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C8 back focus question


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I hope someone may be able to give me the correct figure.

 

I have been trying to image using my C8 with fr and asi2600mc pro. I appear to have really bad coma at 105mm backfocus. I have checked collimation and that appears to be fine so someone has suggested there might be a problem with the mirror but to narrow done the issue, he has suggested that I remove the focal reducer from the equation and take a stacked image of capella at the native fl. I will do this on the next clear night but does anyone know what back focus distance I need when not using a focal reducer on this scope?

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!05mm from the rear of the f6.3 Reducer to the camera sensor is the recommended starting backfocus. 

You should post a short-exposure jpeg of a sub so we can see if that distance is too much or too short.

Without the FR, the distances that Celestron recommend in that link are probably the result of extensive tests, so very Real World.

But I'm not sure what you will prove by testing without the FR.

Post an image with the FR.

Michael

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I'm slowly solving some issues with mine..

Can you confirm what version you have, are you using a reducer and which one? 

Using the Celestron 6.3 reducer and the recommended 105mm doesn't give you the spec'd focal length of 1280mm , 105mm gave me a focal length of 1320mm... I can't remember what back focus distance I'm using but my focal length is now very close to 1280mm and my stars are certainly better..( it's further out than that,further out gives you a reduced focal length)

So for me it's a combination of backfocus distance, good collimation.. I'm using a XLT version, Celestron 6.3 reducer, atik 460ex mono, oag with filterwheel

Edited by Same old newbie alert
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10 hours ago, Same old newbie alert said:

I'm slowly solving some issues with mine..

Can you confirm what version you have, are you using a reducer and which one? 

Using the Celestron 6.3 reducer and the recommended 105mm doesn't give you the spec'd focal length of 1280mm , 105mm gave me a focal length of 1320mm... I can't remember what back focus distance I'm using but my focal length is now very close to 1280mm and my stars are certainly better..( it's further out than that,further out gives you a reduced focal length)

So for me it's a combination of backfocus distance, good collimation.. I'm using a XLT version, Celestron 6.3 reducer, atik 460ex mono, oag with filterwheel

If you could measure your back focus then I would really appreciate it as it is possible that there isn't anything wrong with my mirror or reducer, its just the 105mm back focus is incorrect. I think my collimation is spot on so the poor star shapes are down to the back focus.

 

My version is also the xlt version, with the antares 6.3 reducer, asi2600mc pro. I am not planning to use a filter wheel / drawer as I will be only shooting galaxies in RGB.

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1 hour ago, stargazermanc said:

My version is also the xlt version, with the antares 6.3 reducer,

According to this review, the Antares 6.3 only needs a back focus of 49mm...    f/6.3 Antares Focal Reducer for SCTs- Awesome Astrophotography    However, there seems to be more discussion on the Antares on the web than all the other reducers put together. Some seem to think it's only a reducer and NOT a flattener. Some seem to think it's identical to the Celestron and probably made in the same factory (in which case how can it have a different focal length per the article above).  

There's some useful info on calculating a reducer's back focus at the bottom of this article.. Agena's Guide to Focal Reducers (agenaastro.com)

Graeme 

 

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10 hours ago, jacko61 said:

The OPS question... "does anyone know what back focus distance I need when not using a focal reducer on this scope?"

Graeme

That's not how it's written, never says anything about NOT

It says" 

I have been trying to image using my C8 with fr and asi2600mc pro. I appear to have really bad coma at 105mm backfocus. I have checked collimation and that appears to be fine so someone has suggested there might be a problem with the mirror but to narrow done the issue, he has suggested that I remove the focal reducer from the equation and take a stacked image of capella at the native fl. I will do this on the next clear night but does anyone know what back focus distance I need when not using a focal reducer on this scope?

It says with FR ( focal reducer)
Then goes on the say it's been recommended to remove the focal reducer
Which makes no sense as the whole point is to get the right backfocus.. with and without FR ( focal reducer)  are 2 different measurements..
 
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12 hours ago, stargazermanc said:

If you could measure your back focus then I would really appreciate it as it is possible that there isn't anything wrong with my mirror or reducer, its just the 105mm back focus is incorrect. I think my collimation is spot on so the poor star shapes are down to the back focus.

 

My version is also the xlt version, with the antares 6.3 reducer, asi2600mc pro. I am not planning to use a filter wheel / drawer as I will be only shooting galaxies in RGB.

