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Focal Reducer on Nexstar 6SE

JGM1971

By JGM1971
in Getting Started With Imaging

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JGM1971

JGM1971

Posted
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I'm experimenting with the first steps in Astrophotography and basically looking for a simple answer to whether an f6.3 focal reducer will show "noticeable" benefits with the kit I have?

I know it's AltAz and as a result not really the ideal mount/kit for this, but before I take the plunge into EQ6, Autoguiding, 2nd mortgage then divorce, I'm looking at this as a first step.

I've heard it will cut exposure times a lot, which will be an advantage as I have good / excellent dark skies in Northumberland, so have been able to pick out dust lanes in Andromeda with a 20sec shot on my Canon 1000D attached to the Nexstar, so I'm hoping this will only improve with the focal reducer alone.

Any advice (barring "just spend 5 grand on this, that and the other" as the wife will kill me!

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deckardbr

deckardbr

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Interested in this topic as well.

I have a DSLR and NexStar 6SE. I would like to be able to take some quick images of what I'm looking at using the original configuration (Alt-az mount). Nothing magazine like, but something to keep as I observe the night sky.

JGM, can I ask you: do you do prime focus or EP projection (and if so, how)? What's your current setup? What have you been able to image?

Thanks

Andrea

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ollypenrice

ollypenrice

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If you get the spacing right you'll get F6.3, though if you don't it will end up a bit slower. The difference between F6.3 and F10 is enormous. You'll need about 40% of the exposure time for the same intensity of signal, ignoring the effects of sampling rate for now. You'll get a wider field but you will not, on a target that fits into both the reduced and unreduced frames, get a benefit if you try to crop the focally reduced image and enlarge the main object to the size it appears in the unreduced one. This is called the F ratio myth and gets everyone very worked up. F6.3 will give a different, wider, image but with far better signal to noise ratio than an image at F10 if you accept that the wider field is a part of the bargain.

Olly

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JGM1971

JGM1971

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Hi Andrea,

I've only tried prime imaging, as I got the T adapters and SCT connector when I got the 6SE (knew I was going to at least try some Astrophotography at some point).

So far I've only attempted M31 and the Double Cluster in Perseus. Awaiting the cold winter nights when Orion is up at a work-friendly hour.

Thanks Olly.

That's the sort of confirmation I was looking for. The wider FOV will help, as when I imaged both of the above, they were too wide to fit fully into the Frame.

John

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deckardbr

deckardbr

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John - great! I'm going for the same setup. I mostly do observing, but would like to take the occasional picture of what I see.

I'll try without focal reducer for now, and then make the jump after some practice (I want to see if I can manage to get one picture taken before spending another £100 or so :))

Thanks

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ollypenrice

ollypenrice

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Hi Olly - what do you mean?

Sorry, I wasn't very clear. The 'chip distance' is the design distance between the back of the reducer and the camera chip. If this distance is not respected then in slow systems like SCTs you'll not get the nominal reduction in F ratio but something different, in reality slower. In fast systems you will not get a flat field but distortions in the corners.

Olly

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ollypenrice

ollypenrice

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OK, so you need to take flats. You have a very uneven illumination, brighter nearer the middle, and flats will correct this. You also have a lot of green noise for which there's a freebie. Visit Rogelio's website (Deep Sky Colors, Rogelio Bernal Andreo. It Googles.) His Hasta la Vista Green will nail it.

Olly

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