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Astro_king

C5 vs C8 with 6.3 focal reducer

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Astro_king    48

Hi,

I have setup an observatory for imaging and it is just about tight enough for my C8 on Heq5.

However, I can view planets and moon fine with it, just the dew shield will be a problem on some angles.

This got me thinking, I have a nexstar 5se which is a C5 scope and I was wondering if I attach the focal reducer 6.3 on the C5 and image using a DSLR am I loosing anything for DSOs in comparison to imaging with a 0.63 Focal reduce on the C8?

I understand for planets I will but for DSOs with long exposures on Heq5? This way I can just use the C5 for DSO imaging and C8 for planetary/lunar visual and imaging.

Also, out of curiosity, an ED80 is F6 after 0.85 reducer and so is the C8/C5 after the 0.63 reducer. Why does the ED80 give wider views still compared to the C8/C5? Am I wrong here? shouldn't the field of view be the same if the focal length is the same? because I love the C5 along with the portable Goto mount it came with and don't really want to sell it for an ED80 if I am only gaining things like comma/field curvature/ease of imaging etc. That whole thing is part of the learning curve I would be glad to go through with a C8/C5 as that is where the reward is for me. I used to image with a 200p newtonian taken off a dobsonian base on heq5 before so all this would still be fun for me.

 

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Lockie    3,711
31 minutes ago, Astro_king said:

Why does the ED80 give wider views still compared to the C8/C5? Am I wrong here?

field of view is a function of focal length not focal ratio. The ED80 has a focal length of 520mm with a 0.8 reducer, where's the C5 with a 0.63 reducer will have focal length of 788mm. The shorter the focal length the wider the view for a given eyepiece or sensor. 

Focal ratio is focal length divided by aperture, and this dictates how fast the optics are not how wide the FOV is.

hth 

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Astro_king    48
54 minutes ago, Lockie said:

field of view is a function of focal length not focal ratio. The ED80 has a focal length of 520mm with a 0.8 reducer, where's the C5 with a 0.63 reducer will have focal length of 788mm. The shorter the focal length the wider the view for a given eyepiece or sensor. 

Focal ratio is focal length divided by aperture, and this dictates how fast the optics are not how wide the FOV is.

hth 

Top stuff!

So this should answer if the focal ratio is higher with a C8 compared to C5 with the 0.63 focal reducer than the C5 is going to fare better imaging than the C8!

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Astro_king    48
9 minutes ago, Astro_king said:

Top stuff!

So this should answer if the focal ratio is higher with a C8 compared to C5 with the 0.63 focal reducer than the C5 is going to fare better imaging than the C8!

Actually just calculated and they would have the same focal ratio so C5 won't be better than C8 as the focal ratio would be approximately 6.3. The focal length though would be different so C5 would give a wider field of view with 0.63 reducer.

With 520mm the ED80 will give much wider field of view in comparison to the C8 which would stand at 1280mm which is more than double!

In terms of "FASTNESS" they would be identical with an ED80 sitting at 6.5 Focal Ratio. So all I gain from selling my C5 and buying ED80 is 250mm of focal length. Its best to test and see if the field of view with 0.63 reducer on C5 is decent wide to see how much of sky it fits in. Might not have to fork out money to buy the much loved ED80 then! and I have a goto mount with the C5 to take with me to darker sites if need to!

Let the testing begin!

 

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newbie alert    188

Focal length dictates the size of the object in the fov,hense why you use a focal reducer to give a wider fov...they work like a lense in altering the light cone to a focus point..

My ed80 has a focal length of 600mm ,with a .85 reducer and using a canon gives a nice frame on m45..rosette..m31 (just!) 

As a e.g... my ed80 with reducer is f6.37 and now the focal length is 510mm

My 8 inch  sct with it's  .63 reducer is at f6.3 but now has a focal length of 1262mm so it's still more than double the focal length of the 80ed..

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Lockie    3,711
2 hours ago, Astro_king said:

Top stuff!

So this should answer if the focal ratio is higher with a C8 compared to C5 with the 0.63 focal reducer than the C5 is going to fare better imaging than the C8!

The C5, C6, C8 etc are all f/10. Basically, the focal length increases when the aperture does to maintain the f/10 focal ratio. 

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newbie alert    188
2 hours ago, Lockie said:

The C5, C6, C8 etc are all f/10. Basically, the focal length increases when the aperture does to maintain the f/10 focal ratio. 

Yes..it focal length divided by aperture equals f ratio..

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Lockie    3,711
25 minutes ago, newbie alert said:

Yes..it focal length divided by aperture equals f ratio..

Indeed it is :) I also mentioned this so Astro King has no excuse for forgetting this formula now ;) :icon_biggrin:

6 hours ago, Lockie said:

Focal ratio is focal length divided by aperture, and this dictates how fast the optics are not how wide the FOV is.

 

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newbie alert    188
11 minutes ago, Lockie said:

Indeed it is :) I also mentioned this so Astro King has no excuse for forgetting this formula now ;) :icon_biggrin:

 

Just in case it slipped his mind..!

