Reducer or New Scope?

[Mach13]

Hello and welcome to my first Astro topic..  So I am new to Astronomy... I have a skymax 127 with AZ-GTi mount which I had its maiden outing this week.   I'm using a basic SV205 camera fitted direct to the tube.  My eventual image of the moon is below after stacking et-al..  I've  gray-scaled it as the colour is just  rubbish on that camera.  But as you can see I'm tightly 'zoomed' in.

 

Thing is I'd like to be able to get the whole moon in my FOV.  I've seen YT vids on using reducers but I'm wondering  if anyone here has done it on a 127?

 

If so...

1 How did the image work out?

2 What's the cost of getting that to work V cost of another more suited (smaller i guess) scope?  

 

 

[vlaiv]

Hi and welcome to SGL.

Best and most cost effective way to get full lunar disk image is to do mosaic.

That is done in the same way as you already started - except you take multiple shots of moon (usually 6 to 12 but people have done over 50 panel mosaics). It is a bit more involved as far as processing goes - you need to stitch that mosaic together - but rest is the same.

Smaller scope and focal reducers do work - but you loose resolution and sharpness. Doing mosaics is really the best way to handle full disk shots.

For example, this full disk image was taken with Mak102 and multiple panels:

Actual image is 3084 x 2740 and is quite zoomed in - here is small segment of it:

[Mach13]

Wow Sub Giant... thanks for the prompt reply... that image is STUNNING!  🤩Yep i'm shouting that...  

 

So what resolution did you shoot each segment?

I imagine if you shoot it all in 4K then that's gonna stitch to a whopping image file  🤯

[vlaiv]

4 minutes ago, Mach13 said:

So what resolution did you shoot each segment?

You'll soon find out that we measure things differently when doing astronomy and astro imaging.

For example - resolution when doing planetary imaging is not about pixel count - but rather about what can telescope possibly resolve as there is limit to what telescope, depending on its aperture size - can resolve.

Above image was captured at ~0.36"/px originally and I reduced it to about 0.72"/px in processing (or half the resolution) - as sky at the time of the capture did not support full resolution.

You can see whole image here:

http://serve.trimacka.net/astro/Forum/2020-07-30/moon.png

That means that each pixel represents less than one arc seconds in the sky. Moon is about 30 arc minutes in diameter - that means that we captured at 30 * 60 / 0.72 = 1800 / 0.72 = ~2500px across the diameter.

Each panel was done at rather small resolution (pixel count) I think. Camera that I was using has ~3000x2000 pixels, but with planetary imaging it is very important to get high frame count and for that reason, smaller resolution is often used. I probably used 1280x1024 or something like that for each panel.

[Mach13]

Thanks again.. that's really helpful..  my next session planned then...  I will defo add that my to do list...  

[vlaiv]

In the mean time - I've found technical info on that image:

It was taken at full 3096 x 2080 (ASI 178mc camera) per panel - without ROI (Region of interest - smaller section of chip which produces smaller image) - it was composed out of 9 panels.

[PEMS]

It will not be easy to get a single full image of the moon with the SV205. Seems that the sensor is the IMX179 and that it is 4mm square. From my past days of simple geometry and if the moon is 0.5 degrees that means the maximum focal length of a scope would be 458mm focal length.

Believe the equation is: FL = 4/(Tan(0.5). Allowing for a little space around the actual image it would seem to imply a maximum focal length of around 400mm.

I am not realistically aware of a reducer that would effectively reduce by that amount. I think, only think, that the 127 Skymax is a 1500mm focal length scope so you would need a reducer that in effect quartered the native focal length. They sort of do not exist. Have read of the Starizona items but they are specialised and cost $$$.

Scopes that might are the 60mm ED's that are around, William Optics do one, guess Altair do and maybe a few others. They have focal lengths of 360mm. A Skywatcher 72ED should just squeeze the image on the sensor, but it will be close and assumes that the full sensor size is available.

A mosaic is as said often done but that adds a level of complication. They do not generally just easily and smoothly slot into position next to each other.

You seem to have acquired yourself a scope - 127 that has a long focal length so a large final image, and sensor that is small so needs a small final image to fall on it. They are a bit mismatched. If I have the right geometry idea above then for a 1500mm scope and a 4mm sensor your largest object will be 0.15 degrees or 9 arcmin = 540 arcsec.

[Mach13]

Ah-ha.. You make an interesting point PEMS... So the size of the camera sensor also makes a difference..  I looked it up on Astronomy Tools (thank you Jenham's Astro for that tip) and it seems an appropriate sensor size would have to be 28.3mm to get a full view of the moon on the 127.  That one is fitted to the ZWO AS12600MC Colour Pro and is a galaxy away from my budgetary taste.

That's been really helpful- with the imaged FOV I have at present I'd be looking to stitch a really large number (70+) of images then...

