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Celestron EdgeHD image circle?

astrophotography Celestron imaging dslr
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#1 neoview

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Posted 24 January 2021 - 03:17 PM

Hello all,

 

I'm new to astronomy, and am looking to buy my first telescope. I'm primarily interested in planetary astrophotography, but want something that's good for viewing as well (for my boys). Based on some posts in the Cloudy Nights forums it seems like Celestron's EdgeHD series would be a good choice.

 

From a Celestron white paper (and an answer to a question on their website) it seems that all the EdgeHD scopes have a 42mm image circle. I currently have a Sony a65 with an APS-C sensor which I plan to use, so that's fine. However, I plan to update to a Sony a7r ii or a7r iii which have full frame sensors, so that's a 43+ mm diagonal sensor.

 

Celestron markets their EdgeHD 9.25" and larger scopes as being appropriate for full frame sensors, and they also advertise their M48 T-adapter as producing "better illumination for today's larger full-frame imaging sensors, all the way to the corners". It seems to me though that if your image circle is smaller than your sensor, you will have vignetting no matter how large a diameter the T-adapter is, and the M48 size will not improve illumination at the corners. Am I missing something?

 

Thanks for any insights,

 

-Larry



#2 robbieg147

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Posted 24 January 2021 - 04:03 PM

I would not worry about 1mm to be honest, but it gets more complicated when you start to add reducers.

 

With some of the smaller Edge scopes the image circle will reduce but still cover a APS-C sensor.

 

A lot of flatteners / reducers don't quite cover a full frame mine is about 95% but i don't notice this to be honest, and I usually crop quite a bit anyway.


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#3 mrlovt

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Posted 24 January 2021 - 04:13 PM

It's true, the 1mm won't matter. If you are shooting planetary, you probably won't be using either APS-C or Full Frame, you'll likely go with a planetary camera with a much smaller sensor.  Even if you are using APS-C or full frame for planetary, you'll only be using a small central portion of the sensor anyways.  

 

If you are shooting a wider field and using the entire full frame sensor of the a7r, you will use calibration frames and a little bit of cropping to minimize vignetting. 

 

If you haven't seen it yet, check out this FOV calculator with the cameras in 9.25 EdgeHD, and compare the relative image size of planets and DSOs with different cameras.

 

I'll also throw out there that there's absolutely nothing wrong with an SCT for a first scope, but it's a more complicated instrument than a reflector or refractor.


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#4 Seiten

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Posted 24 January 2021 - 05:30 PM

I have the 9.25" EdgeHD and a D850 full frame DSLR.  If you are planning on doing mostly planetary imaging (vice the moon), then I think you'll be doing alot of cropping.  I often set the camera on tighter FOV (image size) to zoom in a bit.  To put a better way, to scale down the image size since there is alot of image that has absolutely no image data, so I dont care too much about vignette.

 

gallery_334943_14311_33314.jpg



#5 neoview

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Posted 24 January 2021 - 07:58 PM

Thanks all for the replies, I appreciate it. Some comments below...

 

I would not worry about 1mm to be honest, but it gets more complicated when you start to add reducers.

 

With some of the smaller Edge scopes the image circle will reduce but still cover a APS-C sensor.

 

A lot of flatteners / reducers don't quite cover a full frame mine is about 95% but i don't notice this to be honest, and I usually crop quite a bit anyway.

Yep, I noticed on the celestron site that one of the versions (8 inch?) the image circle is reduced when using a reducer.

 

It's true, the 1mm won't matter. If you are shooting planetary, you probably won't be using either APS-C or Full Frame, you'll likely go with a planetary camera with a much smaller sensor.  Even if you are using APS-C or full frame for planetary, you'll only be using a small central portion of the sensor anyways.  

 

If you are shooting a wider field and using the entire full frame sensor of the a7r, you will use calibration frames and a little bit of cropping to minimize vignetting. 

 

If you haven't seen it yet, check out this FOV calculator with the cameras in 9.25 EdgeHD, and compare the relative image size of planets and DSOs with different cameras.

 

I'll also throw out there that there's absolutely nothing wrong with an SCT for a first scope, but it's a more complicated instrument than a reflector or refractor.

After I posted I came around to the point of view that 1 mm is not that much, and in any case I can always crop it. I did come across the FOV calculator just today, and had actually done some hand calculations previously based on the angle subtended by the planetary disk as seen from earth and focal length.

