Reducer f6.3

[Adaaam75]

Guys,

Can I use my DSLR with a Celestron Reducer/corrector on my unguided 9.25 SCT? And would it be worth it or are the reducers only intended for CCD's?

I'm hoping I can take images with this set up as I have seen many fine examples of DSO's taken using a DSLR that are unguided but not sure if a reducer/corrector can be used?

Ta!

[ecuador]

Not only it can be used, but I'd say it is essential for that setup, as it will allow you much more light gathering (2.5x more at f/6.3) , longer exposures (due to less focal length so more tracking tolerance) and a flatter field, correcting significantly the stars in your DSLR frame. See here for more details.

[Adaaam75]

That's exactly what I wanted to hear! The link is ideal but please humour me.... I have my 550d body a Celestron reducer and my 9.25 SCT, what else do I need?

[Davey-T]

One of these and a T ring

Dave

https://www.firstlightoptics.com/adapters/celestron-sct-t-adapter.html

[gonzostar]

I used to use a 8" SCT and found the reducer essential. The only time i didnt use it with my DSLR was taken pics of moon, Saturn and Jupiter. As Dave has mentioned a T ring also needed. Also whats the LP  like in your area. I used a CLS Astronomik filter

[ecuador]

41 minutes ago, Adaaam75 said:

That's exactly what I wanted to hear! The link is ideal but please humour me.... I have my 550d body a Celestron reducer and my 9.25 SCT, what else do I need?

That link to my blog post shows a couple of ways of getting into a good distance from the reducer, and how the exact distance affects the reduction.

[Dave In Vermont]

Although written around the MallinCam-videocams, the principles detailed are universal. Toss a copy in your collection -

Focal Reduction For Dummies.pdf

MallinCam makes some very nice reducers by hand - I have most of them. They are of top-quality:

http://www.mallincam.net/optical-accessories.html

Enjoy!

Dave

[ollypenrice]

On 27/06/2017 at 22:24, ecuador said:

Not only it can be used, but I'd say it is essential for that setup, as it will allow you much more light gathering (2.5x more at f/6.3) , longer exposures (due to less focal length so more tracking tolerance) and a flatter field, correcting significantly the stars in your DSLR frame. See here for more details.

No, the reducer will gather no more light, alas. The light grasp is entirely down to the aperture which is unaffected by the reducer. (Light entering the tube has no idea whether or not there is a reducer further down.) What it does do is concentrate the same amount of light from an object onto a smaller area, making that light more intense and so 'filling' a smaller number of pixels more quickly. This will certainly benefit your DSLR.

You'll get a wider field of view which will be a bonus on many targets and, as Ecuador says, the tracking accuracy you'll need will be reduced. But - and this is a big 'but' - even with reducer you would be imaging at about 0.8 arcseconds per pixel if your DSLR were, say, a 1000D. You may not be familiar with arcsecs per pixel but this unit defines resolution, the ability to capture fine detail, and 0.8 arcseconds per pixel is very fine detail indeed.  Unfortunately fine detail requires fine tracking and this level of resolution is so fine as to be on or beyond the limits of a good budget mount like the EQ6 to deliver even with an autoguider. Without an autoguider your mount is likely to blur the target by over twenty pixels. It could be thirty. That is an awful lot.

The problem is that your focal length is, in practice, too long for a DSLR even with reducer. That said, you can give it a go. I'm just trying to run through the hard facts as they stand. If you are not able to track at the accuracy your mount can deliver you'd get the same level of captured detail at a much shorter focal length and get a much wider field of view into the bargain.

Before putting money into a system which has an unavoidable fundamental flaw you might do better to put the reducer money into some short focal length optics (including a camera lens) which will work effectively on an unguided mount. Just a thought.

Olly

[Adaaam75]

Olly,

Thanks for the warning, I have a C9.25 on an AVX mount and have seen some lovely results on certain DSO's with a DSLR. In short I have a Canon 550d and the above setup and am interested in imaging without an autoguider set up.

Would purchasing a CCD camera (limited budget) be a better option unguided for DSO's?

Can I use a CCD unguided?

[ollypenrice]

28 minutes ago, Adaaam75 said:

Olly,

Thanks for the warning, I have a C9.25 on an AVX mount and have seen some lovely results on certain DSO's with a DSLR. In short I have a Canon 550d and the above setup and am interested in imaging without an autoguider set up.

Would purchasing a CCD camera (limited budget) be a better option unguided for DSO's?

Can I use a CCD unguided?

DSLRs can take great DS images. Check out Maurice Toet. Here's a difficult, faint target shot with a high end modded DSLR. http://www.mauricetoet.nl/Deep-sky/i-VxsgzXb/A

But Maurice is shooting at a very fast F ratio, F2.8, in a Tak Epsilon. And he's guiding (very accurately) at a focal length of about one third of your scope.

