Are Barlow Lenses Bad for Astrophotography? - Forums

Hello all, I have a 400mm refractor that I've been using with a 2X barlow recently to capture higher resolution sub frames of M51 vs. without, however, it's seemed to have caused tons of issues from color rendition to plate solving. I use N.I.N.A to control everything and plate solving with ASTAP works fine when I use the telescope without the barlow, but with the lens installed it fails every time, I've even adjusted the focal length of my scope under the "equipment" section of N.I.N.A to account for the different field of view but it doesn't help. I also notice when stretching my data that it seems there isn't great contrast vs sub frames without a barlow (not on the same target though so it's not an apples-apples comparison) My setup is an ioptron CEM26 with an Astromania SGCMOS Series guide camera and SVBony 60mm guide scope, and without a barlow I'm able to successfully take 2min guided subs with pinpoint stars no problem, but when using the 2X barlow my stars are so faint and slightly triangular shaped which causes issues with Deep Sky Stacker recognizing decent stars to use for alignment. Aside from investing in a higher focal length scope which isn't feasible at this time, should I just ditch the Barlow and throw it in the trash? My camera resolution is 6000x4000, so do I have enough resolution that imaging small galaxies < 1deg as long as I use something like 2X drizzle in DSS will still be worth it? I may be answering my own question already because I've seen the differences in stacking data from different focal lengths, but I'm just curious what everyone else's opinions are, thanks.

There are very few barlow lens out there that can cover such large field without introducing additional aberrations and they are all expensive. You don't state what scope you're using but assuming is around f/6 that means you're shooting at f/12 at least. Not that I can't be done but it needs aperture and the proper image scale for the aperture which I suspect you don't have. Drizzling if you are undersampling (are you?) is a reasonable way forward. And yes, you can ditch that barlow.

As a side note: very few galaxies are larger than 1 degree. In fact I can only think of 2 in the northern hemisphere.

I think you are mistaken that using a 2x barlow gives you higher resolution, it only gives you higher magnification. The resolution of your telescope is purely given by its aperture. If at all, a cheap barlow will reduce your resolution due to bad optical quality.
The thing gets of course a bit more complicated when you factor in the sensor pixel pitch, guiding and seeing, but that's a whole different story.

@andrea tasselli The 80mm scope is by default 400mm @f/5, with the Barlow it’s 800 @f/10. Now come to think of it you’re right, most galaxies are less than 1 degree of sky, to me anything between 30 arc-minutes and 1 degree is the perfect size for a 400mm focal length.

Thanks for your input

Fabian:
@andrea tasselli The 80mm scope is by default 400mm @f/5, with the Barlow it’s 800 @f/10. Now come to think of it you’re right, most galaxies are less than 1 degree of sky, to me anything between 30 arc-minutes and 1 degree is the perfect size for a 400mm focal length.

Thanks for your input

That's fast for a 400mm scope. Still, it is only 80mm. Assuming a pixel pitch of 3.8 um you're around a native resolution of about 2"/pixel. Which is though going for 100mm scope, never mind a 80mm. And you want to go 1"/pixel with an 80mm. It is not going to end well. I had a hard time with an 8" scope with some of the fainter targets at about the same image scale.

Hi @Fabian

I had the same goal in mind some time ago. I wanted to extend my focal length for Astro Photography without busting the bank. And for me it didn't work, too. And I bought a barlow which was not cheap at all. The results were never satisfying. So I sold it at some point. But the increase in exposure time was really dramatic. Also every problem of the original scope gets magnified. Also, the mount and the guiding have to be more precise. I like the deep sky stuff and the Barlow lens didn't work for me. This does not mean, It couldn't work for you, but there is a reason why they didn't show up much in images other than planetary. Cropping the image was always the better choice for me.

This is of course no helpful comment. But it's an experience I made and it seems to match your experience.

Anyway, I hope you find your solution.

