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  #1  
Old 01-09-2017, 08:15 AM
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Andy01 (Andy)
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Galaxy AP, minimum FL required?

Hi folks,
Toying with getting into capturing LRGB galaxies.
I'm experienced with big nebs and NB, but looking for a change.
Obviously will need a longer FL than my two short refractors, (336mm & 660mm)

Maybe I can trade one or both up for a 1000mm newt?
I have a QSI wsg8 with built in OAG and a hypertuned EQ6

Affordable recommendations?

Cheers
Andy
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  #2  
Old 01-09-2017, 09:11 AM
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Atmos (Colin)
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It's more to do with image scale than focal length. For your camera something around the 1000mm range would be good. So a 8" F/5 or 10" F/4 or 12" F/3

http://www.astrobin.com/full/287438/D/
This was shot at 677mm on a night of awful seeing.
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  #3  
Old 04-09-2017, 06:36 PM
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alocky (Andrew lockwood)
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Hi Andy, as Colin says it's all about image scale, so the other question is what's the seeing like at your location? If you only get 2" seeing, then 1"/pixel is probably as good as you need. That would be anywhere between 1000 and 1500mm for that camera.
Cheers
Andrew.
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  #4  
Old 05-09-2017, 07:21 AM
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multiweb (Marc)
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Have you considered using a x2 barlow and give it a go with your refractor first, test the waters with guiding and the mount.
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  #5  
Old 05-09-2017, 08:38 AM
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alocky (Andrew lockwood)
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Quote:
Originally Posted by multiweb View Post
Have you considered using a x2 barlow and give it a go with your refractor first, test the waters with guiding and the mount.
That is quite possibly the best bit of advice I've seen in here! Where's the 'like' button?!
Cheers
Andrew.
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  #6  
Old 05-09-2017, 11:34 AM
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Camelopardalis (Dunk)
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Andy, another means of skinning the cat (image scale) is to decrease the size of your pixels...newer CMOS sensors may be more economical than changing your scope. I've been experimenting with the ASI178 and its 2.4 micron pixels show promise...but it clearly becomes seeing limited, even with my 550mm F/L scope.
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  #7  
Old 11-09-2017, 12:34 PM
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Hi Guys, sorry for the delayed reply, some personal life events took precedence.
I borrowed a 1000 mm f5 newt from a mate to play with last night.
Crikey, not easy for this refractoholic though!
Couldn't get it to focus for visual with my diagonal & eyepiece, so gave up and attached my camera instead.
Trouble is my QSI is a heavy beast, way too heavy for the crayford focusser anyway - I could see it slipping as I watched!
..And the tube is so long that it will easily hit my tripod's legs.

Long story short, I abandoned it and put my small frac back n and was imaging within 15 mins.

Think I'll look into the barlow suggestion, (thanks Marc) and also try using the original flattener that was supplied with my WO - as it's not a reducer, which gives me a FL of 770mm @ f7.0

Stay tuned
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  #8  
Old 11-09-2017, 02:42 PM
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Merlin66 (Ken)
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Andy,
as you've found Newtonians are not designed for use with diagonals....

I think the barlow solution is well worth trialing at least you can then determine the best fl for imaging with your camera.

Check out the CCDCalc page - it allows you to see what FOV you'll get with various fl and cameras - there's a good selection of astronomical objects to allow easy comparison.
http://www.newastro.com/book_new/camera_app.html
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  #9  
Old 11-09-2017, 03:14 PM
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A barlow on a small APO will take forever to get some signal.
Not enough aperture for galaxy imaging.

You can do it but expect to put in massive hours.

Greg.
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  #10  
Old 13-09-2017, 09:04 AM
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Quote:
Originally Posted by gregbradley View Post
A barlow on a small APO will take forever to get some signal.
Not enough aperture for galaxy imaging.

You can do it but expect to put in massive hours.

Greg.
Hi Greg - my take on that piece of advice was that since Andy already owns the camera (v. expensive) and mount (also expensive) he should use the Barlow (cheap) to see if (1) his mount is up to guiding at long focal length, and (2) what the image scale and local seeing delivers at around 1.2m focal length before buying a new imaging ota (v. expensive) that may require another mount (v expensive) or camera.
I don't think anyone was suggesting it as an alternative final solution!
Cheers
Andrew.
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  #11  
Old 13-09-2017, 09:14 AM
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I agree 100% with Andrew....
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  #12  
Old 13-09-2017, 11:07 AM
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I will throw in another suggestion, the Skywatcher MN190 Mak-Newt. Its 1000mm fl and f5,3 so good enough and fast enough for DSOs. It does not need a coma corrector as it has a great front corrector, so its flat field and there are no diffraction spikes as the secondary is mounted on the corrector. Fully baffled, images compare to larger APOs, and it produces true colour rendition. I have had one for years now and will not part with it. The stock focuser is just adequate for imaging with a DSLR but with your camera (and mine), the Moonlight upgrade is recommended (Ron makes a special adaptor kit for the MN190). Its more expensive than a GSO imaging newt of the same size, and heavier (due to the corrector) but well balanced.
Current price is around $1699 from Andrews.
You would need a NEQ6 as a minimum for imaging with it, but with your camera i would suggest a heavier payload mount, i use a CGX these days.
Just my 2 cents. Note i have APOs and Newts, but the MN190 is the easiest to use as you don't need to worrying about reducer correctors, flatteners, or coma correctors - just stick the camera in the focuser and your in business.
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  #13  
Old 13-09-2017, 12:44 PM
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Quote:
Originally Posted by alocky View Post
Hi Greg - my take on that piece of advice was that since Andy already owns the camera (v. expensive) and mount (also expensive) he should use the Barlow (cheap) to see if (1) his mount is up to guiding at long focal length, and (2) what the image scale and local seeing delivers at around 1.2m focal length before buying a new imaging ota (v. expensive) that may require another mount (v expensive) or camera.
I don't think anyone was suggesting it as an alternative final solution!
Cheers
Andrew.
Hey Andrew, Thanks for clearing that up - I was browsing the Bintel website looking at 2x Barlows/Powermates today, but I'm uncertain which to get.

