Barlows - To stack or not to stack...?

[AndyUK]

Sorry to be a pain - I have SO many questions...!

I have a 2x barlow, a 300D DSLR and a TouCam II 840 (and a 0.5 reducer). I found a rather interesting site for field of views of different objects with different scopes, eyepieces, cameras (http://www.12dstring.me.uk/fov.htm) and was playing about with their calculator.

I WAS anticipating buying a 4x ImageMate (or 5x Antares), but then noticed that some people stack barlows (x2 and x3) which would a) be cheaper and provide more options - 2x, 3x and 6x (as I doubt that trying to stack a 4x or 5x with a 2x would give a very good image on a 102 or 130 OTA)

Has anyone got any thoughts / experiences on this?

[Sam]

Andy,

I usually stack my 3 X and 2 X when imaging the moon or the planets. They are both reasonable barlows but certainly not to the standard of the more expensive ones so I can't comment on those. I've had some pretty good results but it is much harder to focus when you start stacking barlows - just takes more time.

Sam

[brianb]

No particular reason not to but you will probably notice quality issues unless both units are good. The better one should be closer to the objective.

Be aware that, when used for imaging, most barlows have a different magnification to that marked on them - because the camera sensor is further from the lens assembly than the focal plane of an eyepiece would be. My "2x" Celestron Ultima SV gives about 2.8x when used with my DMK cameras. Powermates - except for the 5x - are optically designed so that the magnification is nearly independent of the position of the focal plane.

The 4x Imagemate works very very well indeed and is excellent value for money - if you can find one! Highly recommended, I use one for many of my high resolution solar shots.

[AndyUK]

Cheers both - I must admit, my existing 2x barlow probably isn't exactly one of the best (the standard skywatcher issue), and I did wonder if there'd be any noticeable quality degradation stacking it, but reading how many people stack 2x and 3x it made me wonder if I was missing a track...

However, it sounds as though whilst my 2x is okay for viewing, I may have to either upgrade to a better one first (and maybe then consider this question) or forget the 2x entirely for now and go straight to a 4x - I'm really keen to try and get some photo's!

After a quick look on the internet, 2x barlows seem to vary between c. £20 and £390 (Zeiss)... If I had the money (which I don't!) would I really see £370 difference on a SW 102 refractor and 130 reflector?. I know it's a difficult question to answer, but what would be a reasonable amount to spend on a 2x barlow on the basis that I'll probably have to stick with the OTA's I have for at least a couple of years? I guess the answer is as much as I can afford but would I be right in thinking that either Televue or Celestron Ultima would be reasonable (or could I "get away with" something like a Celestron Omni?)

Brian - Thanks for your "technical" explanation about the focal plane magnification. I hadn't considered that (although I was wondering about the effect of the censor size in the camera increasing the effective lens focal length by 0.6?). I note that you "skipped" over ImageMates and went straight to PowerMates - Again, I can see that there's a fairly significant difference between the two in cost and I guess I have the same decision to make, but do you reckon that I'd see that significant a difference with the kit I currently have?

[arad85]

Brian - Thanks for your "technical" explanation about the focal plane magnification. I hadn't considered that (although I was wondering about the effect of the censor size in the camera increasing the effective lens focal length by 0.6?).

It's nothing to do with the sensor size, but it's position from the barlow lens elements. A barlow has a diverging ray set so the further away you get from the lens, the more magnification you get. That's why you can use extension tubes after the barlow lens to get more magnification without using another barlow.

Powermates (apart from the 5x) are designed to have parallel rays emerging after the magnifying lens so magnify at a fixed value no matter how far the sensor is (within reason!!).

HTH...

[glowjet]

I would support what Arad has said, before buying a barlow have a read up on the TeleVue Powermates, they are expensive new, but can be had secondhand at a reasonable price, if you are prepared to wait, the only trouble is they are as rare as hens teeth and you have to keep watching the various for sale sections.

John.

[Uranium235]

What I do to get 4x is to stack the standard 1.25" skywatcher barlow into the 2" Revelation ED barlow (cos that one has a filter thread). It works well enough but just be warned that it will lengthen your optical train quite a lot and therefore add considerable weight - anything up to 500g.

Dont forget too that when youre barlowing up, you will also need to adjust your exposure times to suit

[AndyUK]

Cheers both - I think I understand... So using an extension tube on a 2x barlow would make it into a 2x +"n" barlow? I've seen a 50mm extension tube and also a set of 6, 10 and 15mm advertised... I don't suppose there's an equation to determine how much the magnification would be with extension tubes is there (ie calculating "n"). And if the extension tubes magnify that much (ie by 50%?), is there any difference between an "extended" 2x and a 3x?

An extension tube will also potentially get around a focussing issue I think I'm going to have with the SW 130 as I've been advised that it's not possible to bring it to focus without using a barlow (or presumably extension tube to move the focal plane further away).

(One other thought - Am I in danger of hitting the Dawes limit on these two scopes with such potentially large magnification? The maximum stated practical magnification is 204x for the 102 and 260x for the 130... or am I now getting totally confused???)

It's beginning to sound as though I may expecting too much out of the OTA's / mount that I have and I may be better off using these to "play" with (and taking piggy-backed wide-field images) whilst saving up the money to put towards a smaller aperture APO and EQ5/6 mount for planetary work... Or maybe saving up for / buying a couple of really good barlows now so that I'm future-proofed when I eventually do have the money to move up to the next level.

(I wish I had more money!!!)

[Uranium235]

A 4X barlow on a 130 OTA is pushing it a bit (if you were using a 10mm EP), but there should be someone along soon who can do the maths for ya but as a rule of thumb, apeture rules. Try for a 150mm newt, get used to it and then if all goes well then buy the APO - that way you have options on which scope you want to use for imaging/guiding.... well, thats my plan anyway.. APOs are soooo damn expensive

[AndyUK]

Thanks Uranium - That's really helpful... As you say, the weight of the Canon 300D is now beginning to become cause for concern (especially at the end of the refractor) and as I'm trying to use all this on an AZ mount, my exposure times are going to be limited to only c.30s anyway.

As I noted in my last post on this thread, I think I may be trying to sprint before I can even crawl and therefore have to lower my expectations for now (until I can afford to buy the mount / OTA that will actually do the job).

However, at the very least that revelation ED Barlow would be better than the one I already have (even for viewing) and if I can't use it and the Skywatcher 2x together, then it wouldn't hurt too much to pension off the latter...

[AndyUK]

Thanks again... Yes... The more I look into this, the more I'm thinking I need to get myself an EQ5/6 GOTO mount to really future proof myself (I'm really lazy when it comes to searching the sky - I prefer to set up once and then let the computer do the rest!)

I think I'm going to have to go back to the drawing board and plan this all out better... and then do a lot of saving. But in the meantime, I'm just going to have fun exploring what I can get out of what I have (but without spending too much, unless it's for something that I can take forward and use later...)