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ADC and focal point


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Hi all

Hope this is in the right section. I’m considering purchasing an ADC for planetary imaging with an 8” dob (as well as just visual use). Currently, with the 2x barlow my asi224 focuses just mm above the focuser bottoming out. Will adding the ADC still allow me to reach focus, or does the focal point remain the same meaning I won’t have enough inwards travel? Of course, it’ll depend on the scope so perhaps a more suitable question is whether it shifts the focal point or not, and in which direction.

Another thing is that I’m currently undersampling (f/12 with a 2x, instead of f/18ish). I was going to purchase a 3x barlow as I’m very comfortable tracking at 2x. With the ADC, I believe it will give me another .5x or so, so would I be better off keeping the 2x, or slightly over sampling with the 3x? Assuming I won’t completely fail at tracking! 

TIA

Ross

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It doesn’t shift the focal point so you will need the ADC’s body length of infocus travel (37mm I think) to be able to use it. I’m surprised you each focus with the barlow so far in as normally the barlow will shift the focal point outwards. On my newts I couldn’t use the ADC without the barlow but I could use it with a barlow as it shifted the focal point out far enough. 
 

As for the extra barlow power you get for putting the ADC in between the barlow and camera, you can work out what this will be using this formula:

Barlow Power = (Barlow FL + Distance) / Barlow FL

With distance being the space between the last lens surface of the barlow and the camera sensor. 

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The ADC will shift the focal point by about 34% of the glass thickness, just as your filters and camera-window will do. Now, I have never dismantled an ADC, so have to guess the glass-thickness, could be as much as 3mm per wedge, making the focal point to shift outwards by about 2 x 0.34 x 3 = 2mm. In below drawing n is the breaking index of glass, which is about 1.5168 for Schott N-BK7 glass:

glasbreking.png

The ASI224 has a 3.75 micron pixel size, so optimum focal ratio would be 3 x 3.75 = f/11.25, although, thanks to stacking, good results are achieved when going higher than that (e.g. 4-5 x 3.75 = f/15 - f/18.75. The nice thing about that is that the image will be larger on the chip (and thus on your screen later), the downside is that it costs light (quadratic to the increase in focal length), so exposure times go up. With my C11 I usually image at 3.4 x [pixel size] and then bicubic resize the image if I want it to be larger. With my Lunt I usually image at 3.7 x [pixel size] without resizing.

Nicolàs

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Beware the extra path length of several cm. added by the ADC, which may prevent you reaching focus.  

The difficulty of getting a camera to focus in a Newtonian designed for visual use is well known.  One reason for using a SCT or Mak for planetary imaging is that the  focal range is large, making it much easier to accommodate all sorts of imaging accessories.

Something that owners of the ZWO ADC may not have realised is that both the top and bottom sections unscrew, leaving the centre body with a standard thread at each end! 

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3 hours ago, inFINNity Deck said:

the downside is that it costs light 

No it doesn't. It just distributes it differently.  Rescale the image eg by binning and (other than extra read noise with CMOS cameras) you will be back to where you were

Cheers

Robin

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7 minutes ago, robin_astro said:

No it doesn't. It just distributes it differently.  Rescale the image eg by binning and (other than extra read noise with CMOS cameras) you will be back to where you were

Cheers

Robin

Hi Robin,

you're absolutely right, I did not express myself properly.🤔  Indeed a larger barlow distributes the light differently. If one goes from a 2x barlow to a 4x barlow, the planet will appear twice as big on the chip and the amount of light per pixel will therefore be a quarter of what it was.

Binning could be done, but then we are back to where we started, so is rather pointless (unless no smaller barlow is available).

Examples of my imaging:

Sun with Lunt @ 3.7 x [pixel size]
Jupiter with C11 @ 3.4 x [pixel size] and 200% resize
Mars with C11 @ 3.4 x [pixel size] and 200% resize

Nicolàs

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it was mainly due to the atmospheric conditions. As a matter of fact, I did not expect any data from that session as the seeing was not great. In the end it turned out pretty reasonable and as I prefer not to over-process the images, I left it as it is.

Nicolàs

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9 hours ago, CraigT82 said:

It doesn’t shift the focal point so you will need the ADC’s body length of infocus travel (37mm I think) to be able to use it. I’m surprised you each focus with the barlow so far in as normally the barlow will shift the focal point outwards. On my newts I couldn’t use the ADC without the barlow but I could use it with a barlow as it shifted the focal point out far enough. 
 

