Purchasing a Barlow instead of a new scope

[Jonny_H]

Hi all,

I have a question in the hope that you could assist.

One of my scopes is a SW 150PDS and I have been toying with the idea of replacing it with a larger aperture/longer focal length scope. 

However I have also been looking at Barlows which would be significantly cheaper than a new OTA. However I've never owned one before so not sure one would suit my set-up.

The 150 PDS is F/5 750mm FL scope and I have been looking at, for example, the BST Starguider 2x Barlow - so the end result would be a F/10 1500mm (I believe).

My question is however, what impact would this have on imaging, other than needing to increase my exposure time vs what I am currently doing? Also how difficult is it (or not) to use a Barlow in an AP set-up? My current camera is the 533MC Pro.

Apologies for all the questions....but would very much appreciate any comments.

Thanks,

Jon

[happy-kat]

Are you intending to image the Moon or planets?

If still wanting to image DSO with your setup then the barlow is a quick change for then doing planets. You might want to check the calculations for your setup as it might be you'd want more than x2 barlow.

Adding a barlow doesn't bring the thought is my mount up to the weight of a bigger telescope.

Edited May 30, 2021 by happy-kat

[Jonny_H]

Thanks Kat.

The Barlow on my Newt would be used for both moon and planets. Possibly a bit of DSO work but I have my Esprit 100 for DSO's primarily.

I haven't actually imaged planets yet but what is the 'sweet spot' in terms of focal length?

[happy-kat]

Better answered by someone else to help you, hopefully you'll get a reply for your question.

 

[Jonny_H]

Thanks Kat.

I will await the input of others.

Alternatively I have been looking at something like the StellaLyra 8" f/12 M-LRS Classical Cassegrain.  Although as mentioned above a new scope is significantly more expensive than a Barlow.

[Ags]

The resolution of a telescope is determined only by the aperture not the focal length. You can use a barlow for planetary imaging, but only to match the telescope's native resolution to the pixel size of the chip (more focal length means bigger image scale, which would be a better match for a sensor with big pixels). 

[Louis D]

Calling @vlaiv.  He can probably best answer your imaging question vis-a-vis pixel resolution when adding a Barlow.

[vlaiv]

Indeed, as @Ags mentioned - if you are interested in planetary and lunar there are a few things to consider.

1. For planetary, there is something called critical sampling rate / critical resolution. This means that you should not "zoom in" further than this as it makes no point to do so - you won't be able to record any additional detail as simply - telescope will not resolve it. There is limit to how much telescope will resolve and that depends on aperture.

ASI533 has 3.75µm pixel size and critical sampling rate is achieved at about F/15 (it depends on wavelength of light used, and we usually use green light around 500nm to get this value).

This means that you should not use stronger than x3 barlow lens on your 150 F/5 newtonian

2. You are using fast newtonian and such scopes have rather small diffraction limited field. As soon as you move away from optical axis - coma will start to increase. You need to use coma corrector for larger sensor if you do DSO imaging - but for planetary imaging coma corrector is not good - as it introduces spherical aberration on axis. For planetary imaging you want best sharpness possible.

This means that you won't be able to use whole ASI533 sensor to image the Moon and planets - you'll need to use only central portion of it by using ROI - region of interest.

Coma free field in newtonian is given by this expression:

from: https://www.telescope-optics.net/newtonian_off_axis_aberrations.htm

In F/5 scope that would mean - ~1.39mm or about 2.8mm diameter. If you add x3 barlow - that gets magnified to 8.34mm - or about 1/2 of 16mm diagonal of ASI533.

For lunar - use only half of the sensor if you use x3 barlow. For planetary - use just part of sensor that fits and keeps the planet in FOV - like 640x480px, as that impacts achieved FPS

3. You can use barlow element (not telecentric lens - you can't do this with telecentric) - to dial in magnification. Move sensor away from barlow element - get higher magnification, move closer to sensor - get lower mag. You can dial in how much magnification you want (just be careful that coma free field is always 2.8mm diameter to start with - use x2 barlow and you can only image 5.6mm diameter without coma)

4. Baader VIP is the best barlow to use for imaging. APM has x2.7 coma correcting barlow - which is good option for lunar as it will extend usable field. But you can use simple GSO x2 barlow element as well, it won't make great difference on image quality (much more depends on other factors like collimation of the scope, seeing, way you capture and process and so on)

in the end, don't use barlow for DSO imaging. Although it may seem that you'll get better framing and get closer in when using barlow - and that you can offset that with longer exposure, with DSO imaging similar thing holds as with planetary - there is limit to how much "magnification" you can use. In DSO imaging it is expressed in arc seconds / pixel - or pixel scale, and you want to stay above 1.2-1.4"/px with your setup.

You are already at 1.03"/px with ASI533 and 750mm FL scope - you are already slightly over that sky imposed limit. Using higher sampling rate / resolution will simply result in "zoomed" in - but blurry image devoid of detail when you look at it at 100% zoom level.

 

 

[Jonny_H]

Thanks all and thanks Vlaiv. Always very informative reading your responses.

That is a very valid point you make which stupidly I hadn't thought of. Even with a 2x Barlow this takes me down to .52 "/pixel with my current camera and newt arrangement. Doable but would presumably need some nifty guiding?

I think I need to look into this a little more and do more research - will just stick with my DSO imaging for now 🙂

[vlaiv]

23 minutes ago, Jonny_H said:

Doable but would presumably need some nifty guiding?

If you think about DSO imaging - well, good guiding / poor guiding - that will over sample. Simple truth is - seeing won't allow for such resolution.

Planetary & lunar type / lucky imaging works differently - it beats the seeing by using very short exposures around 5ms or less. It effectively freezes the seeing and while view is distorted - it is not "motion blurred" additionally.

Barlow is good option for those, but for DSO imaging - you are already up there without barlow.