Which scope would produce the best lunar images? Zenithstar 61ii or C90?

[Jay6879]

I have a Zenithstar 61ii and have used to to take a few images of nebula/andromeda etc but recently came across a great deal on a Celestron C90, he seems like a really cool dude and is selling it for $200 (new is almost $400cad). I'd use the C90 for planetary and lunar viewing but I'm mainly curious to know if the C90 would produce a better lunar image than the Zenithstar?

I'd be using a t3i for now until I get a dedicated camera. Obviously the C90 would produce better images of the planets due to focal length but I'm not sure how to compare quality between scopes. Resolution? How is optical quality quantified? 

I'd appreciate any information on this, don't want to miss out on this sweet deal but would prefer to know if it's worth the purchase.

Edited March 15, 2022 by Jay6879

[Adam J]

I would think that the C90 will be very significantly better than a ZS61 for lunar imaging. 

Adam

 

[vlaiv]

C90 will produce much better lunar images than Zenithstar 61.

Due the way planetary / lunar images are captured (lucky imaging technique) and the way they are processed - even if C90 is optically lower quality than Z61 - it will produce better images (unless it's really a lemon or out of collimation).

Level of detail resolved by aperture is proportional to size of aperture. 90mm is x1.5 60mm and there is no way around this.

Just make sure you follow nice lunar imaging tutorial (look up lucky imaging on youtube, or ask questions here) and you should be able to produce some nice images with C90 and guide camera.

(also check out lunar imaging section of SGL and see what people do with small scopes).

[Craney]

or.... look on ASTROBIN.

Plug in the type of scope on the search panel and you should get lots of images (albeit with a range of cameras as well).   Some will be Lunar.

I can speak highly of the C90.   It did give very crisp views of the Moon and was of the right FL to get a full disc with a DSLR.    ( very useful for eclipses ).

[Jay6879]

Thanks for the replies, everything sounds good then. Zenithstar remains my wide field imaging scope, the c90 will become my lunar (and possibly some planetary?) scope.

 

 

16 minutes ago, Craney said:

or.... look on ASTROBIN.

Plug in the type of scope on the search panel and you should get lots of images (albeit with a range of cameras as well).   Some will be Lunar.

I can speak highly of the C90.   It did give very crisp views of the Moon and was of the right FL to get a full disc with a DSLR.    ( very useful for eclipses ).

 

Good to hear that the c90 is a good little scope! And it plays well with a dslr? Just screw it into the t2 built into the rear and you're good to go?

If I wanted to use a barlow with the dslr how would I go about that? And what would be the max barlow I should attempt for imaging do you think? 3x?

[vlaiv]

2 minutes ago, Jay6879 said:

If I wanted to use a barlow with the dslr how would I go about that? And what would be the max barlow I should attempt for imaging do you think? 3x?

That really depends on pixel size, but most likely - you won't need a barlow.

If you image with Zenith star, do you possibly guide? And if so, with what guide camera?

Guide camera is much better option for lunar / planetary imaging.

C90 is a bit shy of being F/14 scope (F/13.89) and that is very well suited for ~3.45um pixel size. So any camera with say 3.3 to 3.75 micrometer pixel size can be used without barlow on C90.

If your DSLR has for example 4.3 - it would need F/17.2 and that means x1.23 barlow - not really feasible thing (you can get x1.5 barlow element and try reducing sensor / barlow distance to get to x1.23).

In any case - look up lucky imaging technique.

This lunar image was taken with 100mm Maksutov telescope (so just 10mm larger aperture than c90, but practically the same focal length):

 

[Alien 13]

I have the C90 and a Z71 and would say that the Mak produces better Lunar images, the Mak also has the benefit of a far more sturdy camera mount.

I still haven't worked out if its best to attach the camera direct or use a 2 inch extension which compensates for the removal of the visual diagonal but it focuses fine both ways although the f/l changes a little.

My first ever image with the Mak, single shot with DSLR.

Alan

[Jay6879]

27 minutes ago, vlaiv said:

That really depends on pixel size, but most likely - you won't need a barlow.

If you image with Zenith star, do you possibly guide? And if so, with what guide camera?

Guide camera is much better option for lunar / planetary imaging.

C90 is a bit shy of being F/14 scope (F/13.89) and that is very well suited for ~3.45um pixel size. So any camera with say 3.3 to 3.75 micrometer pixel size can be used without barlow on C90.

If your DSLR has for example 4.3 - it would need F/17.2 and that means x1.23 barlow - not really feasible thing (you can get x1.5 barlow element and try reducing sensor / barlow distance to get to x1.23).

In any case - look up lucky imaging technique.