As for the extra barlow power you get for putting the ADC in between the barlow and camera, you can work out what this will be using this formula:

Barlow Power = (Barlow FL + Distance) / Barlow FL

With distance being the space between the last lens surface of the barlow and the camera sensor. 

It does actually shift it outwards, as I’m unable to reach focus without it. Rather frustrating as it means I can’t use the ADC by the sounds of things. May be time to invest in a low profile 2” to 1.25” adapter. Thanks for the barlow power info too, will get thinking. 

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5 hours ago, Cosmic Geoff said:

Beware the extra path length of several cm. added by the ADC, which may prevent you reaching focus.  

The difficulty of getting a camera to focus in a Newtonian designed for visual use is well known.  One reason for using a SCT or Mak for planetary imaging is that the  focal range is large, making it much easier to accommodate all sorts of imaging accessories.

Something that owners of the ZWO ADC may not have realised is that both the top and bottom sections unscrew, leaving the centre body with a standard thread at each end! 

Yeah I’ve noticed that with the unscrewing. Might be possible to reach focus if I can screw the camera directly into the top. 

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I've got eyepieces that require over 20mm more infocus than I have even with a low profile Newt focuser.  My solution is to screw the optics section of my GSO coma corrector directly onto the end of the eyepiece and send it deep into the focuser that way.  The CC grabs the incoming light cone sort of like a relay lens and sends it up to the eyepiece where it can then reach focus.  The CC correction isn't perfect, but neither is the eyepiece I'm using it with.

Maybe you could do something similar with a GPC/Barlow element screwed directly to the bottom of the ADC?

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9 minutes ago, Louis D said:

I've got eyepieces that require over 20mm more infocus than I have even with a low profile Newt focuser.  My solution is to screw the optics section of my GSO coma corrector directly onto the end of the eyepiece and send it deep into the focuser that way.  The CC grabs the incoming light cone sort of like a relay lens and sends it up to the eyepiece where it can then reach focus.  The CC correction isn't perfect, but neither is the eyepiece I'm using it with.

Maybe you could do something similar with a GPC/Barlow element screwed directly to the bottom of the ADC?

Being able to screw on the barlow directly would definitely help, but sadly mine doesn’t have the functionality. The stock SW adapter seems to extend ~50mm out from the top of the 2” top, so I’m hoping with a very low profile adapter I should gain enough to use the ADC. Just a cheap one on ebay seems to only extend 10mm above the 2” top, so I’ve got faith.

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46 minutes ago, sorrimen said:

Being able to screw on the barlow directly would definitely help, but sadly mine doesn’t have the functionality. The stock SW adapter seems to extend ~50mm out from the top of the 2” top, so I’m hoping with a very low profile adapter I should gain enough to use the ADC. Just a cheap one on ebay seems to only extend 10mm above the 2” top, so I’ve got faith.

OMG, yes.  Those Synta eyepiece adapters are atrocious.  The whole point of having the focus point so far outside the tube is to facilitate astrophotography, but you can't take advantage of it with their focuser setup.

If they went with a low profile focuser from the factory and moved the focus position in closer to the tube to exclude AP, they could shrink the secondary size and improve visual contrast.  As it is, they've got the worst of both worlds with the current focuser.  Poorer visual contrast and lack of AP backfocus.

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13 minutes ago, Louis D said:

OMG, yes.  Those Synta eyepiece adapters are atrocious.  The whole point of having the focus point so far outside the tube is to facilitate astrophotography, but you can't take advantage of it with their focuser setup.

If they went with a low profile focuser from the factory and moved the focus position in closer to the tube to exclude AP, they could shrink the secondary size and improve visual contrast.  As it is, they've got the worst of both worlds with the current focuser.  Poorer visual contrast and lack of AP backfocus.

Definitely a case of deals between manufacturers. Makes no sense to prioritise AP in a manual dob, not to mention the balance and EP position issues it introduces. 
 

Have you seen anything about cheap adapters with compression ring style locks? I can’t imagine it’s something you can get very wrong but with my eyepieces all having undercuts I don’t want to skimp if there’s well-known risks to the cheaper designs. Certainly look appealing with the £10 price tag and extremely low profile.

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1.25" adapters vary from +16.5mm to -12.7mm, so changing the adapter for anything 1.25" is a way to resolve focuser issues.