 I'd gladly measure it but I don't think it would help as you're using a different reducer.. it will have a different focal length, so therefore a different backfocus, Also with and without a reducer are two different backfocus measurements .. having a APS-C sized sensor will need more precise measurement than a smaller sensor

 

Edited by Same old newbie alert
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56 minutes ago, Same old newbie alert said:

That's not how it's written, never says anything about NOT

It says" 

I have been trying to image using my C8 with fr and asi2600mc pro. I appear to have really bad coma at 105mm backfocus. I have checked collimation and that appears to be fine so someone has suggested there might be a problem with the mirror but to narrow done the issue, he has suggested that I remove the focal reducer from the equation and take a stacked image of capella at the native fl. I will do this on the next clear night but does anyone know what back focus distance I need when not using a focal reducer on this scope?

 
  •  

The very last sentence of the OP that YOU have cut and pasted says exactly what I pasted. . "does anyone know what back focus distance I need when not using a focal reducer on this scope?". I'm not sure why you can't see the word 'not' 8 words from the end. 

Graeme

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7 minutes ago, jacko61 said:

The very last sentence of the OP that YOU have cut and pasted says exactly what I pasted. . "does anyone know what back focus distance I need when not using a focal reducer on this scope?". I'm not sure why you can't see the word 'not' 8 words from the end. 

Graeme

I've not cut anything, I copied and pasted word for word, just that you've not read the first part, 10 words from the start

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1 minute ago, Same old newbie alert said:

I've not cut anything, I copied and pasted word for word, just that you've not read the first part

cut and paste / copy and paste - same thing. Anyway. He asks a very specific question at the end of his post (which I have read fully a number of times BTW).  He wants to know the back focus of his c8 WITHOUT a focal reducer because of some advice he's been given. That was the question I answered with a link to the celestron backfocus page. Whether what he wants to do is of any use is not for you or I to decide; we're just here to answer his question as best we can. 

Graeme

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11 minutes ago, jacko61 said:

The very last sentence of the OP that YOU have cut and pasted says exactly what I pasted. . "does anyone know what back focus distance I need when not using a focal reducer on this scope?". I'm not sure why you can't see the word 'not' 8 words from the end. 

Graeme

Back focus distance when not using a focal reducer.? 

Back from where? with out to reducer there is no back focus distance.

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1 minute ago, Rolfe Lindner said:

Back focus distance when not using a focal reducer.? 

Back from where? with out to reducer there is no back focus distance.

celestron seem to think otherwise....  What are the optimum back focus distances for Celestron SCTs? | Celestron    their definition is the distance from the baffle (i'e' the threaded end of the scope with the visual back removed) to the point of best focus. Whether or not we'd call it that is neither here nor there. It's the distance the OP is asking for at the end of his post. 

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11 minutes ago, jacko61 said:

cut and paste / copy and paste - same thing. Anyway. He asks a very specific question at the end of his post (which I have read fully a number of times BTW).  He wants to know the back focus of his c8 WITHOUT a focal reducer because of some advice he's been given. That was the question I answered with a link to the celestron backfocus page. Whether what he wants to do is of any use is not for you or I to decide; we're just here to answer his question as best we can. 

Graeme

Ok, the question is about backfocus in which taking the reducer off won't solve the question.. it may show a collimation issue but as the backfocus distance will change with and without the reducer the advice isn't sound as it's not linked to the question.. 

 

8 minutes ago, Rolfe Lindner said:

Back focus distance when not using a focal reducer.? 

Back from where? with out to reducer there is no back focus distance.

Really, are you sure??

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1 minute ago, Same old newbie alert said:

Ok, the question is about backfocus in which taking the reducer off won't solve the question.. it may show a collimation issue but as the backfocus distance will change with and without the reducer the advice isn't sound as it's not linked to the question.. 

 

His friend has suggested taking some images without a reducer to see if there is some fundamental issue with the mirrors. I guess he needs to know the focal plane to be able to get these 'non-reducer' images in as good focus as he possibly can. I'd suggest starting with the camera sensor 127mm from the scope 'baffle' then checking star focus with a bahtinov mask and adjusting from there. 

Graeme

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6 minutes ago, Rolfe Lindner said:

No

 

10 minutes ago, jacko61 said:

His friend has suggested taking some images without a reducer to see if there is some fundamental issue with the mirrors. I guess he needs to know the focal plane to be able to get these 'non-reducer' images in as good focus as he possibly can. I'd suggest starting with the camera sensor 127mm from the scope 'baffle' then checking star focus with a bahtinov mask and adjusting from there. 

Graeme

I found this from Celestron 

https://www.celestron.com/blogs/knowledgebase/what-are-the-optimum-back-focus-distances-for-celestron-scts

 

SCTBack focus distance from primary mirror baffle tube lock ring (in)

C5  5 in

C6  5 in

C8  5 in

So I guess you are correct,  but I just use the T-Adaptor !

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