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13 replies to this topic

### #1 rjhat3

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Posted 25 August 2019 - 08:04 AM

I have been using an Orion 120mm F5 scope for a year now. While I have been very happy with my results, I want to upgrade my scope for a narrower field of view. I really love capturing images of galaxies. Unfortunately, this scope is very limited in that area. Whirlpool, Pinwheel, Sombero, and Bodes are all galaxies I'm interested in capturing. My question is this: should I stick with a larger refractor or should I start moving toward an SCT? My current rig is an Orion Atlas mount with a 50mm Orion guide scope and a ZWO ASI 120mm guiding cam. Not really a beginner anymore, but still a novice.

### #2 GoFish

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Posted 25 August 2019 - 08:54 AM

I would first consider the image scale you’re going for. For details on galaxies I’d guess 1 arcsecond per pixel, maybe 0.7?

Formula for image scale (round numbers) = 200p/F. Where p=pixel size (um) on camera and F=focal length of scope (mm).

Example: ASI294 p=4.63. For image scale of 1.0, solve for F=(200)(4.63)/1.0=930mm. So a focal length of 1,000mm might be chosen.

An 8” SCT with 0.63 reducer would give an image scale of a little less than 0.8 with the 294.
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### #3 stargazer60

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Posted 25 August 2019 - 09:00 AM

Wow thanks GoFish you answered my question as well.

### #4 rjhat3

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Posted 25 August 2019 - 10:07 AM

Ha! I have an 8" Meade Lx200 EMC, but it has a cracked corrector plate. I also have a OAG that I have never used. Unfortunately the scope was damaged in moving. I haven't found a replacement for it yet.

### #5 GoFish

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Posted 25 August 2019 - 01:37 PM

Ha! I have an 8" Meade Lx200 EMC, but it has a cracked corrector plate. I also have a OAG that I have never used. Unfortunately the scope was damaged in moving. I haven't found a replacement for it yet.

Ouch!

I’m trying to answer this question for myself as well (I plan to use the 294 camera).  My candidate OTA’s are:

• 8” SCT with 0.63 reducer (0.76 as/px)
• 6” f/5 Newt (1.27 as/px)
• 8” f/5 Newt (0.95 as/px)

I’ve done some unguided short imaging (for EAA) with the C8 with reducer, but using 2x2 binning on the 294. It was good for EAA but I think the scale (1.52 as/px) was a little coarse for fine galaxy detail.

### #6 rjhat3

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Posted 26 August 2019 - 08:36 AM

GoFish,
Just want to make sure I have the math right. So, with the current formula and my current T6s camera and 120mm F/5 scope, this would be (7.3x200)/600 giving me a scale of 2.43?
If this is right, I can take a 2000mm SCT and it would be (7.3x200)/2000 giving me a scale of .73. Is this correct?

### #7 Stelios

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Posted 26 August 2019 - 09:13 AM

GoFish,
Just want to make sure I have the math right. So, with the current formula and my current T6s camera and 120mm F/5 scope, this would be (7.3x200)/600 giving me a scale of 2.43?
If this is right, I can take a 2000mm SCT and it would be (7.3x200)/2000 giving me a scale of .73. Is this correct?

The pixel size of the T6s that *I* have seen is 3.7um, so your image scale would be 206.3*3.7/600 = 1.27"/px. (That's a very good image scale, BTW, unless seeing is exceptional).

If you get an SCT:

At native F/10 : 206.3*3.7/2000 = 0.38"/px

With F/6.3 reducer: 206.3*3.7/1260 = 0.60"/px

You WANT the reducer. 0.38 is too small an image scale unless you live in the Canary islands or Chile. Your seeing will not let you reach it, so you'll get the same detail that you would have by enlarging the image captured at 0.6"/px, but in less than half the time (time required goes by the square of the F/ratio). Also a regular (non-Edge) SCT will not produce a flat field at F/10, so the reducer (which is also a flattener) is recommended for that reason as well.

