Which Barlow?

[slowflow (Barry)]

I would like to buy a barlow and an looking at a TV big barlow 2" or should I spend the extra $100 ona 2" powermate. To be used in a 12" dob with ES 9, 18 & 24mm lens.
Thanks.

[astro744]

Both excellent choices. A Powermate is a four element 'Barlow' and has been designed to let the eyepiece perform exactly as it was intended without increasing the eye relief as a Barlow would do and without possible vignetting which occurs with some Barlow/eyepiece combinations. The Powermate would be my choice, but note it is heavier.

Note your current sizes are 9, 18, 24 and with 2x you will get 4.5, 9, 12, 18 & 24; you effectively gain two extra powers with the 4.5mm being useful on only the best seeing nights. Perhaps consider an eyepiece in the 12-14mm range and one in the 6-7mm range in lieu of 2x Barlow or Powermate.

[astro744]

Do you have the ES 68, ES 82 or ES 100 (or combination thereof)?

You want to try and avoid true fields that are similar even though magnifications are different. e.g. ES 100-9 would have similar true field to ES 82-11. Of course for planets magnification is more important.

Anyway, there are many options. If you do not want to add more eyepieces and want either a Barlow or Powermate, I would still recommend the Powermate so there is no chance of vignetting.

My first choice would be to add more eyepieces to fill the gaps but I have never used ES eyepieces so cannot comment on any of them.

The 2x Powermate however I can comment on and say that it and the 4x are close to being optically invisible. My best 8.75mm eyepiece is my 35mm Panoptic and 4x Powermate but it does require a counterweight on my 10" Dob!

You do add 4 extra elements of glass to the optical train with a Powermate and if you have 6 elements already that is a lot of glass. This can be the difference between seeing a very faint star or galaxy at the limit of perception or not and I have actually noticed this difference but it is almost imperceptible.