This lunar image was taken with 100mm Maksutov telescope (so just 10mm larger aperture than c90, but practically the same focal length):

 

 

Yes I do guide with the zenithstar using an asi 120mm-mini. It's only black and white though, that would be kinda lame for planetary imaging no? As for lucky imaging two years ago I started this whole astronomy thing with a tabletop Starblast 4.5. Despite the objections from people over at cloudynights I stacked a 2x and a 3x barlow on top of each other then a neximage burst and got these images..

 

 

Completely unconventional sure but hey these images blew me away! They were a ton of work, line everything up in sharpcap, hit record for ten seconds or so until it crossed the frame. Move the scope and reorient the planet, hit record for ten seconds or so rinse repeat until I had 5 minutes or so worth of data. It was immensely rewarding though.

So I have a basic understanding of lucky imaging, the processing I could use some more practice with though. This will be the first maksutov ive encountered so that's why I had these questions. Unfamiliar territory! I ousted that little starblast way outside it's comfort zone, was just curious where the edge of thay comfort zone lies with the c90.

I appreciate you taking the time to reply, you've always been helpful! Also the link yubposted has no image? It looks like the link is broken?

 

 

 

[Jay6879]

4 minutes ago, Alien 13 said:

I have the C90 and a Z71 and would say that the Mak produces better Lunar images, the Mak also has the benefit of a far more sturdy camera mount.

I still haven't worked out if its best to attach the camera direct or use a 2 inch extension which compensates for the removal of the visual diagonal but it focuses fine both ways although the f/l changes a little.

My first ever image with the Mak, single shot with DSLR.

Alan

 

Wow that looks amazing considering it's a single shot! I can only imagine what could be produced with lucky imaging.

[Craney]

This is one from my C90Mak.   ( Canon 60D I think..... )

[vlaiv]

7 minutes ago, Jay6879 said:

As for lucky imaging two years ago I started this whole astronomy thing with a tabletop Starblast 4.5. Despite the objections from people over at cloudynights I stacked a 2x and a 3x barlow on top of each other then a neximage burst and got these images..

Very nice images!

You were right to stack barlows (although just using x3 at increased distance would have done the trick). NexImage burst has 3.75um pixel size and ideal F/ratio is F/15.

Starblast is F/4 telescope and it needs at least x4 amplification to get close to F/15.

10 minutes ago, Jay6879 said:

Yes I do guide with the zenithstar using an asi 120mm-mini. It's only black and white though, that would be kinda lame for planetary imaging no?

Maybe for planets, but lunar does not have much color so you'll be fine with mono. You can also experiment with filters if you have any. Narrow band filters suppress seeing effects and are beneficial for lunar.

11 minutes ago, Jay6879 said:

Also the link yubposted has no image? It looks like the link is broken?

Link I posted is to a thread here on SGL where I originally posted the image with all the capture detail.

It opens fine for me - both the SGL thread and also the image posted on that thread. Not sure why you can't see it.

Here is the image uploaded again here:

Do right click / open in new window so you can zoom in fully to see the detail.

[Jay6879]

20 minutes ago, vlaiv said:

Very nice images!

You were right to stack barlows (although just using x3 at increased distance would have done the trick). NexImage burst has 3.75um pixel size and ideal F/ratio is F/15.

Starblast is F/4 telescope and it needs at least x4 amplification to get close to F/15.

Maybe for planets, but lunar does not have much color so you'll be fine with mono. You can also experiment with filters if you have any. Narrow band filters suppress seeing effects and are beneficial for lunar.

Link I posted is to a thread here on SGL where I originally posted the image with all the capture detail.

It opens fine for me - both the SGL thread and also the image posted on that thread. Not sure why you can't see it.

Here is the image uploaded again here:

Do right click / open in new window so you can zoom in fully to see the detail.

 

Wow! That's crisp. Tons of detail. Alright you guys have essentially pushed me over the edge here, I think I'll pick it up. He's selling it for less than half the cost new, I'd like a larger aperture but for the price it seems like it can't be best. Later on down the line I'll upgrade. 

What equation are you using to determine the best f ratio for pixel size?

[vlaiv]

24 minutes ago, Jay6879 said:

What equation are you using to determine the best f ratio for pixel size?

