22 replies to this topic

[Paradoxdb3]

In trying to stick with the holy 5x rule, I use my SkyWatcher SkyMax 180 with a 2x barlow in conjunction with my asi482mc. With a pixel sixe of 5.8 microns, that's about as perfect as I can get. I also have a 2.5x Powermate, which TECHNICALLY pushes me beyond the 5x rule at f/37.5 vs the f/30 with the 2x barlow. But here's what I've noticed between the two lenses. Both produce about the same exposure (maybe the Powermate is ever so slightly brighter), but the barlow makes the target LARGER on the sensor than the Powermate. So is the barlow the better choice? Or is the Powermate still the better way to go?

[Tulloch]

As noted in the FAQ, barlows and powermates don't always produce the magnification expected, it depends on the distance between the back of the barlow/PM and the camera.

 

Tele Vue talks about it here for their barlows

https://www.televue....d=52&Tab=_photo

 

and here for their powermates

https://www.televue....?id=53&Tab=_app

Edited by Tulloch, 23 September 2023 - 04:20 PM.

[MarMax]

You should look for the focal length for your Barlow. It will be a negative number in mm and you can ignore the negative.

 

M = (f + d)/f

 

M = magnification factor, f = focal length of the Barlow, and d = distance from the Barlow lens to the focal plane

 

I use an Antares 2" 1.6x Barlow and its focal length is -116.5mm. Based on this, at a backspacing of 70mm, the magnification is 1.6x. If you space closer the magnification will be less and if you space farther the magnification will be more.

 

2x and 2.5x Powermates, as you can see in the linked information, have an almost straight line for magnification. Changes in back spacing have very little effect on magnification.

Edited by MarMax, 23 September 2023 - 04:38 PM.

[RedLionNJ]

Changes in back spacing have very little effect on magnification.

But since the system is focused by varying the separation between the primary and secondary mirrors, the placement of the barlow field lens relative to the rear cell can have an effect on the native f-ratio of the system before any magnification is applied.

[Tulloch]

But since the system is focused by varying the separation between the primary and secondary mirrors, the placement of the barlow field lens relative to the rear cell can have an effect on the native f-ratio of the system before any magnification is applied.

Yes - my 9.25" SCT has a "native" focal ratio of f/10, but only when the back spacing is around 150mm or so (which you get from a diagonal). Plugging my camera directly into the Visual Back gives me a focal ratio of around f/8.8.

[MarMax]

But since the system is focused by varying the separation between the primary and secondary mirrors, the placement of the barlow field lens relative to the rear cell can have an effect on the native f-ratio of the system before any magnification is applied.

To complete the intent of the statement which you carefully snipped. Changes in backspacing with a 2.5x Powermate have very little effect on its magnification.

 

Yes - my 9.25" SCT has a "native" focal ratio of f/10, but only when the back spacing is around 150mm or so (which you get from a diagonal). Plugging my camera directly into the Visual Back gives me a focal ratio of around f/8.8.

This is why you should learn and understand your SCT and understand the importance of the optimum focus range, which happens to be in the middle of the focuser travel for a C11. You can then adjust your kit so that you reach focus at or near the midpoint and are operating the telescope close to its design f-ratio. Or if you wish to operate out of the "optimum" range you can run some system tests to figure that out as well.

 

And with the above said, you should test the complete kit to verify the system magnification. Since the OP very generally compared a 2.5x Powermate to a 2x Barlow I provided a way to determine the magnification of the Barlow since the Powermate changes very little with backfocus. Knowing the Barlow magnification based on backspacing is a good start to figuring out system magnification.

Edited by MarMax, 23 September 2023 - 05:23 PM.

[Wdh]

While we are on the subject, I’ve been imaging with a C6 and an ASI585MC, which calls for a 1.5X Barlow. I’ve been unable to locate one, so I screw the element of a cheapie Celestron 2X directly to the camera nosepiece. 
 

I’m not sure how the Televue chart would be applied in this case. My crude attempt at calculating is based on measuring the screen size of a Barlow vs. straight image. The Barlowed version is about 1.9x larger in diameter. Is this a valid way to arrive at a rough figure?

[Tulloch]

The straight line goes both ways, so reducing the distance from the zero point reduces the magnification

[dcaponeii]

You should look for the focal length for your Barlow. It will be a negative number in mm and you can ignore the negative.

 

M = (f + d)/f

 

M = magnification factor, f = focal length of the Barlow, and d = distance from the Barlow lens to the focal plane

 

I use an Antares 2" 1.6x Barlow and its focal length is -116.5mm. Based on this, at a backspacing of 70mm, the magnification is 1.6x. If you space closer the magnification will be less and if you space farther the magnification will be more.

 

2x and 2.5x Powermates, as you can see in the linked information, have an almost straight line for magnification. Changes in back spacing have very little effect on magnification.

This is more visual stuff with little relation to imaging.  There is no such thing as magnification when imaging.  5x pixel size gives you the sweet spot for f/ratio.  You can safely ignore everything in the above box.
 

