Imaging smaller galaxies with a 8'' newtonian and a barlow

You need a coma correcting barlow (to replace the existing CC) and the only ones I know of are the APM 2" 1.5x and the 1.25" 2.7x . In theory the 1.5x should fit the bill but its price is quite steep.

Thanks for pointing out the need to replace the coma corrector and for pointing me to that barlow. The result is indeed quite attractive. Those beautiful image Fritz. 

The price tag is indeed quite restrictive. I need to think of it. I also wonder if that kind of barlow would be interesting. It is less expensive, but the magnification factor is a bit high. That could work to kick into planetary. 
https://www.astroshop.eu/barlow-lenses/apm-comacorrecting-ed-barlow-element-2-7-x-1-25-/p,50199#description

To answer Fabio : two years ago, I was hesitating between a newtonian and a RC, both 8''. I went for the newtonian. One thought was that flexibility would be easier to achieve with the newtonian : it would be easier to bring the 200/800 to 1600 than to use a RC with a reduced. I am now thinking of testing this assumption. 

I will inquire and get back with further questions. Many thanks for you input. Any other thoughts always welcome !

I dont think that's a very good idea, unless your location has sub arcsecond seeing regularly. You are already at an aggressive sampling rate of 1.19''/px, which will definitely be seeing limited almost all of the time, and with a barlow your captures would just be slower with no extra detail.

 I looked through your galaxy images and to my eye it looks like you dont have the kind of seeing that could benefit from extra focal length, and in fact you have not really extracted all the detail from 800mm focal length either (not critique mind you, just an observation on your typical sky conditions. The images are very nice!)

You also have to think of the implications when it comes to speed of capture, or speed in general. At twice the focal ratio you need to expose 4 times longer to reach the same signal to noise ratio in a single sub exposure, and if you fail to do this you may run into issues where read noise plays too large a part in the total noise of an image and so you need more data to make up for that loss (the main drawback for slow scopes). Total needed integration time to reach a certain SNR also quadruples, unless you bin x2 but then what was ever the point of using the barlow?

And like was already said, needs to be a coma corrected barlow so not too many options to pick from. There is a reason why nobody is seriously discussing barlows and DSO imaging, it just does the opposite of what we typically want the scope to do.

All being said and done I personally rather than coughing up 500 euros I'd spend a trifle little more and get a cooled camera with smaller pixels, such as the PlayerOne Uranus-C Pro with its 2.9 um pixels, which would yield an increase of image scale of 60%, which should be plenty for typical seeing condition. In the end the barlow would be used only sporadically while the camera can be used all the time and on other scopes/lens too.

Oskari Nikkinen:
I dont think that's a very good idea, unless your location has sub arcsecond seeing regularly. You are already at an aggressive sampling rate of 1.19''/px, which will definitely be seeing limited almost all of the time, and with a barlow your captures would just be slower with no extra detail.

 I looked through your galaxy images and to my eye it looks like you dont have the kind of seeing that could benefit from extra focal length, and in fact you have not really extracted all the detail from 800mm focal length either (not critique mind you, just an observation on your typical sky conditions. The images are very nice!)

You also have to think of the implications when it comes to speed of capture, or speed in general. At twice the focal ratio you need to expose 4 times longer to reach the same signal to noise ratio in a single sub exposure, and if you fail to do this you may run into issues where read noise plays too large a part in the total noise of an image and so you need more data to make up for that loss (the main drawback for slow scopes). Total needed integration time to reach a certain SNR also quadruples, unless you bin x2 but then what was ever the point of using the barlow?

And like was already said, needs to be a coma corrected barlow so not too many options to pick from. There is a reason why nobody is seriously discussing barlows and DSO imaging, it just does the opposite of what we typically want the scope to do.

Thanks @Oskari Nikkinen for this kind comment. A friend of mine imaging nearby indeed replaced its RC by a newtonian for exactly that reason. He also provided evidence with some galaxy image that the outcome with the RC was not better than the outcome with the newtonian. This, supported by your comment, kills the idea it seems. 

My I ask you how did you diagnose the sky quality in my image? I am indeed very curious at this point.

patrice_so:
My I ask you how did you diagnose the sky quality in my image? I am indeed very curious at this point.

Just compared your images to what i have taken with my own 8'' newtonian in good seeing (1-1.3'' fwhm seeing, very rare for me). If your seeing was regularly good, your M101 image for example would look not too different from mine at 1''/px, but since they are different i assume that this kind of seeing is rare for you. Similar deal with our M51 images, i had very good seeing for a short amount of time for that one.

The smallest stars are also disks most of the time, and not pointlike objects. This one is not so useful a metric because stars are often processed separately at will so they could be any size the person processing wants them to be.

Oskari Nikkinen:
I dont think that's a very good idea, unless your location has sub arcsecond seeing regularly. You are already at an aggressive sampling rate of 1.19''/px, which will definitely be seeing limited almost all of the time, and with a barlow your captures would just be slower with no extra detail.

