reducers and barlows

[bomberbaz]

Ok so here is a question regarding the combined effect of reducer and barlows. In simple terms below:

I am aware that if you add a reducer to a OTA, in lowers the F ratio. EG. F10 OTA plus 0.5 reducer = F5

A barlow basically increases magnification by whatever factor the barlow has. EG, 2x barlow on a 20mm EP gives you a equivalent 10mm EP

My query is if I screw a 0.5 reducer into a EP and then slot it into a 2x barlow, In theory the magnification is not altered but what is the effect on F ratio.  

Cheers

Steve

 

 

[Merlin66]

They "compensate" each other...the f ratio remains unchanged.

(I wouldn't recommend using this configuration - added glass, added problems for no gain)

 

[bomberbaz]

6 minutes ago, Merlin66 said:

They "compensate" each other...the f ratio remains unchanged.

(I wouldn't recommend using this configuration - added glass, added problems for no gain)

 

This was as I thought, thansk for the reply so quick.

steve

[MarkAR]

Along the same lines, what about imaging with a 0.8 reducer/flattener and a 2x or even 4x 2" TeleVue Powermate.

[Louis D]

You'll have vignetting with widest field eyepieces because the barlow will spread the beam outward, allowing some of the ray bundles to slam into the tube walls and be lost forever.  The reducer will then recollimate the central diverging rays and bring them back closer to their original angles of incidence with the exception of those edge bundles which are gone forever.  As a result, the edge near the fieldstop will fuzz out to darkness.  I actually use a barlow/reducer combination to reduce the power of my home-brew binoviewer OCA/GPC.  I put a Meade 140 2x nosepiece on the front of my 1.25" diagonal and a 0.5x reducer on the nosepiece of my binoviewer.  I get about 0.7x and severe vignetting in 26mm Plossls with this setup, but the views are wide.  Without the FR, I get 3.0x, so a massive reduction in power.  For my Dob, I put 45mm of spacer rings between the two and get exactly 1.0x and severe field curvature but no vignetting at 26mm.  32mm Plossls vignette anyway due to the 22mm clear aperture of the binoviewer, so those lost edge bundles aren't a big deal for this application.

[Merlin66]

Not a recommended combination.....

The end result would be a 0.8 x 2 (or 4) = 1.6 magnification.

Before you ask.....you can stack the TV Powermates to give addition magnification - two x2 PM will give x4. (I confirmed this while solar imaging)

 

[Louis D]

4 minutes ago, MarkAR said:

Along the same lines, what about imaging with a 0.8 reducer/flattener and a 2x or even 4x 2" TeleVue Powermate.

You're probably okay because the Powermates are telecentric magnifiers that don't leave the ray bundles diverging.

[MarkAR]

Cheers guys.

I've been contemplating the Powermates to up my focal length. Altair Wave 115 EDT is 805mm, have the 0.79 F/R on it at the moment and am also considering the 1.0x F/R.

[Merlin66]

Hmmmm

Silly question....but why not just remove the reducer??

 

[MarkAR]

5 minutes ago, Merlin66 said:

Hmmmm

Silly question....but why not just remove the reducer??

 

Not daft at all. It also works as a field flattener. Also I don't have any suitable extensions to get to the right focal length. 

Saying that, I have heard that the Altair Wave series can get away without the flatteners/ reducers much depending on the camera FOV as well.