach the same signal strength. Quintuplets are there to get rid of the image curvature and cover a larger than 1/2" sensor properly. Basically an apo/semiapo + flattener or reducer kit does the same.

[MonsterMagnet]

Thanks for all the help on this thread.

I have picked up a StellaMira 90mm ED Triplet second hand which I think will be a great scope for me going forward.

MM