Question about Barlow's and PF Photography

[Bernoulli]

One of you optical gurus out there will know this, I'm sure.

I've been taking pictures of the moon through a 4" apo refractor with FL = 900. At prime focus, I get a moon that's 1800 pixels across. If I put a 2X Barlow in front of the camera, the image is almost 6000 pixels across, or 3.3 times bigger.

So instead of doubling my FL from 900 to 1800 mm, it's almost 3000 mm. That's a bit more than I need.

Why would a 2X Barlow more than triple the focal length on my telescope? It does seem to double my eyepieces, but why is it stronger with prime focus?

[chris.bailey]

I assume you are addding the Barlow direct rather than through a diagonal? The actual magnification will vary depending on where in the optical path the barlow sits, if its closer to the focal plain than designed for then the magnification factor will be greater.

[Bernoulli]

Correct, no diagonal. Telescope, camera vs. telescope, barlow, camera.

Not that I'm not getting good results (see below), it's just more magnification than I really need.

[troypiggo]

May have something to do with where it's sitting in your optical train?

Assume with eyepieces it's scope, barlow, eyepiece.

With imaging it's scope, barlow, T-adapter etc, camera. Maybe with the camera connectors between camera and barlow it's giving some extra magnification?

[A.S.I.G.N. Observatory]

That's a spectacular image.

[chris.bailey]

It sure is a great moon image. You can even see the lava ridges in the mares.

Its images like this that truly show that its a matter of when as opposed to if and now Bruce Willis is getting a bit old to do anything about it....

[Bernoulli]

Thanks guys.

I made a little breakthrough in imaging over the past few weeks. I got a 4" apo refractor (Orion 100 ED I think it's called) and it way outperforms my 6" Mak and my 8" SC for lunar photography. It's the lack of scattered light that really makes the difference.

I've got a thread over at the astronomy and celestial section showing some other shots:

http://photography-on-the.net/forum/showthread.php?t=700029

I had noticed some great shots through fairly small refractors in this forum and that's what pushed me to get one. It was money well-spent.

4" refractor should resolve about 2.5 km on the moon before it's diffraction limited. I got some nice prime focus shots which, at 900 mm, gave me about 1.9 km/pixel. I wanted to see if I could do better, so I tried eyepiece projection, and that was just muddy, probably due to the considerable number of lenses in the eyepieces. So I tried putting just a 2X Barlow in and shooting "prime focus" and got the results I described above.

Actually, I'm really happy with the 2X Barlow and the 3000 mm effective focal length. As you said, the images are fine even if they are a bit over-sampled at only 0.6 km/pixel. But I'm just trying to understand the optics a bit better about why I got so much magnification.

[Celestron]

Have you read this about Barlows (http://www.astronomics.com/main/category.asp/catalog_name/Astronomics/category_name/8M71WR24K8K48LAX1LJ4J9WXK7/Page/1) from astrononomics ? What i know about any 2x barlow is that if you insert it into the diagonal it becomes a 2x power for the EP . If you insert the barlow in front of the diagonal it becomes 3x power . I have done this with my C8 but i do not prefer it at all because it actually degrades seeing for me because the power is actually more than my scope is made for . Barlows should be compatable with your scopes optics , EPs' and diagonals . Thats why that all my EPs' that are not Plossis are Ultimas EP and my barlow is an Ultima . They are compatable and parfocal to each other which leaves me with very little focusing or none at all :) . Refractors usually can handle very high power especially if the optics are good optics . This is why you can produce very good images with your barlow setup either way in front or behind your diagonal . This is an advantage of a refractor . I have a SV AT1010 80mm 480mmFL called the TANK . It can handle a 3x barlow very easily because SV has great optics even in their achromatic Refractors . SCs' on the other hand have different optics (mirrors combined with lens) and they are better with compatable EPs' and barlows . The reason too much power is not necessary is because you look past most DSOs' . The reason we observe with low power is so we avoid do this and the DSO's look better and focus better . Planets are best used with high power EPs' preferably without a barlow but in refractors the more the power the better . Long story short . If you insert the barlow at PF without the diagonal it's still 2x power for the EP . The only time you triple the power is barlow first , diagonal next , then EP . It has to do with the barlow -diagonal setup .