CS

Christian

@Christian Großmann thank you for sharing your experiences, they do sound indeed similar to mine. Theoretically, my mount and guiding should be able to handle such a high focal length, perhaps I just don't have my guide settings optimized although that's a separate issue itself. I've realized that unless I really shell out the money and get something like a high-end Schmidt Cassegrain with low f-stop, getting amazing detail from small galaxies will be impossible.

CS

Fabian

Fabian:
@Christian Großmann thank you for sharing your experiences, they do sound indeed similar to mine. Theoretically, my mount and guiding should be able to handle such a high focal length, perhaps I just don't have my guide settings optimized although that's a separate issue itself. I've realized that unless I really shell out the money and get something like a high-end Schmidt Cassegrain with low f-stop, getting amazing detail from small galaxies will be impossible.

CS

Fabian

I think you are missing another point (like I did). To get those beloved details, the seeing had to be taken into account. I wanted a big scope with a long focal length and therefore a long range, too. I opened a thread to have a discussion about what to buy. But instead people convinced me, that the details I tried to get from our usual locations are not possible due to physics and the weather condition or the athmospere. All the things that people teached me could be experienced with my rig. So I accepted the fact, that it is really hard and expensive to reach that goal. It is kinda sad, but those beautiful details of tiny galaxies are really challenging.

I hope I guessed your interest right.

Christian Großmann:
Fabian:
@Christian Großmann thank you for sharing your experiences, they do sound indeed similar to mine. Theoretically, my mount and guiding should be able to handle such a high focal length, perhaps I just don't have my guide settings optimized although that's a separate issue itself. I've realized that unless I really shell out the money and get something like a high-end Schmidt Cassegrain with low f-stop, getting amazing detail from small galaxies will be impossible.

CS

Fabian

I think you are missing another point (like I did). To get those beloved details, the seeing had to be taken into account. I wanted a big scope with a long focal length and therefore a long range, too. I opened a thread to have a discussion about what to buy. But instead people convinced me, that the details I tried to get from our usual locations are not possible due to physics and the weather condition or the athmospere. All the things that people teached me could be experienced with my rig. So I accepted the fact, that it is really hard and expensive to reach that goal. It is kinda sad, but those beautiful details of tiny galaxies are really challenging.

I hope I guessed your interest right.

While we are digressing from the original question these are very important remarks every beginning astrophotographer should take to heart. We see all these beautiful images and then try to reach the same results with some shortcuts, while these images are the result of several years of practice slowly increasing the skill level. Even worse than trying with inadequate equipment is the attempt to achieve the aim by buying all the expensive gear without an appropriate level of competence. Been there, tried that too, failed miserably ...

Thank you for sharing your experiences. I wished I had this insight already when I started this wonderful hobby half a year ago. Have fun, accept your limitations for now and keep improving step by step.

Clear skies
Wolfgang

Interesting topic, and one that I think most folks starting out wonder about when they start adding up the price tag of all the equipment they want. The general advice folks give is mostly true (don't bother with the Barlow). If you don't want to do the math (to see if your pixel size will be below the Dawes limit or seeing conditions), you can always experiment and see what works. If you don't want to experiment, you can review my quick experiment from when I got my big scope last year. I had to wait a month for the focal reducer to arrive, so I captured some data with a 2x Barlow and compared it on the same target with the reducer a few weeks later. I've aligned and cropped the images for comparison.

Observations: With the reducer, there is substantially less noise and the stars are in focus across the field. With the barlow, the stars are slightly smaller and I can resolve a tiny bit more detail, but there is much more noise and the stars are not round on the right side of the image.

Conclusion: Save the Barlow for planetary imaging or targets so small that you would crop most of the image anyways (maybe the core of the cat's eye nebula), but don't bother for most deep sky targets.

15 x 180" Antlia 3.5nm Ha with 0.74x reducer

15 x 180" 3.5nm Antlia Ha with a 2x Celestron X-Cel Barlow

Scope: Sharpstar 140ph
Camera: ZWO 294MM Pro
Target: M16

Interesting to see the responses to this.

