Barlow with an astrocamera?

[900SL]

I'm toying with the idea of using a Barlow with an image train for Lunar (or even experimenting with Solar) as follows:

GT71 with a 1x field flattener only (but see below), 2x Barlow, appropriate filter and a 533MC pro

My simple understanding (as I'm new to Barlows) is that this will double the focal length to 820mm (subject to position), decrease the aperture to F12 and drop the image scale to around 1'/px, which is presumably below the resolution limit of the GT71.

I have a few questions before I send any money to Flo as follows:

1: Can I use the GT71 without a field flattener given that the Barlow will double the size of the image circle and I will only be imaging the center of the field with the 11mm 533 sensor? Or am I likely to see a lot of field curvature to the edges? 

2: the GT71 has a 2" eyepiece adaptor behind the focuser so I assume a 2" Barlow can be mounted here and the position adjusted?

3: My 533MC came with a  1.25" adaptor and I have a bunch of M48/M42 adaptors and spacers, I'm not sure what is the best way to mount the 533MC to the Barlow and whether I need to consider backfocus

4: I have a 178MM incoming if FLO ever get round to shipping my order   (waiting on a Neumann tilt adjuster) and would this be worth using for lunar at 2x2 binning with appropriate filter?

 

Or am I wasting my time

 

Thanks in advance for any consideration

 

 

 

[vlaiv]

1) That is preferred way of using a barlow on a scope. You don't need field flattener as central part of the field is usually quite flat. It is the corners that have issues.

Barlow magnifies the image and you end up with only small portion of central field on the sensor with it - so there is really no need for flattener as that part is really flat.

2) 2" barlow is probably overkill for 99% of use cases - just use normal 1.25" barlow.

3) You can either use 1.25" nose piece that you will insert into barlow, or you can use T2 adapters and spacers if you happen to have barlow with detachable barlow element. Second option is preferred way of using barlow as you can adjust barlow to sensor distance.

Barlow element magnification varies with its distance to focal plane - and in case of imaging this means barlow / sensor distance. If you make this distance smaller - you will get less magnification from barlow, if you increase this distance - magnification will increase. There is only one position where barlow works "as prescribed" (like x2 barlow).

Ideally - you want to dial in wanted magnification factor (does not need to be x2) depending on pixel size.

If you want to do planetary imaging (solar/lunar included) - and want to experiment with lucky imaging, then there is simple formula to follow: needed F/ratio of your setup needs to be 4 x pixel size (unless you are using narrowband filters - then there is different formula).

This means that for ASI533 and its 3.75um pixel size - you need F/15 as optimum F/ratio. While for ASI178 and its 2.4um pixel size - you only need F/9.6

Your scope is F/5.9 so you'll need ~ x2.5 barlow for ASI533 and x1.5 barlow for ASI178.

This is why using barlow element and variable distance helps - you can dial in needed F/ratio

4) Sure you can use ASI178 for lunar, but you won't need to bin x2. If you are planning on using some sort of NB filter to help with seeing (you can try Ha, or SII or even OIII, or something like Baader Solar Continuum filter), then you need to use slightly different formula for F/ratio.

F/ratio = pixel_size * 2 / central wavelength

Where central wavelength is central wavelength of used filter in microns (same units as pixel size). If you want to use Ha NB filter than central wavelength will be 0.656um. For OIII it will be 0.5um and for Baader Solar Continuum it will be 0.54

This does not make big difference, but there is some difference

Ha F/ratio for ASI178 is F/7.3

OIII F/ratio for ASI178 is F/9.6

Solar continuum F/ratio for ASI178 is F/8.9

Just a note - longer the wavelength, less detail will be "available" (this is represented as lower F/ratio needed) - but atmosphere will be more "stable" (atmospheric dispersion also depends on wavelength and longer wavelengths are less distorted / less bent), so it is a compromise. Most people use Ha filter, but I think that Solar continuum should give best results for both solar (white light) and lunar.

 

 

[900SL]

Thanks Vlaiv, I don't use awesome lightly but that was an awesome response. Many thanks (again)

[900SL]

On 13/09/2022 at 22:56, vlaiv said:

1) That is preferred way of using a barlow on a scope. You don't need field flattener as central part of the field is usually quite flat. It is the corners that have issues.