[Wdh]

The straight line goes both ways, so reducing the distance from the zero point reduces the magnification

Thanks, but if I’m reading it right I would have to reduce the distance from the Barlow top surface to the imaging surface to less than zero millimeters to get down to 1.5 with a 2x.

“For astro-imagers, the above chart indicates the magnification for each Barlow (2x, 2x Big & 3x) as a function of distance from the Barlow top surface to the imaging surface (CCD or film).”

Does that seem right?

[MarMax]

This is more visual stuff with little relation to imaging.  There is no such thing as magnification when imaging.  5x pixel size gives you the sweet spot for f/ratio.  You can safely ignore everything in the above box.
 

Thank you for your most enlightening commentary. I've only seen the bold statement about 1,000 times here.

 

Regardless of what you are doing, if you would like to know how much a Barlow magnifies, you use the formula in the box. What you do with it from there is on you. The formula is correct.

 

And as Tulloch stated, the line goes both ways. A Barlow can help you both ways to get you to the magical 5x. And the Barlow does this by producing different MAGNIFICATION based on back spacing.

 

I thought CN was a place to help others, but apparently the rules of this Forum are different. The glass is always half empty here. I'm out.

[Tulloch]

Yes, the Barlow element is at the top/front, then there’s an empty tube behind it. The back end of the tube is zero.

[Wdh]

Yes, the Barlow element is at the top/front, then there’s an empty tube behind it. The back end of the tube is zero.

Ah, thanks! At the moment I’m using the element screwed to the nosepiece of my 585MC. The image size with the Barlow is about 1.9 times the image without it, at the same ROI. Is that roughly indicative?

[Tulloch]

Hmm, should be less than that. Have you tried it at full length?

[Wdh]

Hmm, should be less than that. Have you tried it at full length?

Nope- I have very limited room for extra components in the imaging chain. It’s an EVO alt/az mount, and at high angles I’ve only got 5 inches under the end of the C6 tube. I have an ADC in the chain, and I’m using a star diagonal (bad practice, I know) so that I can do initial alignment with an eyepiece. I’d like to have a flip mirror instead, but there’s really no room. 
 

I’ll give the Barlow a test at full length just for comparison. It’s a Celestron that came with a cheapie filter kit, and just says 2x on the side. I’d like to find a real 1.5x that I can fit in the chain.

 

Thanks for the suggestions and clarification. 

[Tulloch]

I have an evo too, so I understand your pain. I use a 2,5x power mate with the adapter, an Adc and camera, and I have to push the ota right to the front of the mount so the back of the camera doesn’t hit the mount at high elevation angles.

I’ve only hit the mount once, no damage done.

[Tulloch]

You can push the c6 further up. The Evo likes it front heavy

[Wdh]

You can push the c6 further up. The Evo likes it front heavy

I’m at the end of the dovetail already. Maybe I can get another inch. 

[RedLionNJ]

Thank you for your most enlightening commentary. I've only seen the bold statement about 1,000 times here.

 

Regardless of what you are doing, if you would like to know how much a Barlow magnifies, you use the formula in the box. What you do with it from there is on you. The formula is correct.

 

And as Tulloch stated, the line goes both ways. A Barlow can help you both ways to get you to the magical 5x. And the Barlow does this by producing different MAGNIFICATION based on back spacing.

 

I thought CN was a place to help others, but apparently the rules of this Forum are different. The glass is always half empty here. I'm out.

We should be using the word "amplification" here, not magnification, to reduce the ambiguity. Don is correct - "magnification" has no place in imagng.

[Paradoxdb3]

Okay, so just to reiterate...the barlow is better to use in this scenario than the Powermate is better to use in this scenario than the barlow is better to use in this scenario.... Got it! 😁

[Ittaku]

Powermates with their extra glass substantially decrease transmission relative to regular barlows and particularly blue which are the photons we already catch the least of with our camera sensors. I'd actually tend towards steering people away from them for planetary now, having seen their transmission curves.

[Tulloch]

We should be using the word "amplification" here, not magnification, to reduce the ambiguity. Don is correct - "magnification" has no place in imagng.

I'm not really fussed about the terminology, according to my dictionary, magnify means "make (something) appear larger than it is". And that's what the barlow does, makes the image on the camera sensor larger than without it. 

[Mike_Kar]

Nope- I have very limited room for extra components in the imaging chain. It’s an EVO alt/az mount, and at high angles I’ve only got 5 inches under the end of the C6 tube. I have an ADC in the chain, and I’m using a star diagonal (bad practice, I know) so that I can do initial alignment with an eyepiece. I’d like to have a flip mirror instead, but there’s really no room. 
 

I’ll give the Barlow a test at full length just for comparison. It’s a Celestron that came with a cheapie filter kit, and just says 2x on the side. I’d like to find a real 1.5x that I can fit in the chain.

 

Thanks for the suggestions and clarification. 

You might want to try these.  I've used use both the 1.25x and 1.5x barlows and like them a lot. 

https://agenaastro.c...150xcommon.html