In fact, I was hoping to barlow my new scope when it arrives. Some comments suggest that barlowing is counterproductive, especially in light of limited aperture, which makes sense. In my case, I have, or will have, a 12 inch Newtonian and because it is 1200mm f.l. I thought I might be able to gain something with a 2X barlow. But I have not explored my pixel pitch, etc. yet. In fact, I could go through all that, but I am hoping to just throw in the barlow and give it a shot. The 12 inch imaging train will use a coma corrector for times that I want to illuminate my C-sized sensor. Under more normal conditions, my other camera has such a small sensor, I will not be using the coma corrector. So for use as a 2400mm telescope, I will have the coma corrector followed by the barlow. If anyone has done that, I would be happy to hear. Otherwise, I will post something here. Just don't hold your breath, since the telescope has not yet shipped and the weather here stinks anyway.

Alan

I have a 2x ed barlow and a TV 3x and only use them for lunar and planetary imaging. I had thought about trying them on tight globular clusters to resolve the core but just haven’t gotten around to switching things over.
In my case I use 10” & 12” f/5 newts. If your using that small of an aperture your image will definitely be dim and need much longer integration time.
I have to agree with the majority and just save your money for a new scope down the line.

I have a TeleVue 2x, 4x and an ES 5x barlow and tried them out - my personal viewpoint is - congruent to most prewriters - use high power barlows for planets and not for deep-space. My personal limit is to use an excellent 2x barlow as maximum magnification changer for deep-space imaging.

What those extenders do is to increase the magnification and to reduce the numerical aperture, meaning the resolving power of the scope. From a certain aperture upwards (increasing number), the diffraction hits in and you loose details - called empty magnification.

I would say, anything above F11 or 12 results for deep-space in empty magnification and not increase details. For my scopes, as example, for the Explore Scientific APO 127/952, the TeleVue 2x extender is the absolute maximum, already giving some emptiness… For my F4,7/1250 Newton, the 2x works still fine..
I am sure, you can see, what I ment!

Best regards and CS,

Georg

Something else that can be done (I did it in the past) is instead of a focal reducer or field flatterer you could use a paracor which would give you a slight magnification factor plus It's excellent for flattening out the field. At least that's been my experience with them.

Georg N. Nyman:
I have a TeleVue 2x, 4x and an ES 5x barlow and tried them out - my personal viewpoint is - congruent to most prewriters - use high power barlows for planets and not for deep-space. My personal limit is to use an excellent 2x barlow as maximum magnification changer for deep-space imaging.

What those extenders do is to increase the magnification and to reduce the numerical aperture, meaning the resolving power of the scope. From a certain aperture upwards (increasing number), the diffraction hits in and you loose details - called empty magnification.

I would say, anything above F11 or 12 results for deep-space in empty magnification and not increase details. For my scopes, as example, for the Explore Scientific APO 127/952, the TeleVue 2x extender is the absolute maximum, already giving some emptiness... For my F4,7/1250 Newton, the 2x works still fine..
I am sure, you can see, what I ment!

Best regards and CS,

Georg

I had never heard of this phenomenon but it does make physical sense, for my setup theoretically I am actually making useful magnification because I'm F10, however, it could be said the results from it are still not worth it. Thank you for your advice. CS

Georg N. Nyman:
I would say, anything above F11 or 12 results for deep-space in empty magnification and not increase details. For my scopes, as example, for the Explore Scientific APO 127/952, the TeleVue 2x extender is the absolute maximum, already giving some emptiness... For my F4,7/1250 Newton, the 2x works still fine..

Magnification is an empty word as far as imaging is concerned. It makes no sense at all. Same as saying that a given focal ratio is some kind of limit per se. It is not. What matters is the image scale regardless of the focal ratio and the aperture. Those are the only two parameters that have any bearing in assessing whether a given set-up is more or less efficient in producing deep sky images.

andrea tasselli:
Georg N. Nyman:
I would say, anything above F11 or 12 results for deep-space in empty magnification and not increase details. For my scopes, as example, for the Explore Scientific APO 127/952, the TeleVue 2x extender is the absolute maximum, already giving some emptiness... For my F4,7/1250 Newton, the 2x works still fine..