 

I currently have a 300 mm Tamron telephoto zoom and a Manfrotto 475b tripod, and have taken a few pictures of the skies with that setup: a partial solar eclipse, the moon, and most recently comet Neowise. The results were ok, but it made me want more... sharper images, more detail, and less chromatic aberration. 

 

I was originally looking at 80 f/6 mm triplet APO refractors, but based on my FOV calculations I decided I wanted more focal length. I then focused (no pun) on Maksutov Cassegrain scopes since it seemed like one could get more for the money. However, subsequently I saw a post indicating that the field curvature of the EdgeHDs was lower than typical Maksutov Cassegrains, and that sounded good to me. 

 

I have the 9.25" EdgeHD and a D850 full frame DSLR.  If you are planning on doing mostly planetary imaging (vice the moon), then I think you'll be doing alot of cropping.  I often set the camera on tighter FOV (image size) to zoom in a bit.  To put a better way, to scale down the image size since there is alot of image that has absolutely no image data, so I dont care too much about vignette.

 

[...]

Nice picture! That's the kind image I am hoping to acquire. smile.gif

 

When I said planetary astrophotography I actually meant the moon, planets, and comets. 


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#6 SimonMiller

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Posted 26 January 2021 - 03:35 PM

I have the EdgeHD 800 and also the EdgeHD 800 0.7x Reducer.

 

There are differences between the EdgeHD 800 0.7x Reducer and the 9.25 EdgeHD 9.25 Reducer (and bigger versions):

 

1. Physical size of the Reducers are increased from 2" to 3.25";

2. Diameter of the glass on the back of the Reducers are increased from 35mm to approximately 60mm;

3. Image circle of the image circle of the Reducers are increased from 26mm to 42mm (approximately 0.7x * 43mm for the EdgeHD 800);

4. Back Focus that was 105mm after the Reducer is now 143mm with the bigger Reducers (it doesn't change).

 

NB There is only one version of the Celestron OAG:

 

1. The minimum prism height of the Celestron OAG is 15mm off the centre line (30mm diameter);

2. The Celestron OAG has a thickness that is much more than the "thin" ZWO and QJY OAGs.

 

I bought (in this order):

 

1. EdgeHD 800;

2. EdgeHD 800 0.7x Reducer;

3. Celestron OAG.

 

NB I returned the Celestron OAG and bought a  QHYOAG-M.

 

With Celestron you can choose any two of EdgeHD 800 + EdgeHD 800 0.7x Reducer + Celestron OAG.

 

If you try to use all three then you will notice:

 

1. Celestron OAG sees only half of the image circle of the back of the 0.7x Reducer;

2. You cannot use a DSLR with the Reducer. The back focus required is 105mm and the BF available is 113mm (approximately);

3. Celestron OAG is very "thick" when you add up the body and adapters;

4. Refractors have a Reducer with a back focus of approximately 60 - 65mm;

5. Back focus with EdgeHD 9.25" is retained and is easy to accommodate;

6. To work with an OAG solution that also "works" with refractors, the ZWO or QHY OAGs work for both.

 

To use an OAG with the EdgeHD 800, you really are forced to buy a non Celestron OAG. There is an option to vary the adapters and this results in (much to much) additional cost.

 

When buying the EdgeHD XXXX, you should consider if you will want a Reducer and of course an OAG and then if there will be a DSLR. For the best outcome consider one of the bigger EdgeHD and if you have a refractor (and want a common OAG solution) a non Celestron OAG.



#7 speedster

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Posted 26 January 2021 - 08:03 PM

You can have all 3 + DSLR + more.  8" EdgeHD back focus is 133mm.  Celestron OAG SCT adapter = 25.3, body = 29, male adapter = 12.5, T-ring = 10mm,  DSLR flange focal length (full frame) = 44, for a total of 120.8.  You have 12.2mm left over to play with.  Use the included female adapter instead of the SCT adapter and you're down to 100.  If you need a little more space, use a thin T-ring and pick up another 8mm which leaves you with 41 mm to spare. Add a filter wheel at 22mm and you still need 19mm of spacer.

 

Back focus with the reducer is only 105mm which is 5 more than your 100 but not enough for a filter wheel or whatever.   Going mirrorless APS-C instead of full frame DSLR gets you another 26mm for other things in the train.




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