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New toys arriving, trying to understand imaging train order

astrophotography cassegrain catadioptric equipment imaging planet
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#1 Seralyn

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Posted 19 January 2021 - 10:58 PM

Hey folks,

 

I have the Sky Watcher 127mm Maksutov-Cassegrain (SkyMax 127) and I've been plugging my ZWO ASI224mc directly into the rear of it (sometimes with a crappy single element non-branded 10 dollar barlow) for my planetary work thus far. I have just ordered a TeleVue 2.5x Powermate 1.25", ZWO ADC, and a Baader Plaentarium UV/IR Cut filter. I am trying to figure out what order all this stuff should be in and am looking to my betters for advice. My best guess is:

OTA > ADC > UV/IR Cut > Powermate > ASI 224

 

But it's only a guess. How do you more experienced planetary imagers do it?


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#2 KTAZ

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Posted 19 January 2021 - 11:26 PM

I had the same question when I started planetary.

 

Per the support team at ZWO:

 

"ADC will performance better with long focal ratio. Barlow will extend the focal ratio, so we it is better to place ADC after Barlow, which you will get Camera>ADC>Barlow>SCT." Oh well, Katherine's English is not perfect, but she was kind enough to reply! 

 

The UV/IR cut can go wherever you like, but I thread mine right onto the nosepiece of the camera where you slide it into the ADC.

 

Like this:

 

D646Tvg.jpg


Edited by KTAZ, 19 January 2021 - 11:31 PM.

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#3 Seralyn

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Posted 20 January 2021 - 01:28 AM

I had the same question when I started planetary.

 

Per the support team at ZWO:

 

"ADC will performance better with long focal ratio. Barlow will extend the focal ratio, so we it is better to place ADC after Barlow, which you will get Camera>ADC>Barlow>SCT." Oh well, Katherine's English is not perfect, but she was kind enough to reply! 

 

The UV/IR cut can go wherever you like, but I thread mine right onto the nosepiece of the camera where you slide it into the ADC.

 

Like this:

 

[ ]

Thank you so much~! This is very helpful smile.gif



#4 Seralyn

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Posted 20 January 2021 - 01:29 AM

But I've also heard that you can get better focus when the barlow is screwed directly into the ASI...any comment about that?



#5 jesco_t

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Posted 20 January 2021 - 04:14 AM

But I've also heard that you can get better focus when the barlow is screwed directly into the ASI...any comment about that?

Magnification of a Barlow depends on the distance between it and the camera sensor. The larger the distance, the higher the magnification. Higher magnification makes focussing more difficult.

 

That's why I tried to make my imaging train as short as possible by "disassembling" my Barlow and inserting the lens element only into a compression ring in front of the ADC. This gives me a magnification of only 1.8x.

 

ZWO ASI224MC + Omegon ADC +  TS Shorty 2x Barlow for planetary imaging

 

A big advantage of operating the ADC at high f-ratios is that it becomes less sensitive. You need larger amounts of lever opening to achieve the same amount of correction. This sounds bad, but is actually good. With the Barlow after the ADC, my setting is only +/-20° for planets 10° above the horizon. If they cross 30° elevation, I can't use the ADC anymore because even the minimal setting is overcorrecting. With the Barlow in front of the ADC, my setting for 10° elevation of +/-60° lever opening and even at 40° elevation I can still use the ADC with +/-10° lever setting.

 

Jesco


Edited by jesco_t, 20 January 2021 - 04:15 AM.


#6 Kokatha man

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Posted 20 January 2021 - 06:31 PM

The Televue 2.5x Powermate has a negative response to distance https://www.televue....b=_app#MagChart although this might not provide you with the "optimum" image scale for your scope/camera setup which settles at anything from 5x to 7x the camera's pixel-sizing. (possibly on the higher value for colour cameras, although I always recommend that "5x" figure...ie, 5 x 3.75 = approx. f19)

 

Jesco has given a good resume of the situation & I have pikkies detailing means of setting up imaging trains & using only the lens element of barlows (with graphs) for those who'd like to know more on these matters...let's know if you do & I'll try & dig them up. ;)


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#7 RedLionNJ

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Posted 22 January 2021 - 09:28 AM

Note with the SkyMax 127, you have a native focal ratio of f/12.  You want to be aiming for around f/18-f/20, so even a 2.5x (even a 2x) amplification is a bit much.

 

Lower-powered amplification can sometimes be achieved (as mentioned above) by removing barrels, or replacing with shorter ones.  The other solution is to find (or custom order) a 1.5x barlow.

 

Grant


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#8 Kokatha man

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Posted 24 January 2021 - 07:34 PM

But I've also heard that you can get better focus when the barlow is screwed directly into the ASI...any comment about that?

Re-visiting this thread after posting on a similar thread here I should make the point that some barlows (Televue especially) use their own threads & although I have cut complementary ones for fittings to screw the Televue's lens elements directly into the ZWO cameras, this thread-cutting is not an easy exercise to undertake...

 

Re-reading Jessco's reply to you I see he appears to be advocating the approach I articulated here on CN some time back with an annotated diagram, using a readily obtained & inexpensive ZWO ADC fitting that can be purchased separately from the ADC... wink.gif

 

https://www.cloudyni...3-handy-hint-1/  

 

I'll also include the graph I extrapolated from the Televue site to demonstrate what moving the lens element closer to the camera sensor achieves in terms of amplification, noting that most barlows follow a similar path.

 

Televue2XGraphModified.jpg

 

Looking at the graph "0" represents the amplification of a 2x barlow at the back-surface of the black body (ie, 2x where an ep or camera would be inserted & clamped via the thumbscrew) such that the black body (roughly 80mm in length from memory) creates said 2x.....with the entire black section of the 2x barlow removed & the lens element placed as near to the sensor as possible, the magnification drops to about 1.25x.

 

Assuming the 80mm length of the black body section is correct (too lazy to check atm wink.gif ) the graph shows that with that removed & the lens element by itself 40mm from the camera sensor an amplification of 1.5x is obtained... wink.gif


Edited by Kokatha man, 24 January 2021 - 07:37 PM.

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