Magnification is an empty word as far as imaging is concerned. It makes no sense at all. Same as saying that a given focal ratio is some kind of limit per se. It is not. What matters is the image scale regardless of the focal ratio and the aperture. Those are the only two parameters that have any bearing in assessing whether a given set-up is more or less efficient in producing deep sky images.

I need to disagree because the term "empty magnification" is a technical term in optics. It describes a situation where no more information is made visible just the spatial distance between information content is made wider.

andrea tasselli:
Georg N. Nyman:
I would say, anything above F11 or 12 results for deep-space in empty magnification and not increase details. For my scopes, as example, for the Explore Scientific APO 127/952, the TeleVue 2x extender is the absolute maximum, already giving some emptiness... For my F4,7/1250 Newton, the 2x works still fine..

Magnification is an empty word as far as imaging is concerned. It makes no sense at all. Same as saying that a given focal ratio is some kind of limit per se. It is not. What matters is the image scale regardless of the focal ratio and the aperture. Those are the only two parameters that have any bearing in assessing whether a given set-up is more or less efficient in producing deep sky images.

It's not that difficult, resolving power or angular resolution is proportional to lambda/D (Rayleigh criterion), where D is the aperture. If you look at the Dawes limit it's 120/D, where D is the aperture (mind the different units).

If your pixel scale is much lower or much higher than that you are under or oversampling. That's were the focal length (or magnification) and pixel pitch comes into play. Pixel scale does not depend on D.

Neither of those parameters are meaningless...

Adding a barlow can be adequate if it has good optical quality, but since you need to expose x^2 longer it comes at a hefty prize.

Have you considered doing a 2x drizzle instead?

The term might have a meaning in the realm of visual optics since you are comparing it against the perceived angular size with naked eyes. But in imaging makes no sense whatsoever to talk about "magnification". You only talk about image scale and that's it.

andrea tasselli:
The term might have a meaning in the realm of visual optics since you are comparing it against the perceived angular size with naked eyes. But in imaging makes no sense whatsoever to talk about "magnification". You only talk about image scale and that's it.

If I give you a 4inch scope and and 8inch scope with the same focal length and the same camera, i.e. the same image scale, you'll take the 4inch because it's lighter?

D. Jung:
If I give you a 4inch scope and and 8inch scope with the same focal length and the same camera, i.e. the same image scale, you'll take the 4inch because it's lighter?

What's that got to do with anything I've written so far in this thread? Nothing. The image will be 4x as bright (for the 8") for the same exposure (of the 4") but that is about it. Depending on the seeing it might also be able to afford a better angular resolution if the imaging parameters are right. SO what?

Incidentally, writing:

D. Jung

Adding a barlow can be adequate if it has good optical quality, but since you need to expose x^2 longer it comes at a hefty prize.

Is factually wrong. If you add a barlow or a 2x focal extender to be more precise, the image scale doubles which means the exposure needs to be quadruple in order to have the same intensity of the one without said extender.

andrea tasselli:
D. Jung:
If I give you a 4inch scope and and 8inch scope with the same focal length and the same camera, i.e. the same image scale, you'll take the 4inch because it's lighter?

What's that got to do with anything I've written so far in this thread? Nothing. The image will be 4x as bright (for the 8") for the same exposure (of the 4") but that is about it. Depending on the seeing it might also be able to afford a better angular resolution if the imaging parameters are right. SO what?

Incidentally, writing:

D. Jung
Adding a barlow can be adequate if it has good optical quality, but since you need to expose x^2 longer it comes at a hefty prize.

Is factually wrong. If you add a barlow or a 2x focal extender to be more precise, the image scale doubles which means the exposure needs to be quadruple in order to have the same intensity of the one without said extender.

This is getting really silly, x^2 is the common way of expressing the square... Besides that, adding a 2x would half the pixel scale, not double it.

This conversation is getting to a point where optical science is getting ignored by some contributors - if they even understand it. I quit here this discussion, it has become of no value.

Peace guys - in particular in these days …! Thank you for your contributions, very helpful indeed.

Clear skies
Wolfgang