Barlow magnifies the image and you end up with only small portion of central field on the sensor with it - so there is really no need for flattener as that part is really flat.

2) 2" barlow is probably overkill for 99% of use cases - just use normal 1.25" barlow.

3) You can either use 1.25" nose piece that you will insert into barlow, or you can use T2 adapters and spacers if you happen to have barlow with detachable barlow element. Second option is preferred way of using barlow as you can adjust barlow to sensor distance.

Barlow element magnification varies with its distance to focal plane - and in case of imaging this means barlow / sensor distance. If you make this distance smaller - you will get less magnification from barlow, if you increase this distance - magnification will increase. There is only one position where barlow works "as prescribed" (like x2 barlow).

Ideally - you want to dial in wanted magnification factor (does not need to be x2) depending on pixel size.

If you want to do planetary imaging (solar/lunar included) - and want to experiment with lucky imaging, then there is simple formula to follow: needed F/ratio of your setup needs to be 4 x pixel size (unless you are using narrowband filters - then there is different formula).

This means that for ASI533 and its 3.75um pixel size - you need F/15 as optimum F/ratio. While for ASI178 and its 2.4um pixel size - you only need F/9.6

Your scope is F/5.9 so you'll need ~ x2.5 barlow for ASI533 and x1.5 barlow for ASI178.

This is why using barlow element and variable distance helps - you can dial in needed F/ratio

4) Sure you can use ASI178 for lunar, but you won't need to bin x2. If you are planning on using some sort of NB filter to help with seeing (you can try Ha, or SII or even OIII, or something like Baader Solar Continuum filter), then you need to use slightly different formula for F/ratio.

F/ratio = pixel_size * 2 / central wavelength

Where central wavelength is central wavelength of used filter in microns (same units as pixel size). If you want to use Ha NB filter than central wavelength will be 0.656um. For OIII it will be 0.5um and for Baader Solar Continuum it will be 0.54

This does not make big difference, but there is some difference

Ha F/ratio for ASI178 is F/7.3

OIII F/ratio for ASI178 is F/9.6

Solar continuum F/ratio for ASI178 is F/8.9

Just a note - longer the wavelength, less detail will be "available" (this is represented as lower F/ratio needed) - but atmosphere will be more "stable" (atmospheric dispersion also depends on wavelength and longer wavelengths are less distorted / less bent), so it is a compromise. Most people use Ha filter, but I think that Solar continuum should give best results for both solar (white light) and lunar.

 

 

 

Nothing is ever easy in Astro, or the 'practice of accumulating millions of adaptors, extension tubes and brackets'

I've finally got round to getting a Baader Hyperion 2.25 x from FLO. This is a 3 element design and supposedly has good correction for photography, and is adjustable (more on that later)

It comes in 3 pieces and you use the T2 adapter 'B' for a DSLR connection with 55mm backspace from the back of the 'B' adapter, or you can screw the lens element directly into a ZWO 1.25" nose piece for direct connection to an ASI camera for 28mm backspace (29mm + 12.5mm - 13.6mm)

 

 

 

Here's where the fun starts.

With the 55mm backspace on a Nikon D5600 DSLR or 28mm backspace ASI 178mm, mounted in the 1.25" eyepiece clamp on the GT71, there isn't enough rack on the focuser to achieve focus

 

Additionally, at 55mm backfocus, the barlow magnification is not 2.25x but 2.7x, according to this article

https://www.baader-planetarium.com/en/blog/barlow-lenses-their-magnification-factors-and-working-distances/

 

The 28mm ASI back space should give around 2.3 x mag. I tried a 60mm long M42 extension tube and 1.25" adapter direct into the rear of the GT71 eyepiece clamp, with another 1.25" clamp at the far end , so around 80mm total length and this Heath Robinson contraption got me in focus eventually, racked out a fair distance

To get something workable I think I need a WO M63 to M48 adapter to connect directly to the focus tube. I have various M48 to M42 adapters and I can use a spare M42 to 1.25mm eyepiece clamp before the barlow.

 

 

 

 

[900SL]

Finally had a chance to see if this contraption works. Took an hour to find focus but first effort with the TS CF 90 APO and the Hyperion barlow and ASI 178MM seemed to work OK 

First attempt at Autostakkert, trying my hand at lucky imaging