So I'm thinking in terms of my current setup - reducer v new scope = New scope AND probably new camera  (The SV205 was only ever something to get me started)

 

As a starting point then

A new (reasonable budget) camera such as a ZWO 120 would expand my imaged FOV and mean I could stitch a reasonable number of images (approx 14) - which sounds intriguing and much more do-able

A new (or used ) scope with a smaller focal length will certainly help

Much to think about....👍

[M40]

Looking at the FOV calculator, using something like a 13mm eyepiece with the 127 will give you a view of the whole of the moon, so vlaiv, would eyepiece projection work? Cost of one or two adapters might be worth a try if it can achieve focus.

[vlaiv]

3 hours ago, M40 said:

Looking at the FOV calculator, using something like a 13mm eyepiece with the 127 will give you a view of the whole of the moon, so vlaiv, would eyepiece projection work? Cost of one or two adapters might be worth a try if it can achieve focus.

I would suggest going for Afocal instead of doing eyepiece projection.

Eyepiece projection makes it easy to reach focus with DSLR type camera as it utilizes a lot of back focus - and in process it magnifies image. It can be made to act as reducer - but that tends to put focus position extremely out - and eyepieces are not optimized to do that - so image will be very blurry even in center. Sensor also needs to be mounted very close to eyepiece for that to work.

Afocal method is better for this sort of thing - but it does require special adapters and camera lens.

There are couple of things that need to be taken into account when contemplating afocal setup. By the way - here is schematic of afocal setup:

This means that CS/C mount lens will be needed and adapter for SV205 since it has 1.25" filter thread - something like this:

https://www.firstlightoptics.com/adapters/astro-essentials-low-profile-125-nosepiece-to-fit-c-mount.html

Next thing to worry about is CS/C flange distance and how far sensor is with SV205. ASI cameras are built to accept CS or C mount lenses as is - 12.5mm or 17.5mm flange distance, but this SVBony - I'm not sure as there are no specs for that.

Anyway, you need such adapter, CS/C mount lens of suitable focal length and way to secure camera with such lens against eyepiece.

Say you take 32mm eyepiece - that will give roughly x47 magnification. Moon will be around 23.5° - say we take 25° to have some room left.

As far as I can tell - SVBony camera has 1/3.2" sensor size - which gives x7.6 crop factor. Using this calculator:

https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/#{"c":[{"av":"8","fl":6,"d":3048,"cm":"0"}],"m":0}

we can see that with 6mm lens - we can comfortably put 25° (in fact 29.5° in vertical direction) on sensor.

There you go: 32mm plossl and 6mm C/CS mount lens will give you suitable reduction to put whole lunar disk on sensor at once.

Only thing left to do is to match exit / entrance pupil (this is not strictly needed - but not matching them will result in lost light and we don't want to loose light if we can help it).

127mm / x47 = 2.7mm

Exit pupil will be around 2.7mm and since we use 6mm lens - we need F/2.2 (6mm / 2.7mm) or faster lens in order not to loose light.

Perhaps something like this:

https://www.aliexpress.com/i/32988089350.html

Do take care to position lens at exit pupil of eyepiece for best performance.

In the end, I want to stress that SV205 is really not best type of camera to do high quality lunar shots - because it does not support RAW mode and uses compression to stream video. That introduces artifacts and prevents stacking from working properly.

 

[M40]

Superb reply as always vlaiv 👌

[Nik271]

I regularly image the moon with the same scope and a DSLR with a crop sensor, the whole moon disc  fits perfectly. So one solution can be to purchase a dirt cheap second hand crop sensor DSLR camera 😀

[Mach13]

🤯  Wow that's even more food for thought... challenging for my limited knowledge base but that's cool.

 

Really appreciate all the comments  and the effort that's obviously gone into them...👏

 

[Mach13]

On 25/06/2021 at 18:45, Nik271 said:

I regularly image the moon with the same scope and a DSLR with a crop sensor, the whole moon disc  fits perfectly. So one solution can be to purchase a dirt cheap second hand crop sensor DSLR camera 😀

I decided to go for a DSLR after all... Arrives tomorrow, then just gotta figure how to use it...

 

Thank you everyone 👍👏

[Mach13]

DSC_5468_lapl5_ap87834.bmp b.bmp

On 25/06/2021 at 18:45, Nik271 said:

I regularly image the moon with the same scope and a DSLR with a crop sensor, the whole moon disc  fits perfectly. So one solution can be to purchase a dirt cheap second hand crop sensor DSLR camera 😀

Thanks so much for your comment.. so i went for a DSLR.. steep learning curve on how it to use it but finally started to get images I like... this is from Sunday 18th July 21 the full image is 46mb... 😅

Edited July 19, 2021 by Mach13

[Nik271]

Glad to be of help! This is a pretty good image with lots of detail visible. The 127 Mak can do even a bit better when the Moon is higher in the sky (usually in winter or spring) and of course with good seeing and stacking.

 

Nik

[Mach13]

Thanks. much appreciated... Will keep at it..👍

[Mach13]

My attempt at a Mineral Moon effect