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AT92 w AT2” FF and EOS Ra - poor performance

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#1 skaiser

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Posted 26 January 2021 - 01:08 AM

Hi all

 first light with my new AT92 , Astro tech 2” FF and Eos RA.

Maybe I’m expecting too much from this combination?

the stars are ok from center point out about 30% of the FOV.

then they get progressively more  elongated as you go farther out to the edge of FOV.

I figured the field flattener won’t take care of the full FOV but did not think it would start failing that quickly.

Am i expecting too much to be able to use a full frame imager with this scope or do I need a different field flattener.

Thanks for any insight on this.

take care

steve



#2 balu01

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Posted 26 January 2021 - 01:18 AM

You can get elongated stars for not having your flattener in the right distance from the camera sensor.

But first and foremost the 2" AT flattener simply does not provide a proper FF image circle and more likely the AT92 does not either without vignetting.

Very few telescopes provide real full frame performance without significant vignetting and a truly flat field.

 

You should be able to take very good shots with an aps size sensor with that setup , however it is best to get a "screw on " flattener.

 

Simply put yes you are expecting too much with the particular setup.


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#3 sg6

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Posted 26 January 2021 - 04:23 AM

Have you positioned the sensor at the correct position? That rotten thing termed "back focus".

 

Apparently for the 2" it is 57mm optimally and has a tolerance of +/-4mm meaning anything from 53 to 61mm. The RF mount has a flange distance of 20mm, meaning you have to find around 37mm more between the flattener and the sensor. How are you accomplishing that - 37 seems a fair amount to accomplish.

 

Next full frame = 36x24 = 44mm diagonal so 44mm diameter image circle. The flattener seems to avoid mentioning a specific image circle - which is poor. Lets face it the image circle size is 100% relevant to buying a flattener.

 

In the Review of the flattener it says:

"This one performs well over a 40mm image circle in all three small RCs (6", 8", and 10")."

 

That seems to be the only mention of an image circle, and is insufficent for a full frame sensor. I will agree it says in RC's but cannot think why the image circle should alter greatly. So to me it reads that the flattener is unable to support a full frame sensor. Or at least a good chance it cannot.

 

The eample image on the description page says:

"The image shows the corner stars across the entire APS-C sized sensor"

That is not a full frame.

 

If it supports and is designed for a 44mm image circle I expect it to be stated plainly and up front. It isn't.

 

Any flattener that covers 44mm will by definition be close on a full frame, really one needs 48 - 50mm image circle. And as ever I don't think such really exists. Manufacturers try to get by with the smallest margin they can.

 

So what spacers are incorporated between the flattener and the flange of the DSLR?

Do they add up to around 37mm?



#4 skaiser

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Posted 26 January 2021 - 02:46 PM

Hi

 Thanks for the feedback.

This is my 2nd try at using a Refractor Scope (Old hand with Newtonian and SCT)

I read some of the postings talking about tuning the FF distance.

That may be what I need to do to get better results.

Last night I shot a series of test shots on the AT92.

Using my Canon 60D and the EOS Ra.

Looking at the RA shots  and comparing FF with no FF i saw very little change in distortion.

I would expect a significant difference .

Here is the FF setup I'm using.

AT FF 1
The EOS RA uses a Adapter to change the connection from the R to the standard EF .
Canon documentation says this puts the face of the adapter at the "standard Canon distance of the regular AP C cameras
I tested this with my standard EF lenses and the images appear to be Clear and sharp.

I assumed the Thread Adapter and EOS T-ring would set the camera at the proper distance to the FF

My imaging results makes me think I need to tweak the spacing from FF to Imager.

I need to do some measurements to see what the 60D looks like compared to the EOS Ra.

 

If I need more distance I can always 3D print some spacer rings to go on the FF .

I'll let you know what I find.

Take care


Edited by skaiser, 26 January 2021 - 02:47 PM.



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