 I looked through your galaxy images and to my eye it looks like you dont have the kind of seeing that could benefit from extra focal length, and in fact you have not really extracted all the detail from 800mm focal length either (not critique mind you, just an observation on your typical sky conditions. The images are very nice!)

You also have to think of the implications when it comes to speed of capture, or speed in general. At twice the focal ratio you need to expose 4 times longer to reach the same signal to noise ratio in a single sub exposure, and if you fail to do this you may run into issues where read noise plays too large a part in the total noise of an image and so you need more data to make up for that loss (the main drawback for slow scopes). Total needed integration time to reach a certain SNR also quadruples, unless you bin x2 but then what was ever the point of using the barlow?

And like was already said, needs to be a coma corrected barlow so not too many options to pick from. There is a reason why nobody is seriously discussing barlows and DSO imaging, it just does the opposite of what we typically want the scope to do.

For imaging galaxies and small PN, 1.19" is never going to be enough. Focal length or a higher resolution will always net a better result in even average seeing(2"-3"). At 2" seeing you want to be at less than 1" ideally, .5"-.75". With the small size of pixels these days on devices and screens, 4x is ideal to me. I'd rather ALWAYS oversample than ever undersample.

If you're too oversampled you can always use integer resample. The inverse, drizzle does F'all for our purposes.
With decon or even say blurxterminator, the more pixels you have the better the result!
I can prove this time and time again having many telescopes at multiple focal lengths.




To the OP, a barlow isn't a great route compared to a telescope with a larger native focal length but they do work. I played around with newts and barlows in the past but have since used only RCs in the past couple years. They're also more mechanically superior. I have yet to find a scope better than an 8" RC.

Matthew Proulx:

Oskari Nikkinen:
I dont think that's a very good idea, unless your location has sub arcsecond seeing regularly. You are already at an aggressive sampling rate of 1.19''/px, which will definitely be seeing limited almost all of the time, and with a barlow your captures would just be slower with no extra detail.

 I looked through your galaxy images and to my eye it looks like you dont have the kind of seeing that could benefit from extra focal length, and in fact you have not really extracted all the detail from 800mm focal length either (not critique mind you, just an observation on your typical sky conditions. The images are very nice!)

You also have to think of the implications when it comes to speed of capture, or speed in general. At twice the focal ratio you need to expose 4 times longer to reach the same signal to noise ratio in a single sub exposure, and if you fail to do this you may run into issues where read noise plays too large a part in the total noise of an image and so you need more data to make up for that loss (the main drawback for slow scopes). Total needed integration time to reach a certain SNR also quadruples, unless you bin x2 but then what was ever the point of using the barlow?

And like was already said, needs to be a coma corrected barlow so not too many options to pick from. There is a reason why nobody is seriously discussing barlows and DSO imaging, it just does the opposite of what we typically want the scope to do.

For imaging galaxies and small PN, 1.19" is never going to be enough. Focal length or a higher resolution will always net a better result in even average seeing(2"-3"). At 2" seeing you want to be at less than 1" ideally, .5"-.75". With the small size of pixels these days on devices and screens, 4x is ideal to me. I'd rather ALWAYS oversample than ever undersample.

If you're too oversampled you can always use integer resample. The inverse, drizzle does F'all for our purposes.
With decon or even say blurxterminator, the more pixels you have the better the result!
I can prove this time and time again having many telescopes at multiple focal lengths.




To the OP, a barlow isn't a great route compared to a telescope with a larger native focal length but they do work. I played around with newts and barlows in the past but have since used only RCs in the past couple years. They're also more mechanically superior. I have yet to find a scope better than an 8" RC.

There is chapter and verse of theory regarding optimal sampling, all of which i think is rather useless today with BlurXterminator so wont say anything about that. However i think your advice of increasing focal length is just not right, because if we are oversampled at a focal length of X then we will still be oversampled at a focal length of 2X and so no additional detail can be seen and the system has just gotten slower - so only negatives can come from that.

Sure youre not mixing aperture and focal length here? Because i will agree with the statement that a larger aperture will always resolve more than a smaller one in any given seeing. But just a blanket statement of "more focal length is better" couldn't be further form the truth in my opinion.

Once optimum sampling has been reached (which is tied to several factors) no further increase of image sampling will result in increase in resolution, just an increase of the object's size. At the expense of the SNR.

Oskari Nikkinen:

Matthew Proulx:

Oskari Nikkinen:

Matthew Proulx:

Oskari Nikkinen:
I dont think that's a very good idea, unless your location has sub arcsecond seeing regularly. You are already at an aggressive sampling rate of 1.19''/px, which will definitely be seeing limited almost all of the time, and with a barlow your captures would just be slower with no extra detail.

 I looked through your galaxy images and to my eye it looks like you dont have the kind of seeing that could benefit from extra focal length, and in fact you have not really extracted all the detail from 800mm focal length either (not critique mind you, just an observation on your typical sky conditions. The images are very nice!)

