Is there a decent 3.5x barlow?

[kalasinman]

As I have a QHY5L-II, been considering some planetary imaging with it. The tutorial I saw indicated that sufficient magnification/scale would be achieved if I used a barlow which would increase the FR by the formula pixel size times FR of scope I get 3.75 x 5 = 18.75. This would be achieved by using a 3.5x barlow. Is there a decent one? Just use a 4x?

[michael.h.f.wilkinson]

Siebert optical make telecentric Barlows (like powermates) at that scale:

http://www.siebertoptics.com/SiebertOptics-barlows.html#Ad%202

These are 2". A 4x would also work. Being a small percentage over the "optimum" is not a great deal. I think the QHY5L-II has the same pixel size as the ASI120MC I have, and that works well at F/20 with my 2x TeleXtender from Meade.

[ronin]

I would not take the formula as absolute.

When I was looking at cameras and collecting together some information on angles and pixel size I found that there was a factor of 5 across different opinions. Really did come across as a bit insane to find such a spread.

It makes deciding either difficult or easy.

[kalasinman]

I would not take the formula as absolute.

When I was looking at cameras and collecting together some information on angles and pixel size I found that there was a factor of 5 across different opinions. Really did come across as a bit insane to find such a spread.

It makes deciding either difficult or easy.

Gotcha. I'm used to cropping molehills into mountains already (working with camera lenses)  so getting in the ballpark (or on the green) is good enough at this point.

[kalasinman]

Siebert optical make telecentric Barlows (like powermates) at that scale:

http://www.siebertoptics.com/SiebertOptics-barlows.html#Ad%202

These are 2". A 4x would also work. Being a small percentage over the "optimum" is not a great deal. I think the QHY5L-II has the same pixel size as the ASI120MC I have, and that works well at F/20 with my 2x TeleXtender from Meade.

Thanks.