Why don’t you reduce weight by just using less focal length on the telescope and adding barlows or smaller focal length eyepieces on the telescopes

[kingsbishop]

Here is something that I don’t understand if you want to reduce weight of the telescope how come people don’t use less focal length on telescopes and instead using smaller focal length eyepieces or barlows I understand that it would effect the eye relief but that wouldn’t be a big deal especially if your using a camera so what is the real reason do you lose light or something can someone please tell me?

[Cosmic Geoff]

Short answer: if this worked, somebody would have tried it already. 🙂

Longer answer: adding a Barlow does not reverse the various aberrations (spherical, colour, coma) introduced by using a short focal ratio. Flattener/correctors available for short-focus imaging telescopes are usually wincingly expensive.  For visual, you can use a shorter focal length eyepiece, but these tend to be more elaborate than what you could get away with on a f10 telescope.

I have tried adding a x2 Barlow to my f5 Startravel.  It seemed to improve the image quality a bit, but not enough to make it equivalent to a f10 Evostar. 

There is a budget Newtonian telescope which incorporates a Barlow-type lens to shorten the overall length. It has a terrible reputation as a useless newbie trap.

[KP82]

What you're looking for has already been achieved by scopes with folded optics (ie. cassegrain) that offer much better image quality than your proposed barlow solution. The weight of cassegrains depend on the size of their mirrors, but they are already as compact as possible compared to newts or refractors with an equivalent aperture.

The aforementioned budget newt (aka bird jones) produces poor views mainly because of its spherical mirror. But even if it had a parabolic one, it would be extremely sensitive to collimation errors and adding a barlow wouldn't change anything.

Edited December 13, 2022 by KP82

[Louis D]

There are very short focal length Dobs made to be more ergonomic to observe with.  Here's a Webster 28" f/2.7:

Is this what you are suggesting?  The problem is that it is difficult to make short focal length scopes with the same optical quality as longer focal length telescopes.  As a result, they are more expensive aperture for aperture.

Edited December 14, 2022 by Louis D

[Ratlet]

53 minutes ago, Louis D said:

There are very short focal length Dobs made to be more ergonomic to observe with.  Here's a Webster 28" f/2.7:

Is this what you are suggesting?  The problem is that it is difficult to make short focal length scopes with the same optical quality as longer focal length telescopes.  As a result, they are more expensive aperture for aperture.

That was a fantastic read Louis.  Thanks for sharing it.  Would love to experience a scope like that.

[Nakedgun]

~

Wait. Isn't this the "longest-thread-title-in-the-history-of-SGL" thread?

Just wondering.

 

.

[niallk]

On 13/12/2022 at 03:18, kingsbishop said:

Here is something that I don’t understand if you want to reduce weight of the telescope how come people don’t use less focal length on telescopes and instead using smaller focal length eyepieces or barlows I understand that it would effect the eye relief but that wouldn’t be a big deal especially if your using a camera so what is the real reason do you lose light or something can someone please tell me?

At least for a dob, the main weight that is cumbersome is the mass of the mirror.  To first order, it is more a function of aperture than focal length.  Reducing focal length exacerbates other challenges and increases cost - like the 28" Webster above.

[Louis D]

I think the OP was asking, "Why do we put up with long f-ratio scopes just for high power usage?  Why not just buy shorter f-ratio scopes and use Barlows or short focal length eyepieces to get to higher power."  And the partial answer is, that would be great if optical quality didn't suffer at shorter f-ratios, particularly in refractors.  However, as I stated above, shorter f-ratio scopes of high quality get real expensive real fast.

[Deadlake]

Short focal length refractor can equal long focal ratio scopes for optical performance.

However the main issue (ignoring cost) is weight.

The len's cell is very heavy and on a small scope can make balancing harder. 

At the same time having a fast scopes, allows for rich field views and adding a Barlow allows for high contrast views on planets. 

It does give you two scopes in one.

Look at AP it scopes are around F6.3 in speed and are great for AP or visual use and very portable which a f7.7 scope for Takahashi will not give you.

 

[scotty38]

Like most of the other threads I think you're all talking to yourselves anyway 🤣