Televue? Powermate Other? Presumably they have screw threads at each end for affixing between the scope/flattener/camera

I'm assuming a barlow/powermate will slow the system down by a stop at least, so I'm guessing my 770mm f7.0 refractor becomes f8 or 9 @ 1540mm FL? That seems comparable to some of the affordable RC type scopes, and it's only a few hunderd bucks.

Quote:
Originally Posted by glend View Post
I will throw in another suggestion, the Skywatcher MN190 Mak-Newt. Its 1000mm fl and f5,3 so good enough and fast enough for DSOs. It does not need a coma corrector as it has a great front corrector, so its flat field and there are no diffraction spikes as the secondary is mounted on the corrector. Fully baffled, images compare to larger APOs, and it produces true colour rendition. I have had one for years now and will not part with it. The stock focuser is just adequate for imaging with a DSLR but with your camera (and mine), the Moonlight upgrade is recommended (Ron makes a special adaptor kit for the MN190). Its more expensive than a GSO imaging newt of the same size, and heavier (due to the corrector) but well balanced.
Current price is around $1699 from Andrews.
You would need a NEQ6 as a minimum for imaging with it, but with your camera i would suggest a heavier payload mount, i use a CGX these days.
Just my 2 cents. Note i have APOs and Newts, but the MN190 is the easiest to use as you don't need to worrying about reducer correctors, flatteners, or coma correctors - just stick the camera in the focuser and your in business.
Thanks Glen, that's a very positive rap for a beast I know next to nothing about! Not sure I want to spend big $ upgrading my mount, OTA on this project yet, but good to know what's out there.
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  #14  
Old 13-09-2017, 01:12 PM
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Merlin66 (Ken)
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Andy,
Depending on spacing (for Barlows) the magnification will be as advertised ie x2, x2.5, x3 etc.
This means your f7.5 scope would end up f15 (1200mm fl), f18.75 (1500mm fl), f22.5 (1800mm fl) - much more than just a "stop" (That's one reason I went for the TMB x1.8 Barlow on my f6 system)
Again most Barlows are 1.25" or 2" fitment and may or may not have a T thread for camera connection.
My Powermates are 1.25" and 2" fitment but do have a T thread adaptor.

If the Barlow is (or can be) positioned closer to the camera than design distance, the magnification will be reduced. NB This doesn't apply to Powermates which have the same magnification at any/all distances.

Hope that helps

Last edited by Merlin66; 13-09-2017 at 01:18 PM. Reason: detail added
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  #15  
Old 13-09-2017, 03:03 PM
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Slawomir (Suavi)
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I think that astro-physics Barlow can also be positioned at various distances to change magnification from 1.8x up.
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  #16  
Old 13-09-2017, 03:09 PM
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Merlin66 (Ken)
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OT...
I'm currently testing the Baader Hyperion x2.25 Barlow with various refractors/ filters for solar imaging.
So far, it looks very promising.
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  #17  
Old 13-09-2017, 03:52 PM
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I disagree with the sampling argument as it ignores the intrinsic resolution of the instrument. If were stuck with average seeing limits all the time, then most high res planetary imagery would be impossible.

I frankly find images taken at around 1100mm a bit of a yawn. Rarely is there any filigree detail.

Ive also found galaxies to be a mix of deep sky and planetary imaging. Image scale AND brightness helps enormously IMHO. I'd suggest something at the FL 2500mm mark would be about right.

Assuming you dont want to be at F10, then that also means a fair sized aperture.

Now we are talking big scope....and big mount....assuming you don't want it wobbling all about the place.

So. Careful what you wish for...it could lead to aperture fever.
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  #18  
Old 13-09-2017, 04:29 PM
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Quote:
Originally Posted by Merlin66 View Post
Andy,

This means your f7.5 scope would end up f15 (1200mm fl), f18.75 (1500mm fl), f22.5 (1800mm fl)
Yikes! Hmmm - well that's not going to work is it!

Quote:
Originally Posted by Peter Ward View Post

I frankly find images taken at around 1100mm a bit of a yawn. Rarely is there any filigree detail.
I'd suggest something at the FL 2500mm mark would be about right.
Assuming you dont want to be at F10, then that also means a fair sized aperture.
Now we are talking big scope....and big mount....assuming you don't want it wobbling all about the place.
So. Careful what you wish for...it could lead to aperture fever.
Aaaaahgghhh - thanks anyway Peter - it's going to hard to get tthat past SWMBO though
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  #19  
Old 13-09-2017, 05:07 PM
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10" or 11" SCT???
I use the C11 at f10 for spectroscopy on a NEQ6 mount. No issues, no drama
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  #20  
Old 13-09-2017, 05:17 PM
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Drizzle...a poor man's solution to undersampling
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