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APS-C versus full frame sensors and telescope compatability

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#1 dmelv

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Posted 19 September 2021 - 11:47 AM

1) The backfocus and the image "spot" which the telescope produces is in play here. 

 

2) How does one know how to choose a camera/sensor which will not vignette but will not have excessive spillover either

 

3) When picking a telescope what is the correct method for doing so to obtain optimum performance in this regard?

 

4) Is a full format better in any way?

 

 

 

I use an APS-C sensor camera on my 72ED refractor and is just happens to be compatible. But I was lucky.  How can one be intelligent in a choice and not guess?


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#2 StephenW

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Posted 19 September 2021 - 12:11 PM

Most telescope specs will include how big the "image circle" is. For full frame, which is 36x24mm, the image circle needs to be big enough to fully cover the larger axis (i.e. 36mm).

In practice, you want the image circle to be bigger than your imaging chip to reduce the effects of vignetting and other aberrations that tend to occur near the edge of the imaging circle.
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#3 DJL

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Posted 19 September 2021 - 12:15 PM

I felt bad about the photons spending all those thousands or millions of years crossing space just to miss the sensor inside my telescope. In the end I chose APS-C rather than full frame, to be flexible for different scopes.


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#4 Complexmystery

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Posted 19 September 2021 - 01:14 PM

1) The backfocus and the image "spot" which the telescope produces is in play here. 

 

2) How does one know how to choose a camera/sensor which will not vignette but will not have excessive spillover either

 

3) When picking a telescope what is the correct method for doing so to obtain optimum performance in this regard?

 

4) Is a full format better in any way?

 

 

 

I use an APS-C sensor camera on my 72ED refractor and is just happens to be compatible. But I was lucky.  How can one be intelligent in a choice and not guess?

1. Backfocus refers to the distance from the back of the scope to the focal plane. OR, it refers to the distance in which your camera's sensor must be from the lenses in a reducer/field flattener. Both of these come into play in some circumstances. What scopes are you thinking about and we can discuss further about this matter.

 

2-3. This matter is subject to many characteristics of the combination. Ideally, the larger the imaging circle/illuminated field a telescope offers, the easier it is to determine the camera. If a scope can only support a 30mm imaging circle, using a full 24x36 sensor is useless as your stars will not be round near the edges of the frame. Typically, a scope which can support a full 42mm imaging circle will also be able to illuminate said circle. Something VERY important in my opinion is the choice of threaded/bolted connections going from the back of the scope to the camera. M48 can vignette a 24x36 sensor at fast F/ratios-thus making sure your scope can fully illuminate your sensor useless as the connection to the scope is the limiting factor and not the optics.

 

4. As DJL said above, not using the photons that your scope collects is a bit of a disservice. Besides a larger field of view which captures these "spilled over" photons, there is no benefit to using a full frame sensor. Some scopes cannot natively support a full frame sensor like the GSO RC's. So, going with one is useless unless you intend to use a flattener. In some cases, it is better to use a smaller sensor and reduce the imaging circle of the system using a focal reducer. This allows you to use a faster scope and smaller sensor to give you a FOV comparable to a native slower scope and full frame sensor. 

Here is a recent example of this: https://www.cloudyni...25-f53-qhy268m/

(Thanks @Rouzbeh)

 

To sum it up: Find a scope that specifically states an IMAGING/CORRECTED(not illuminated) field of the size of the sensor in which you intend to use. If you plan to use a "full frame" 24x36 sensor which has a 43mm diagonal, you will need a scope with an imaging circle of at least 40+mm. General rule of thumb is to go with the largest possible(M54+) connections in order to not limit your scopes ability to fully illuminate your chip. I've used this with decent success in the past for estimates (it doesn't have to be filters, the same idea applies of apertures in the system): https://astronomy.to...ccd_filter_size Obviously you can use smaller connections when using smaller chips (M48 should be fine for APSC and even full frame with slower systems). 
 

Edit: I forgot to mention that some scopes like the GSO RC’s and many refractors have the ability to illuminate a large imaging circle, BUT most will need some sort of field flattener. The specs of the field flattener is where backfocus and clear aperture/illumination come into play independent of the scope. 
 