Your scope would do better with a CCD because of the option of larger pixels or binning or both but it will remain a tall order and will not deliver effectively without autoguiding. You just do need to autoguide long focal lengths. 

One of the new CMOS cameras might be an option.

Olly

[mftoet]

45 minutes ago, ollypenrice said:

DSLRs can take great DS images. Check out Maurice Toet. Here's a difficult, faint target shot with a high end modded DSLR. http://www.mauricetoet.nl/Deep-sky/i-VxsgzXb/A

Thank you, Olly. But "unfortunately" this image has been captured with an Atik 11000 CCD...

Here is an example of a faint object captured with a DSLR: a Canon EOS 5D Mark II which can be considered noisy by today's high-end DSLR standards. 

http://www.mauricetoet.nl/Deep-sky/i-S6xNktX/A

[ollypenrice]

48 minutes ago, mftoet said:

Thank you, Olly. But "unfortunately" this image has been captured with an Atik 11000 CCD...

Here is an example of a faint object captured with a DSLR: a Canon EOS 5D Mark II which can be considered noisy by today's high-end DSLR standards. 

http://www.mauricetoet.nl/Deep-sky/i-S6xNktX/A

Oops, sorry Maurice! But I think that my point still stands, given your corrected link. You demonstrate that a DSLR at fast F ratios can deliver very fine astrophotos.

Olly

[Adaaam75]

1 hour ago, ollypenrice said:

DSLRs can take great DS images. Check out Maurice Toet. Here's a difficult, faint target shot with a high end modded DSLR. http://www.mauricetoet.nl/Deep-sky/i-VxsgzXb/A

But Maurice is shooting at a very fast F ratio, F2.8, in a Tak Epsilon. And he's guiding (very accurately) at a focal length of about one third of your scope.

Your scope would do better with a CCD because of the option of larger pixels or binning or both but it will remain a tall order and will not deliver effectively without autoguiding. You just do need to autoguide long focal lengths. 

One of the new CMOS cameras might be an option.

Olly

Okay mate,

let me throw my Sky Watcher 130p in the mix! F6.5 with my Canon 550d attached and mounted on my AVX mount. Would that still need decent autoguiding or can I take short shots and stack them?

OR, what would I need in terms of suitable autoguiding? I do have a 9 x 50 finderscope and a Celestron 70 travel scope. Mixed views on whether these can be used as a finder scope. But what camera would be sufficient?

[ecuador]

10 hours ago, ollypenrice said:

No, the reducer will gather no more light, alas. The light grasp is entirely down to the aperture which is unaffected by the reducer. (Light entering the tube has no idea whether or not there is a reducer further down.) What it does do is concentrate the same amount of light from an object onto a smaller area, making that light more intense and so 'filling' a smaller number of pixels more quickly. This will certainly benefit your DSLR.

Of course it will gather more light. It will gather more light on the sensor, i.e. more photons will fall on it (about 2.5x times more) which is what you are saying as well 

And the unguided C9.25 setup might not be that bad, it has given me much better results for small targets (say M81/82 or smaller) than my 130PDS (when that also unguided) with just a tiny ZEQ25 mount at its limits, so I wouldn't say it is not worth trying. I had bought it only for planetary, and thought I'd try it for DSO on that small mount just for kicks, but the results were much better than I expected.

[ollypenrice]

6 hours ago, ecuador said:

Of course it will gather more light. It will gather more light on the sensor, i.e. more photons will fall on it (about 2.5x times more) which is what you are saying as well 

And the unguided C9.25 setup might not be that bad, it has given me much better results for small targets (say M81/82 or smaller) than my 130PDS (when that also unguided) with just a tiny ZEQ25 mount at its limits, so I wouldn't say it is not worth trying. I had bought it only for planetary, and thought I'd try it for DSO on that small mount just for kicks, but the results were much better than I expected.

No, I think it important to distinguish between gathering more light and concentrating the same amount of light onto fewer pixels, as focal reducers do. *

The relationship between F ratio and exposure time comes from the camera world in which it is unambiguously correct. It is correct because the F ratio is changed by changing the aperture. When we go down by 'one F stop' we are opening the iris so that the lens has twice the available surface area and collects, therefore, twice as much light. Not surprisingly this halves the exposure time.

But what happens when we change the F ratio not by changing the aperture but by changing the focal length?  The critical variables are aperture and focal length. F ratio is merely a derived variable.

The excellent imaging book, Lessons from the Masters, has a first chapter by Stan Moore looking at this in detail. His main points can be found here. http://www.stanmooreastro.com/f_ratio_myth.htm

This isn't playing with words. Increasing aperture and reducing focal length are two different things.

