Using a Barlow for imaging

[Mal22]

Hi everyone, 

Now that we’re in galaxy season I’m weighing up whether or not to add my 2x Barlow lens to the imaging train to get a bit more up close and personal with the likes of M51, M101 and M81 + 82. 
 

What are the considerations in doing so? I know that it will double the F ratio and therefore I would need twice as much imaging time, but is that a worthwhile trade off to get a closer up image? Or is sticking with the non-Barlow field of view and going for a closer crop when processing a better approach? I tend to get good guiding over long exposures (with guide scope/camera) with good stars so I think that will still be ok. 
 

My set up is a WO Zenithstar 73 and ZWO ASI294MC Pro. Below is the current FOV on M51 without a Barlow, and an image I did with this set up last summer (before I knew how to use PixInsight properly…)

Thanks for any advice! 

 

 

[woldsman]

I doubt a Barlow will help you. As well as dramatically increasing imaging time, the Barlow could introduce significant optical aberration unless it is an expensive four element design. The practical limits you are working with are set by your scope’s aperture and seeing conditions. Your resolution limit with the scope is about 1.6 arc seconds. The imaging scale of your camera (if pixel size is 4.6 microns) with the zenithstar 73  is 2.2 arc seconds. Good seeing is 2 arc seconds. So your equipment seems well suited and the best approach is to image at a dark site with good seeing. A Barlow won’t improve resolution and so you might as well crop the image obtained without it in far less time.  Don’t use a reducer though as you could end up under-sampling (blocky stars). 

Edited April 8 by woldsman

[Mal22]

1 hour ago, woldsman said:

I doubt a Barlow will help you. As well as dramatically increasing imaging time, the Barlow could introduce significant optical aberration unless it is an expensive four element design. The practical limits you are working with are set by your scope’s aperture and seeing conditions. Your resolution limit with the scope is about 1.6 arc seconds. The imaging scale of your camera (if pixel size is 4.6 microns) with the zenithstar 73  is 2.2 arc seconds. Good seeing is 2 arc seconds. So your equipment seems well suited and the best approach is to image at a dark site with good seeing. A Barlow won’t improve resolution and so you might as well crop the image obtained without it in far less time.  Don’t use a reducer though as you could end up under-sampling (blocky stars). 

Thank you, very helpful response! 
 

So my misguided assumption was that using a Barlow would ultimately yield more detail in the galaxy, but clearly that’s not the case? 20 hours with a 2x Barlow would get no more detail and resolution than 10 hours without. 
 

That settles it then. No Barlow. I’m in the UK too which it seems is just one big permanent cloud now anyway so a decision to double the necessary imaging time would have needed real justification…

[900SL]

A 2x barlow requires 4x the image time. Its an area thing. Only useful on bright targets, lunar etc

[woldsman]

2 hours ago, Mal22 said:

Thank you, very helpful response! 
 

So my misguided assumption was that using a Barlow would ultimately yield more detail in the galaxy, but clearly that’s not the case? 20 hours with a 2x Barlow would get no more detail and resolution than 10 hours without. 
 

That settles it then. No Barlow. I’m in the UK too which it seems is just one big permanent cloud now anyway so a decision to double the necessary imaging time would have needed real justification…

Others may have a different opinion but, yes, that’s what I think. I don’t see how you can better the resolution you are currently achieving. You would need more aperture to capture more photons. While a Barlow would improve sampling a bit the cost is x4 imaging time & potentially aberrations. On the Moon and larger planets, there would be contrast and a Barlow could help - but not on dim galaxies. 

[Clarkey]

You might improve resolution with a barlow, but at the cost of much more time. The set up as it is gives a pixel scale of around 2.2 arcsecs/px which is fine for widefield imaging. It is probably a little bit undersampled for galaxy imaging, but not excessively. With UK seeing you cannot get more detail than the atmosphere will allow. I image with an RC8 with a reducer and bin 2x2, which gives a scale of 1.3 arcsecs/px which I find is about right. Either use the scope as it is and crop out the sky - or get more focal length. For a 294 something around 1000mm would be ideal.