Powermate with ext tubes with/without diagonal little focus change??

[Cliff]

I am a little unsure of what is happening,

I have a 76mm x 570mm refractor a total of approximately 100mm ext tubes , a 4X Powermate then 40mm plossil (or other eyepiece) to reach focus without a diagonal.

Now experimenting as the weather is lousy, I tried putting a diagonal into the powermate and then the eyepiece in the diagonal and only see about 3mm difference in focus position with a 1.25" prism or 6mm with a 2" mirror diagonal?

My understanding was the diagonals took up 50-100mm of the focus distance.

Anyone like to enlighten me ? Not used extension tubes with barlows or powermates before. I did discover the powermate needs to go at the end of the extension tubes not at the telescope end to get any focus at all.

thanks
Cliff

[Merlin66]

Cliff,
Welcome to the world of the Powermate/ Telecentric extender.


The quick answer is that the spacing of the eyepiece/ camera BEHIND the PM is of no real consequence. The magnification changes very little.
The PM will need to be placed slightly inward if the backfocus changes dramatically.


So, whether you use a diagonal, spacer etc AFTER the PM doesn't do much to the optical path.
HTH

[Cliff]

Thanks Ken,

Very interesting Barlows are totally different am I right ?
So if this is so then my Combo Quark for solar shouldn't make a lot of difference as it is placed after the powermate also ?
This is the reason I went down this path in the first place trying to get focus with the 4X Powermate using the Quark just had no Sun to try it out.

Still lots to learn never ends really.
Cliff

[Don Pensack]

As you can see from this chart, the magnification changes very little behind the PowerMates with different back focus distances:
https://www.televue.com/engine/TV3b_page.asp?id=53&Tab=_app
The 4x PowerMate gets to 4.45x at 100mm away from the lens.

[Renato1]

Thanks. That's interesting to know.
Regards,
Renato

[Merlin66]

The back focus scale used in the TV docs has a zero point some 55mm from the top surface of the lens housing. The rear of the lens is 10mm below this surface.
So, the distance behind would be the actual measurement from the top surface of the lens housing minus 65mm.
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For astro-imagers, the above chart indicates the magnification for each Powermate™ (2x, 2.5x, 4x & 5x) as a function of distance from the Powermate™ eyepiece holder top surface to the imaging surface (CCD or film). (Imaging through a DSLR with the Powermate™ T-Ring Adapter and camera T-Ring — without added extensions — brings the focus position to 0mm.)

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