Spatial cutoff frequency:

https://en.wikipedia.org/wiki/Spatial_cutoff_frequency

+ Nyquist

If you rearrange things, put two pixels per cycle as per Nyquist and plug in say 500nm (being good wavelength to represent visible spectrum) - you get that F/ratio is pixel size * 4 (actual formula is F/ratio = 2 * pixel size / 0.5um - because 500nm = 0.5um and you need pixel size and wavelength in same units so 2 * pixel_size / 0.5 = 4 * pixel size)

If you want to solve for particular wavelength - like when using Ha narrowband filter or IR pass filter - then you would use actual wavelength of interest. For Ha that would be

F/ratio = 2 * pixel size / 0.656 = pixel_size * 3.05

(because Ha is 656nm)

[Elp]

Well for comparison, heres my single shot with a Z61:

[vlaiv]

@Jay6879

I see that you are confused

Here is simple equation

F/ratio = 2 * pixel_size / wavelength

Where wavelength and pixel size are in same units - say micrometers. For wavelength you should put 0.5um (which is the same as 500nm - wavelength of green light), unless you have reason to use other value. If you do that - equation simplifies to

F/ratio = pixel_size * 4

[Jay6879]

On 15/03/2022 at 16:59, vlaiv said:

@Jay6879

I see that you are confused

Here is simple equation

F/ratio = 2 * pixel_size / wavelength

Where wavelength and pixel size are in same units - say micrometers. For wavelength you should put 0.5um (which is the same as 500nm - wavelength of green light), unless you have reason to use other value. If you do that - equation simplifies to

F/ratio = pixel_size * 4

 

Welp....I completely bailed on everything I talked about in this thread. Instead I bought a wicked cheap used Orion Apex 102, an asi224mc and an ir cut filter. Just need a 2x barlow and I'm ready to rock! 

[vlaiv]

9 hours ago, Jay6879 said:

 

Welp....I completely bailed on everything I talked about in this thread. Instead I bought a wicked cheap used Orion Apex 102, an asi224mc and an ir cut filter. Just need a 2x barlow and I'm ready to rock! 

Don't think you need x2 barlow.

Start without it.

That scope is F/13 and by the formula I gave above, as ASI224 has 3.75um pixel size, ideal F/ratio is 3.75 * 4 = F/15. You are already at F/13 and would thus need 15/13 =  ~x1.153 barlow. Using x2 barlow will put you at about F/26 - that is much further away from F/15 than F/13 is.

Even using x1.3 barlow will put you at F/16.9 - which is still too much and it would be better to just use F/13 as is.

[Jay6879]

5 hours ago, vlaiv said:

Don't think you need x2 barlow.

Start without it.

That scope is F/13 and by the formula I gave above, as ASI224 has 3.75um pixel size, ideal F/ratio is 3.75 * 4 = F/15. You are already at F/13 and would thus need 15/13 =  ~x1.153 barlow. Using x2 barlow will put you at about F/26 - that is much further away from F/15 than F/13 is.

Even using x1.3 barlow will put you at F/16.9 - which is still too much and it would be better to just use F/13 as is.

Ah ok, I was hoping to get the images a little bigger with the bsrlow. What are the benefits of imaging at f/13 over f/26? Will it be not as sharp at f/26?

[vlaiv]

5 hours ago, Jay6879 said:

Ah ok, I was hoping to get the images a little bigger with the bsrlow. What are the benefits of imaging at f/13 over f/26? Will it be not as sharp at f/26?

Yes, you will get bigger images - but void of small detail. It is a bit like enlarging already existing images in software - image does get bigger but it is lacking detail and looks blurry.

There is another very important downside to using slower F/ratio than needed - that is signal to noise.

You need to achieve good SNR in your stack in order to be able to sharpen image to retrieve detail that is there, and if your SNR is low - you won't be able to do that because of noise. You will be tempted to use longer exposures to get good SNR - and that is a no-no in planetary imaging as you want to freeze the seeing - that means using exposures up to 5ms and not longer.

So - no benefits, only downsides. Use F/13.

[Jay6879]

On 01/04/2022 at 06:17, vlaiv said:

Don't think you need x2 barlow.

Start without it.

That scope is F/13 and by the formula I gave above, as ASI224 has 3.75um pixel size, ideal F/ratio is 3.75 * 4 = F/15. You are already at F/13 and would thus need 15/13 =  ~x1.153 barlow. Using x2 barlow will put you at about F/26 - that is much further away from F/15 than F/13 is.

Even using x1.3 barlow will put you at F/16.9 - which is still too much and it would be better to just use F/13 as is.

You were correct, at least for the lunar images. The asi224 with the Apex 102 is REALLY zoomed in. I tried my hand and creating a mosaic for the first time and it somewhat worked out..I had massive stacking artifacts on one panel that ruined the full image so I had to crop it and was left with this..

 

I also purchased a used (mint condition!) 2x Celestron Ultima SV barlow for thr planets later on in the season. Thanks again for the help!