You also have to think of the implications when it comes to speed of capture, or speed in general. At twice the focal ratio you need to expose 4 times longer to reach the same signal to noise ratio in a single sub exposure, and if you fail to do this you may run into issues where read noise plays too large a part in the total noise of an image and so you need more data to make up for that loss (the main drawback for slow scopes). Total needed integration time to reach a certain SNR also quadruples, unless you bin x2 but then what was ever the point of using the barlow?

And like was already said, needs to be a coma corrected barlow so not too many options to pick from. There is a reason why nobody is seriously discussing barlows and DSO imaging, it just does the opposite of what we typically want the scope to do.

For imaging galaxies and small PN, 1.19" is never going to be enough. Focal length or a higher resolution will always net a better result in even average seeing(2"-3"). At 2" seeing you want to be at less than 1" ideally, .5"-.75". With the small size of pixels these days on devices and screens, 4x is ideal to me. I'd rather ALWAYS oversample than ever undersample.

If you're too oversampled you can always use integer resample. The inverse, drizzle does F'all for our purposes.
With decon or even say blurxterminator, the more pixels you have the better the result!
I can prove this time and time again having many telescopes at multiple focal lengths.




To the OP, a barlow isn't a great route compared to a telescope with a larger native focal length but they do work. I played around with newts and barlows in the past but have since used only RCs in the past couple years. They're also more mechanically superior. I have yet to find a scope better than an 8" RC.

There is chapter and verse of theory regarding optimal sampling, all of which i think is rather useless today with BlurXterminator so wont say anything about that. However i think your advice of increasing focal length is just not right, because if we are oversampled at a focal length of X then we will still be oversampled at a focal length of 2X and so no additional detail can be seen and the system has just gotten slower - so only negatives can come from that.

Sure youre not mixing aperture and focal length here? Because i will agree with the statement that a larger aperture will always resolve more than a smaller one in any given seeing. But just a blanket statement of "more focal length is better" couldn't be further form the truth in my opinion.

Focal length gives you increased resolution with the same sensor. "I" am not mixing anything here. 1.19" is simply limiting your resolution for absolutely no reason. My esprit 120 is exactly 1.19" with the reducer at 640mm focal length and it does not achieve anywhere close to what I do at 2000-2400mm focal lengths. In someone's dreams perhaps but not in reality. 

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Your Esprit has an aperture of 120mm, whereas i am assuming you are talking about your 250 or 300mm instruments with the 2000-2400mm focal lengths. Its like comparing apples and oranges. The focal length has almost nothing to do with the amount of resolved detail in this case.

Since you mention a reducer for the Esprit its a nice segway to another topic which is the blur that comes from corrective optics and reducers. Most correctors/flatteners/reducers are a compromise between sharpness in the center of the field of view and in the edges of the corrected field. The end result is that none of the field of view is close to the diffraction limit, but the edges are tolerable. With a larger aperture instrument the diameter of the diffraction limited field of view is larger 'even without' a corrector, and with a corrector the tradeoff is significantly smaller (could be no tradeoff for modest sensor sizes). This fact makes your comparison of a small 120mm aperture refractor with a reducer and an enormous reflector rather silly. Not sure we could have a more unfair comparison even if we tried.

Focal length has a 'chance' of giving you more resolution, if we are not oversampled to begin with ( i will argue that at 1.19''/px we almost certainly are, OPs images support this theory).

It's not though, the only difference here is the amount of photons they collect in the same amount of time. We're talking about resolution. Unless the device is diffraction limited in this case the Esprit is .97" limited. I can take a 10" and reduce it to the point where my focal length is 640mm (1.19") and it will not resolve the same detail as a 10" at 1000mm (.78") using the same sensor. There's just not F'ING way around this. 

I dont know how many times I have to say this but 1.19" is NOT enough unless you are always above 3" seeing which is hard to believe anywhere. If you are, you may as well get into the hobby of knitting. 


Many of you dont have as bad seeing as you think. You have poor guiding, poor focus, poor collimation, tilt etc. My images get better every year and my sky isn't getting better. My knowledge and tools are. I'd rather be in the situation where I can bin if the seeing is bad or take advantage of good seeing which almost everyone is going to get at some point.

Matthew Proulx:
Focal length gives you increased resolution with the same sensor. "I" am not mixing anything here. 1.19" is simply limiting your resolution for absolutely no reason. My esprit 120 is exactly 1.19" with the reducer at 640mm focal length and it does not achieve anywhere close to what I do at 2000-2400mm focal lengths. In someone's dreams perhaps but not in reality. 



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You don't have 120mm aperture when imaging at 2000mm FL though. You have 300mm, which gives you 0.4" maximum resolving power (instead of 1" which is the case at 120mm). Subsequently, the larger focal length allows you to use that power. So would a camera with smaller pixels. Or Drizzle (less optimally). 

If you put a 3X barlow on that 120mm eprit, you will not see the same improvement at all. What you will see will be very similar to putting the 640mm image in Photoshop and scaling up 300%. 

In the OP's case, the theoretical maximum resolution of his telescope is 0.6" but due to pixel size and focal length he can only achieve 1.19". So there is room to increase focal length, although it is very likely he is already oversampling at 1.19" due to local atmospheric conditions which are the limiting factor.