(Planetary) Could I get away with using extension tubes instead of buying more barlows?

[JokubasJar]

Ok so I have a slight problem. I have a ZWO 224mc which has a pixel size of 3.75. As everyone knows if we times that by 6 (average seeing conditions) we get 22.5 which is the optimal f ratio for my camera. My scope is an 8 inch f/5.9. So to get to 22.5 I need a 3.8x barlow, we can round that to 4x. The problem is that Televue and other manufactures of barlows suitable for planetary imaging have 4x barlows, but they cost so much that it would be hard to justify spending the same amount of money I spent on all of my other accessories combined, on one single barlow. I know that good seeing is rare and that a 3x is what I would use more often but I don't want to waste the potential of the scope on the few good nights. I see so much images from scopes like c14s that look like they were taken with a c6 because the user undersampled the planet. Therefore my question is could I, instead of splurging on a bunch of barlows I will barely use, buy one decent 2x or 2.25x barlow and then use extension tubes to reach my desired magnification. I assume there might be vignetting but that doesn't really matter to me since I can just center the planet. But other than that, is the image quality affected by using this method? I did read somewhere that this wouldn't work with a powermate because they work differently than barlows do. So if I were to do this, what is a good quality 2x to 2.5x barlow that I could buy to do this? Also, I intend to use an ADC, how does that affect all of this? Finally, If I for example have a 2x barlow, how long of an extension tube would I need for it to be a 3.8x, or a 4x?

[Mr Spock]

You might find it difficult to reach x4. Here's some measurements I did using the lens from A Meade 140 and various Svbony extension tubes.

Length	X
70mm	2.11
70 + 56	2.36
70 + 70	2.64
70+70+56	3.10

 

As you can see, I only got up to x3.1 and that was an unwieldy length. You might get the result with a 'shorty' Barlow, but you would have to experiment with the tube lengths to get it exact. There is a formula to do this but I can't remember it off the top of my head. Someone will be along who knows what it is.

[JokubasJar]

10 hours ago, Mr Spock said:

You might find it difficult to reach x4. Here's some measurements I did using the lens from A Meade 140 and various Svbony extension tubes.

Length	X
70mm	2.11
70 + 56	2.36
70 + 70	2.64
70+70+56	3.10

 

As you can see, I only got up to x3.1 and that was an unwieldy length. You might get the result with a 'shorty' Barlow, but you would have to experiment with the tube lengths to get it exact. There is a formula to do this but I can't remember it off the top of my head. Someone will be along who knows what it is.

Ok, I have decided on a 3x televue barlow. They have a graph on their website that shows the relationship between the distance and magnification for their barlows (not powermates), and in order to achieve 4x I need about 45mm of extension. Now my question is how do I get that. Which extension tubes am I supposed to buy? My whole image train is 1.25''.

[Mr Spock]

17 minutes ago, JokubasJar said:

Which extension tubes am I supposed to buy? My whole image train is 1.25''.

The Svbony 70mm 1.25" extension tube top measures exactly 40mm  About as close as you will get I think. You could always not put it fully in by 5mm if you wanted to be spot on.

[JokubasJar]

1 hour ago, Mr Spock said:

The Svbony 70mm 1.25" extension tube top measures exactly 40mm  About as close as you will get I think. You could always not put it fully in by 5mm if you wanted to be spot on.

Thanks a lot. This looks like it will work, but I keep wondering, does the adc change anything? Does it act like an extension tube or does the barlow magnification stay the same?