#41  
Old 11-10-2015, 02:54 PM
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Visionoz (Bill)
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Applaud you Allan for your perseverance in nutting this out finally

Been following your adventures with this 10" f4 Newt astrograph that you're building with great interest.

Noticed that originally you did try to use a 40mm extension (to lift up your mirror) and now after your last foray it seems to need a 49mm extension lift; well you must have a patience of a monk (don't know whether these days there are any patient ones left! ) - good on'ya to be able to resolve it to your satisfaction and looking forward to more images after this

HTH
Cheers
Bill
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  #42  
Old 11-10-2015, 07:09 PM
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Quote:
Originally Posted by Visionoz View Post
Applaud you Allan for your perseverance in nutting this out finally

Been following your adventures with this 10" f4 Newt astrograph that you're building with great interest.

Noticed that originally you did try to use a 40mm extension (to lift up your mirror) and now after your last foray it seems to need a 49mm extension lift; well you must have a patience of a monk (don't know whether these days there are any patient ones left! ) - good on'ya to be able to resolve it to your satisfaction and looking forward to more images after this

HTH
Cheers
Bill

Thanks Bill,
I'll go with what I've got now but those flats
show that I'm 30% down in intensity the worst corner.
That is really unacceptable.
In the long run I'll have to change my imaging train to try & fix it.

cheers
Allan
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  #43  
Old 11-10-2015, 08:36 PM
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I was thinking of using my Baader MPCC which has a 55mm back focus.

In theory this would give me better flats as the camera sensor would be closer to the focuser.
My RCC1 that I use now has a back focus of of 91.5mm which allows me
to use a TS9 OAG , a QHY9 filter wheel, the camera nose piece & a Varilock spacer.
This very long 91.5 mm is probably why my flats are 30% down in the worst corner.

Can it work?

Calculation.

CCD Back focus 15mm from CCD surface to M42/0.75 screw plane -
( that is from the camera manual.)


The distance from the glass output side of the coma corrector to the front face of the camera is about 65mm.
( I measured that & it's more than expected because of my TS9 OAG )
add 15mm for the depth of the KAF8300 sensor chip.

64 + 15 = 79 mm

subtract the length required for the Baader MPCC

79 55 = 24mm = too far from the required 55mm

Removing the nose piece & using an adaptor would subtract the 12mm
nose length but add say 3mm for an adaptor so only gain 9mm.
therefore 24 - 9mm = 15 mm = still 15mm too long for the Baader MPCC.

So removing the nose piece of the QHY9 still wouldn't work with the Baader MPCC.

Is there a coma corrector that would work for better flats?

cheers
Allan
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  #44  
Old 11-10-2015, 10:38 PM
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I did a check of my RCC1 imaging train with my old 8" f6 Newt.
& compared it with the new 10" f4 Newt.
Imaging train is here:
https://www.flickr.com/photos/247194...in/photostream

What a surprise - the flats from the old Newt. are no better.
I include stretched pictures to more easily see the problem.
It's a surprise as the KAF8300 sensor only has a 22.5 mm diagonal -
that's only 11.25 mm off axis - not much!

Can anyone else post their flats in a similar way for comparison please?

cheers
Allan
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  #45  
Old 12-10-2015, 03:02 PM
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there are a few apertures down that train that could be limiting the illumination at the corners with an f4 light cone. However, I think that 30% reduction should still be quite manageable with flat fielding - after doing this, the main effect will be a slight drop in SNR in the corners and maybe a bit of extra diffraction around bright stars - but so what? All it means is that you have an 8.5 inch scope in the corners and a 10 inch one in the centre of the field. Even the super scope designers accept significant vignetting http://planewave.com/wp-content/uplo...-spot-size.pdf

the RCC1 is the only CC I know of that will accommodate that length of image train.

Last edited by Shiraz; 12-10-2015 at 08:33 PM.
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  #46  
Old 12-10-2015, 03:18 PM
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the only CC I can think of between the 55mm and 91mm baader options is the wynne which was about 69mm (and 2.5" not 2"). but yes, it aint cheap, currently on sale for 671euros.
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Old 12-10-2015, 10:09 PM
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Quote:
Originally Posted by Shiraz View Post
there are a few apertures down that train that could be limiting the illumination at the corners with an f4 light cone. However, I think that 30% reduction should still be quite manageable with flat fielding - after doing this, the main effect will be a slight drop in SNR in the corners and maybe a bit of extra diffraction around bright stars - but so what? All it means is that you have an 8.5 inch scope in the corners and a 10 inch one in the centre of the field. Even the super scope designers accept significant vignetting http://planewave.com/wp-content/uplo...-spot-size.pdf

the RCC1 is the only CC I know of that will accommodate that length of image train.

Thanks Ray,
Yes - you're right -
I can still have a lot of fun with this telescope as is.
If I put a galaxy in the middle & then crop the frame
it would make no difference.

cheers
Allan
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  #48  
Old 12-10-2015, 10:15 PM
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Originally Posted by rustigsmed View Post
the only CC I can think of between the 55mm and 91mm baader options is the wynne which was about 69mm (and 2.5" not 2"). but yes, it aint cheap, currently on sale for 671euros.

Hi Russell,
You mean this one?

http://www.teleskop-express.de/shop/...g-sensors.html

Looks really nice but I would need a 2.5" motorised focuser.
It's starting to get expensive.
I reckon I could get that to work & I would be at 4 x 0.95 = f3.8

f 3.8 would be very nice indeed - even faster.

Just found out - this corrector needs to be 170mm above the tube
so my secondary mirror would be too small.

I'll stick with what I've got for now.


cheers
Allan

Last edited by alpal; 12-10-2015 at 10:58 PM.
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  #49  
Old 14-10-2015, 12:38 AM
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Update Tuesday night 13.10.15,

what a good night - I just took 57 x 2 minute frames binned 1x1 of NGC 253.
It's a Gigabyte of LRGB data to process before I make flats, bias & darks.
FWHM was under 3 arc seconds sometimes.

This will be fun to process.
I think it will come out well.

cheers
Allan
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