Barlows - when 2 x 2.5 does not = 5!

[Dennis]

Hi

I’ve just analysed some of the Saturn images that I managed to grab over the last couple of days because I noticed that the relative image scales were not quite what I had expected when using similar configurations.

The x2.5 Powermate and x5 Powermate were used in the same system with the same configuration of C9.25, JMI NGFCM motofocuser and Philips ToUcam 840K. Yet the X5 Powermate image is more that twice the image scale of the x2.5 Powermate image.

As expected, when the x2.5 Powermate is used with the addition of a Meade 647 Flip Mirror in the imaging train, the amplification is greater than without the flip mirror. So, 2 x 2.5 does not = 5 it seems!

Cheers

Dennis

[iceman]

Also, the 5x can be actually longer when there's an extension - be it an extension tube, filter wheel, etc.

I usually run my 5x as a 6x or even more with the addition of the filter wheel and adapters for the DMK.

[sheeny]

I think the 5x powermate and most barlows have diverging exit rays so entending the light path after the barlow or Powermate increases inage size. I'm sure though that the 2.5x Powermate has converging exit rays so extending the light path actually reduces image size (from 2.5x).

So with the 2.5x Powermate you'll get best image scale with the EP or camera direct into the Powermate. Most other barlows or powermates I think you get more image scale with extension tubes of some sort.

I'm sure some one like DP posted a link or a copy of a Powermate chart that showed all this...

Al.

[Dennis]

Hi Al

What I find puzzling is the lack of a linear x2 multiplication factor between the top (x5) image and the bottom (x2.5) image. These were taken using the same, simple configuration. That is the ToUcam was plugged directly into the x2.5 and the x5 and then the Powermates were just plugged straight into the focuser in turn? Both bottomed out in the focuser, and the ToUcam bottomed out in both Powermates.

It looks like the x2.5 is behaving closer to a x2.2 system?

Cheers

Dennis

[sheeny]

Yes, well, that's a disappointment! i know the image scale of my 2.5x is bigger than with my 2x barlow, but I've never done a check to see by how much... (actually I haven't used the barlow much since getting the Powermate.):confuse3:

I'd like to get a 4x Powermate for planetary imaging with the C8, but 4x is only available in 2". I'm not sure whether 5x would be able to be used too often unfortunately... f/50??? hmmm, have to be great seeing to use it!

Al.

[Dennis]

Hi Al

Yeah, my x5 Powermate only comes out 2 or 3 times a year, mainly for multiple stars. RobertT uses his x4 Powermate to great advantage on planets using the DMK (previously ToUcam).

Cheers

Dennis

[sheeny]

Yeah but, as Dad says in The Castle: "It's what you do with it!":thumbsup: :D

Al.