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ollypenrice    17,056

Beware of focal ratio in astrophotography. It doesn't work in the same way as it does with camera lenses where the F ratio is altered by varying the aperture. If you vary the F ratio by changing the focal length you enter a different world... Read up on 'The F ratio myth.' http://www.stanmooreastro.com/f_ratio_myth.htm

Olly

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Astro_king    48
9 hours ago, ollypenrice said:

Beware of focal ratio in astrophotography. It doesn't work in the same way as it does with camera lenses where the F ratio is altered by varying the aperture. If you vary the F ratio by changing the focal length you enter a different world... Read up on 'The F ratio myth.' http://www.stanmooreastro.com/f_ratio_myth.htm

Olly

Wow this article is totally against everything I have learned in this thread!

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Lockie    3,711
45 minutes ago, Astro_king said:

Wow this article is totally against everything I have learned in this thread!

Are you sure? All you've learnt in this thread is that field of view is a function of focal length, and f-ratio is focal length divided by aperture.

 

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Astro_king    48
52 minutes ago, Lockie said:

Are you sure? All you've learnt in this thread is that field of view is a function of focal length, and f-ratio is focal length divided by aperture.

 

Yes, my comment was a bit exaggerated :)

What I should have said was that what I have understood from the article is that based on the writer's opinion, the focal ratio doesn't matter when imaging which is not against something I leaned in this thread. However, I don't have the equipment to test this as both my scopes are F10s reduced to 6.3. I would need a fast newtonian at F4 maybe to test this with my SCT at F10.

 

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ollypenrice    17,056

Here's an image taken by Julian Shaw using a 6 inch refractor at F15. He used an Astro Physics Advanced Barlow.

SOMBRERO%20JULIANS%20DATA%20V2%20CROP-L.

We must remember that on this target  the photons which interest us come from the Sombrero Galaxy. The Barlow does not alter the number of photons from the galaxy and neither would a focal reducer.

However, in a system in which the focal reducer is used, say, to turn an image of M42 into an image of M42 plus Running Man (and we want both objects) then we do get more useful photons with the reducer because it brings the Running Man into the image. The new photons come from the Running Man.

The caveat concerns getting the faint signal over the read noise. If you can't expose for long enough to do this without the focal reducer then the focal reducer will help you. 

Olly

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Lockie    3,711
10 hours ago, ollypenrice said:

The caveat concerns getting the faint signal over the read noise. If you can't expose for long enough to do this without the focal reducer then the focal reducer will help you. 

I'm glad you included this caveat because this is exactly what I've found in the past when using my C8 Edge at f/10, then at f/7.

I was only able to take short exposures owing to the badly bodged NEQ6 I was sold (they saw me coming!). At f/10 I couldn't even detect the target, but with the same exposure @ f/7 the object suddenly leaped out at me! This was a few years back now, but I remember being quite surprised by the difference.

I obviously wasn't getting my signal above the noise floor at f/10! Think I was using 60 second exposures an old self modded Canon 350D...don't laugh ;)   

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Lockie    3,711
12 hours ago, Astro_king said:

Yes, my comment was a bit exaggerated :)

I don't have the equipment to test this as both my scopes are F10s reduced to 6.3. I would need a fast newtonian at F4 maybe to test this with my SCT at F10.

 

Hiya, you should be able to test this by using your SCT's at f/10, then add the 0.63 reducer but keep the exposure time the same. A lot may depend on how long the exposure time is! i.e getting your signal above the noise floor as Olly pointed out.

I think it would be interesting to try imaging with and without the reducer for various exposure times to see where the benefit of f/10 on small targets kicks in?

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ollypenrice    17,056
10 hours ago, Lockie said:

I'm glad you included this caveat because this is exactly what I've found in the past when using my C8 Edge at f/10, then at f/7.

I was only able to take short exposures owing to the badly bodged NEQ6 I was sold (they saw me coming!). At f/10 I couldn't even detect the target, but with the same exposure @ f/7 the object suddenly leaped out at me! This was a few years back now, but I remember being quite surprised by the difference.

I obviously wasn't getting my signal above the noise floor at f/10! Think I was using 60 second exposures an old self modded Canon 350D...don't laugh ;)   

Yes, I strongly suspect that this would be your problem. It's commonplace in CCD imaging to go as far as 30 minutes per sub and a few go even longer.

Olly

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Astro_king    48
On 13/10/2017 at 22:19, Lockie said:

Hiya, you should be able to test this by using your SCT's at f/10, then add the 0.63 reducer but keep the exposure time the same. A lot may depend on how long the exposure time is! i.e getting your signal above the noise floor as Olly pointed out.

I think it would be interesting to try imaging with and without the reducer for various exposure times to see where the benefit of f/10 on small targets kicks in?

Yes, I intended to image at F10 for small targets but didn't think about imaging the same target at different F ratios just by removing the reducer :)

Cheers

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One thing to not regarding FOV is that it is not just the focal length of the scope and the size of the chip that determine it. Internal baffles can limit the maximum chip size you can use. With the C8, the effective image circle when using a focal reducer is just 28mm, in the C5 there is more internal baffling and the image circle will be smaller, so it can only really handle 1.25" EPs (unlike the C8 which can handle 2" EPs).  A back of the envelope calculation suggests that that would means an image circle not much bigger than 18mm. Given the differences in focal length, the true FOV if your chip supports the 28mm of teh C8 should be roughly the same with the C5 and the C8

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