If the scope is a SkyWatcher, the stock focuser takes either a tall 1.25" or tall 2" focuser to focus normally.

But if the adapter is removed and the visual accessory inserted directly in the focuser ring on top of the focuser, a HUGE amount of in-focus can be gained.

 

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The problem as I understand it with the Synta focuser is that it is not natively 2" without either tall adapter, so you can't use a stock, low profile 2" to 1.25" adapter.  It's a bit larger:

Does anyone make low profile versions of these Synta adapters?

IMG_0040.jpgspacer.pngspacer.png

The Lumicon Low Profile 2 Inch Prime Focus Adapter almost looks correct if you need a male T2 thread:

spacer.png

Edited by Louis D
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1 hour ago, Don Pensack said:

1.25" adapters vary from +16.5mm to -12.7mm, so changing the adapter for anything 1.25" is a way to resolve focuser issues.

If the scope is a SkyWatcher, the stock focuser takes either a tall 1.25" or tall 2" focuser to focus normally.

But if the adapter is removed and the visual accessory inserted directly in the focuser ring on top of the focuser, a HUGE amount of in-focus can be gained.

 

I experimented with this as the camera has a 2” slot, but sadly the barlow compression screw sticks out further than 2”.  Very useful tip though. 

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1 hour ago, Louis D said:

The problem as I understand it with the Synta focuser is that it is not natively 2" without either tall adapter, so you can't use a stock, low profile 2" to 1.25" adapter.  It's a bit larger:

Does anyone make low profile versions of these Synta adapters?

IMG_0040.jpgspacer.pngspacer.png

The Lumicon Low Profile 2 Inch Prime Focus Adapter almost looks correct if you need a male T2 thread:

spacer.png

Oh god that just adds to the complication! Would you think it’s significantly larger enough to cause off-centre issues with a standard 2” to 1.25”? Just having tested it, the wiggle room with the asi224 (my only 2” test) is no greater than the supplied 1.25” adapter, so in the very least it doesn’t seem that the Synta one has compensated for the diameter and it wouldn’t be much, if any, of a downgrade centring-wise. Would be ideal to avoid having to scout out two sufficiently low profile adapters. 

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On 06/09/2022 at 16:45, sorrimen said:

Oh god that just adds to the complication! Would you think it’s significantly larger enough to cause off-centre issues with a standard 2” to 1.25”? Just having tested it, the wiggle room with the asi224 (my only 2” test) is no greater than the supplied 1.25” adapter, so in the very least it doesn’t seem that the Synta one has compensated for the diameter and it wouldn’t be much, if any, of a downgrade centring-wise. Would be ideal to avoid having to scout out two sufficiently low profile adapters. 

You can use a conventional 2" to 1.25" adapter in the focuser to lower the 1.25" eyepieces a lot.

The ring on the focuser may or may not (depending on vintage of the scope) have a 2" clear aperture.  Earlier ones were <2" in I.D.

If it does, a 2" eyepiece can be used in the focuser directly without the 2" adapter provided.

If it doesn't, you'll have to find an after market ring to replace the one on the focuser.  That is readily available in the US, but I don't know about the UK.

Edited by Don Pensack
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On 06/09/2022 at 16:45, sorrimen said:

Oh god that just adds to the complication! Would you think it’s significantly larger enough to cause off-centre issues with a standard 2” to 1.25”? Just having tested it, the wiggle room with the asi224 (my only 2” test) is no greater than the supplied 1.25” adapter, so in the very least it doesn’t seem that the Synta one has compensated for the diameter and it wouldn’t be much, if any, of a downgrade centring-wise. Would be ideal to avoid having to scout out two sufficiently low profile adapters. 

The ring on the focuser is 2" in I.D., but the tube of the focuser below that ring is slightly larger than 2" in ID.  That's why it is important that the ring have 2 screws to bind the eyepiece, where with or without a brass split ring inside the ring on the focuser.

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34 minutes ago, Don Pensack said:

The ring on the focuser is 2" in I.D., but the tube of the focuser below that ring is slightly larger than 2" in ID.  That's why it is important that the ring have 2 screws to bind the eyepiece, where with or without a brass split ring inside the ring on the focuser.

Oh great, that saves me some worry. I’m praying the additional 40mm or so of infocus with a new standard adapter works out. Thanks for following up.

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