Making 5x Barlow from 2x or 3x Barlow

[Manoocher]

I have read "if you have a 2X, you can make it a 3X or even 5X by putting a suitable length spacer in between". Can anyone comment on this and the size of spacer? Thanks. Manoocher

[sunslayr]

Depends on the barlow, as long as it isn't a telecentric barlow or powermate. You probably will have to experiment. If you have a camera it should be pretty easy to figure out the exact magnification with plate solving but you could probably estimate it with an eyepiece. I would imagine 30 mm would probably give you 4x out of a 2x.

[astro744]

See https://en.m.wikipedia.org/wiki/Barlow_lens

When used on a refractor a 2x Barlow inserted into the star diagonal will produce 2x but if the star diagonal is inserted into the Barlow the extra distance of the star diagonal gives 3x (approx. and dependet on optical length of diagonal). I’ve never tried this or used extension tubes and simply buy and use the appropriate Barlow magnification. I’m not sure if there is any vignetting when adding an extension. Note a 3x Barlow is not double the length of a 2x (e.g. Tele Vue) so there must be a difference in focal length of the Barlow doublet.

[Shiraz]

If any use, this is a pretty good summary of the formulas you could use to calculate magnification for conventional Barlows. It also deals with the telecentric ones (like the TV Powermates), that don't have much(or any) variation in magnifcation. http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/BarlowLens.pdf

Practical data can be found on the Televue site, which has some useful tables that give magnification factors for their Barlows. https://www.televue.com/engine/TV3b_page.asp?id=52&Tab=_photo

[Hans Tucker]

Buy yourself a Baader Flat Field Converter (FFC) :D