Olly

* Edit: The caveat being that the object in question fit on the chip without reducer. If the wider FOV brings in another 'desired object' then, yes, new useful light is indeed brought onto the chip. But if all you bring in is unwanted background sky which you then want to crop out then you haven't gained much. It's a complex issue. In a nutshell, if you want the wider FOV then the reducer is useful. It will also be useful if you are struggling to expose for long enough to get the signal above the noise floor.

[RichLD]

43 minutes ago, ollypenrice said:

No, I think it important to distinguish between gathering more light and concentrating the same amount of light onto fewer pixels, as focal reducers do. *

The relationship between F ratio and exposure time comes from the camera world in which it is unambiguously correct. It is correct because the F ratio is changed by changing the aperture. When we go down by 'one F stop' we are opening the iris so that the lens has twice the available surface area and collects, therefore, twice as much light. Not surprisingly this halves the exposure time.

But what happens when we change the F ratio not by changing the aperture but by changing the focal length?  The critical variables are aperture and focal length. F ratio is merely a derived variable.

The excellent imaging book, Lessons from the Masters, has a first chapter by Stan Moore looking at this in detail. His main points can be found here. http://www.stanmooreastro.com/f_ratio_myth.htm

This isn't playing with words. Increasing aperture and reducing focal length are two different things.

Olly

* Edit: The caveat being that the object in question fit on the chip without reducer. If the wider FOV brings in another 'desired object' then, yes, new useful light is indeed brought onto the chip. But if all you bring in is unwanted background sky which you then want to crop out then you haven't gained much. It's a complex issue. In a nutshell, if you want the wider FOV then the reducer is useful. It will also be useful if you are struggling to expose for long enough to get the signal above the noise floor.

Very well explained Mr Penrice. I've lost count of the number of debates I've had about this with fellow (non-astro) landscape photographers.

You're absolutely right. This is not a matter of semantics! ?

[ollypenrice]

By the way, I just worked this out to be sure. If we image a flats panel with and without 0.63 reducer we do pull in about 2.5x the light because of the increased surface area of the panel which illuminates the chip. However, out on the sky the amount of new and 'wanted' light brought in by the reducer is target-specific. If the additional FOV brings in nothing but dark sky then the increased light will, of course, be far less than 2.5x that at native.

Olly

[Davey-T]

And for the mathematically challenged amongst us how does the 6.3 f/r affect the arcsec / per pixel ?

250mm, 2.5 meter f/l,  f10.

Dave

 

[ollypenrice]

21 minutes ago, Davey-T said:

And for the mathematically challenged amongst us how does the 6.3 f/r affect the arcsec / per pixel ?

250mm, 2.5 meter f/l,  f10.

Dave

 

Being mathematically challenged myself I go here for an online cribsheet: http://www.12dstring.me.uk/fovcalc.php

Olly

[ecuador]

On 6/30/2017 at 10:42, Davey-T said:

And for the mathematically challenged amongst us how does the 6.3 f/r affect the arcsec / per pixel ?

250mm, 2.5 meter f/l,  f10.

Dave

 

It is a linear relationship: arcsec/px = 206.3 * sensor pixel size μm / Focal length mm. So applying a 0.63x reduction means 1/0.63x=1.6x in arcsec/px (or if you prefer, arcsec/px after = arcsec/px before * original f-ratio / new f-ratio). If you happen to have my polar scope alignment app for iOS, there is a calculator for most things, including that

[pete_l]

32 minutes ago, ecuador said:

It is a linear relationship: arcsec/px = 206.3 * sensor pixel size μm / Focal length mm.

Rather than using 206.3, for all real-world calculations, 200 * the number of microns gives you the FL in mm.

So 200 * 5.4 (μ) gives about 1000mm focal length for the "ideal" set up. However, + or - 50% still yields perfectly acceptable results.

[ollypenrice]

Nobody doubts that a focal reducer puts more object photons onto a pixel than a native system. What this body wants to know is how a focal reducer brings in more object photons, per se, from a discrete object.

[michael8554]

Light is a wave and a particle and err,  yes, passing through the f6.3 raises it's energy state, uhmm, well it's obvious really, hmm, think I'll get back to you on that one......

Albert E.

[newbie alert]

Best thing to do is to try it and see..you will need a reducer for dso's...

I luv my sct for imaging..just don't expect real long exposures like what can be achieved with fast fracs and newts...

This was done with a 9.25 sct and a ccd..not mine I might add!

[Merlin66]

long exposures come down to effective guiding and sky background.

Many amateur spectroscopists use SCT's at f10 and work with total exposures of > 3 hours on some targets.

It can be done.