Note that imaging with an SCT has some extra issues. There's mirror movement issues (so an OAG is required) and auto-focus issues. By the time everything is added up you *may* be better off with something like a 900mm APO (say a 130mm F/7 which rings in at 910mm). This will give you an 0.84 image scale, which is probably the limit of your seeing, and will let you image the object you are interested in (and smaller ones) very well.

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### #8 rjhat3

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Posted 26 August 2019 - 09:45 AM

This is terrific info! Thanks for the help. The correlation between Camera and focal length of scope make much more sense. I can now see why some camera and scope combinations are great and others not so great. My problem in my current area is light pollution. Although it is not terrible, it’s not great either. I just moved here five months ago. The area that I came from had fairly low light pollution. I guess I need to buy a power supply and do some traveling. Again, thank you very much for the info.

Edited by rjhat3, 26 August 2019 - 09:48 AM.

### #9 GoFish

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Posted 26 August 2019 - 10:57 AM

The pixel size of the T6s that *I* have seen is 3.7um, so your image scale would be 206.3*3.7/600 = 1.27"/px. (That's a very good image scale, BTW, unless seeing is exceptional).

If you get an SCT:

At native F/10 : 206.3*3.7/2000 = 0.38"/px

With F/6.3 reducer: 206.3*3.7/1260 = 0.60"/px

You WANT the reducer. 0.38 is too small an image scale unless you live in the Canary islands or Chile. Your seeing will not let you reach it, so you'll get the same detail that you would have by enlarging the image captured at 0.6"/px, but in less than half the time (time required goes by the square of the F/ratio). Also a regular (non-Edge) SCT will not produce a flat field at F/10, so the reducer (which is also a flattener) is recommended for that reason as well.

Note that imaging with an SCT has some extra issues. There's mirror movement issues (so an OAG is required) and auto-focus issues. By the time everything is added up you *may* be better off with something like a 900mm APO (say a 130mm F/7 which rings in at 910mm). This will give you an 0.84 image scale, which is probably the limit of your seeing, and will let you image the object you are interested in (and smaller ones) very well.

(Note that Stelios refined the image scale equation to use a constant of 206.3, vs my previous use of 200. The value of 206.3 has 4 significant figures and will yield a more precise result at the expense of no longer being able to do the calc in my head  )

### #10 rjhat3

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Posted 26 August 2019 - 12:22 PM

Can anyone tell me what the significance of the number 206.3 is? I understand and apply things better when I know what it’s origin is and can even help me remember it.😁

### #11 GoFish

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Posted 26 August 2019 - 01:45 PM

Can anyone tell me what the significance of the number 206.3 is? I understand and apply things better when I know what it’s origin is and can even help me remember it.

180 deg = pi radians, so 1 rad = 180/pi = 57.296 deg

1 deg = 60 x 60 = 3,600 arcseconds

1 mm = 1,000 um (microns)

Putting all together:  57.296 x 3,600 / 1,000 = 206.26  (deg/rad) (arcseconds/deg) / (um/mm)

Or something like that.

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### #12 RJF-Astro

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Posted 26 August 2019 - 04:18 PM

Another option might be the 6" ritchey-chretien. With 0,67x reducer the f/l is 939mm and the ratio F6.

I use it with the ASI1600 at 3.8um, The image scale is about 0.83. Combined with a HEQ5 and separate guidescope, it works great. Some collimation is required, but I did not find it too difficult with the right guide. And this is my first reflector/cassegrain type scope. Maybe something to consider..

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### #13 rjhat3

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Posted 26 August 2019 - 10:11 PM

Some interesting info...given the formulas above and a distance of 1500 light years to the Orion Nebula, if we were looking at a solar system the same size as ours, it would be roughly 1/10th of one pixel with a pixel ratio of 1! Don’t know if I did the math correctly, but that is astounding.

### #14 View2

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Posted 26 August 2019 - 10:29 PM

I use the ASI 1600 on an F 7.5 refractor 952 mm focal length 127 mm diameter. I have a focal reducer of 0.7 x for wide fields, but don't use it for Galaxies except clusters. My image scale is 1.17 with the reducer and I think it is 0.96 without it. I like the scale

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