-Josh 


Edited by Complexmystery, 19 September 2021 - 02:52 PM.

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#5 andysea

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Posted 19 September 2021 - 01:33 PM

Also do your research because a lot of times scopes that claim to be able to produce a corrected ~43mm circle, in real life can not match that claim. The ones that do it competently are usually pretty pricey.


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#6 jsmithsd

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Posted 19 September 2021 - 01:38 PM

I recently moved from the ASI071MC-Pro (APS-C sensor) to the ASI2400-MC Pro (Full Frame Sensor). I have a William Optics 98FLT and William Optics 132FLT as my imaging telescopes.  I have had great success for several years with the ASI071MC-Pro on these telescopes using the William Optics AFR-IV Flattener / Reducer and the 68II Flattener, The ASI071MC-Pro is a great camera.

 

Moving to a full frame sensor camera like the ASI2400MC-Pro requires a study of your telescopes and accessories. The full frame sensor is 36mm x 24mm but has a diagonal of 43.27mm.  The telescope must provide at least that size or greater of fully illuminated image circle to avoid vignetting the image frame.  You need to go to a full 48mm or greater imaging train diameter as well.

 

I had to upgrade to the William Optics Flat7 and Flat 68III to get a true image circle of 44.4mm. These flatteners provide a fully illuminated frame on the 132FLT but I still see a very slight vignetting on the 98FLT.  Too slight to worry about as I can process that out.  These flatteners are mechanically solid and no chance of image train sag or tilt when slewing to different parts of the sky.

 

You need to use 48mm (2-inch) filters and get the filter as close as possible to the sensor in your optical train. That was the easy part. I mounted the ASI2400-MC Pro directly to the ZWO EFW without using threaded adapters. I have a dark filter, clear filter, IDAS D1 filter, and the Optolong L-eNhance filter loaded in the filter wheel.  The Optolong L-eNhance filter is awesome with both of these cameras, I highly recommend.  The ASI-071MC-Pro has an AR window so you need a UV/IR cut filter but the ASI2400MC-Pro has a UV/IR cut window so you need a high quality clear filter with AR coating (Astronomik).

 

Sensor tilt and curvature was the next challenge.  When I first installed the ASI2400MC-Pro on the 98FLT with the Flat7 0.8x Flattener / Reducer I had horrible sensor tilt and field curvature.  Stars were bloated on one side of the frame and tight on the other side. I found the included M54 to M48 adapter from ZWO induced more sensor tilt.  I ended up using the included M54 to M48 21mm extender and using the adjustment screws on the tilt plate I have almost adjusted out the sensor tilt but the curvature is still in need of a few clear nights to get the spacing dialed in. I am using CCD Inspector to assist with adjusting out the sensor tilt and curvature. Plan to use 5-10 nights adjusting the sensor tilt and dialing in the flattener spacing.  I am sure more experienced astro-imagers could do this faster but I am learning as I go. I am having fun and that is what this hobby is about.

 

I still love my ASI071MC-Pro camera but the ASI2400MC-Pro camera is truly amazing.  This camera has virtually no dark current. With the gain set to 148 the sensor does a mode switch and dark current drops to 1.5e. I only cool the camera to 0C and it is very stable.  I will eventually try the ASI2400MC-Pro on my AstroTech 12RCT and see what field flattener I will need on that telescope but for now I am really enjoying the wide fields of a full frame sensor.  Imaging at f/5 (495mm focal length) I can get the entire Veil Nebula, both east and west sections in one frame. I have also captured the Lagoon Nebula and Trifid Nebula in one frame.

 

Moving to a full frame sensor camera can be an enjoyable experience but plan for the challenges ahead.  Your telescope must be able to provide a 43.3mm image circle after your flattener / reducer and you need to match the camera pixel size to your focal length.  Too small of a pixel with a medium focal length telescope (600mm to 950mm) will put your resolution on the sky well below your normal seeing conditions. You will need to use 48mm or greater diameter filters and imaging adapters. 

 

But once you get it all dialed in... it is amazing !


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