Bintel Astrograph Newt back-focus

Lee · #1 30-03-2013, 09:35 AM

I picked up a 200mm f/4 "imaging-optimised" Newt from Bintel this week, the carbon fibre version. Only had some minimal first light last night, in short lived sucker holes, but did notice some concerns that may affect some people.....
If you buy one of these to use your DSLR for prime focus work, I suspect there still may not be enough in-focus. My little Atik-420 CCD reached focus no problem - but its sensor is 13mm inside the unit. It reached focus with the focuser racked out at 10mm or so, add the depth of the 1.25" adaptor (10-12mm or so), maybe a couple of mm for the 1.25mm nose piece flange on the camera - infinity focus is likely falling approximately 35mm or so outside the fully racked in focuser tube....

From what I've measured/read, DSLRs tend to have their sensors 40-odd mm inside.....

Low-profile focusers will likely still be needed.... unless my measurements are off/wrong..... I'll have to add a moonlite to get enough back-focus to use my filter wheel.....

So - what actually defines an "astrograph" newtonian??

To add - my first impressions are favorable - the scope has a nice finish, the focuser feels very sturdy, without noticeable slop, and coma on my small chip not particularly noticeable - once I get some decent longer exposures I may revise this though!

pmrid (Peter) · #2 30-03-2013, 10:51 AM

Quote:

Originally Posted by Lee

I picked up a 200mm f/4 "imaging-optimised" Newt from Bintel this week, the carbon fibre version. Only had some minimal first light last night, in short lived sucker holes, but did notice some concerns that may affect some people.....
If you buy one of these to use your DSLR for prime focus work, I suspect there still may not be enough in-focus. My little Atik-420 CCD reached focus no problem - but its sensor is 13mm inside the unit. It reached focus with the focuser racked out at 10mm or so, add the depth of the 1.25" adaptor (10-12mm or so), maybe a couple of mm for the 1.25mm nose piece flange on the camera - infinity focus is likely falling approximately 35mm or so outside the fully racked in focuser tube....

From what I've measured/read, DSLRs tend to have their sensors 40-odd mm inside.....

Low-profile focusers will likely still be needed.... unless my measurements are off/wrong..... I'll have to add a moonlite to get enough back-focus to use my filter wheel.....

So - what actually defines an "astrograph" newtonian??

To add - my first impressions are favorable - the scope has a nice finish, the focuser feels very sturdy, without noticeable slop, and coma on my small chip not particularly noticeable - once I get some decent longer exposures I may revise this though!

I've encountered the same issue with other so-called astrograph-newtonians. Even the Vixen R200SS - trumpeted as an imager's scope par excellence - had a focuser you instantly wanted to remove and even with a low-profile Moonlite, still needed the mirror moved up the tube - in my case by 15mm. With a Bintel 8" F5, the GSO focuser was utter rubbish and the mirror would need to come 40mm up-tube to make it workable with any kind of filter/imaging system. Not sure why this is but it seems a recurring theme.

Peter

astroboy · #3 30-03-2013, 01:03 PM

Hmm , I had exactly the opposite problem .
I have a Bintel 8" " astrograph " that to reach focus with an FT focuser , Parracor 2 and DSLR I had to extend the tube 100mm .
Maybe the new CF version would reach focus with a Coma corrector.

gbeal · #4 30-03-2013, 02:19 PM

I have the Astro Tech 8" imaging newt, and echo Zane's comments, the focus point is way too far from the tube miles out.
It's a love hate/relationship, I'm sticking it into storage for a while, and using the refractor.
Gary

Lee · #5 30-03-2013, 03:20 PM

I suppose if you could make a refractor to the same aperture/weight/size/cost as a newt, we wouldn't bother with newts....

I picked up a Baader MPCC also, although didn't use it last night, they are supposed to not affect back-focus? Wait and see....

MrB (Simon) · #6 30-03-2013, 04:27 PM

Quote:

Originally Posted by Lee

I picked up a Baader MPCC also, although didn't use it last night, they are supposed to not affect back-focus? Wait and see....

I've not noticed any difference.
Did you get the mkII or the new mkIII?

Lee · #7 30-03-2013, 04:30 PM

MkIII....

MrB (Simon) · #8 30-03-2013, 06:20 PM

Good stuff.
I got one too but haven't had the chance to use it yet.
Will be interesting to see how it stacks up against the mkII.

Lee · #9 30-03-2013, 09:40 PM

Some measurements taken from the OTA...

Distance from OTA radius plate surface to infinity focus plane - 107mm
(one 1mm thick luminance filter in place)
Height of fully racked in focuser - 72mm
Available back focus 35mm...

Enough for your average CCD (?what about filter wheels), doubtful on the DSLR....

So a low profile moonlite, which are apparently 38mm high will buy another 34mm of back focus.... for a maximum of 69mm back focus....

naskies (Dave) · #10 22-07-2013, 01:23 PM

I've noticed that most reasonably priced (for the average consumer) "imaging" scopes only provide a tiny amount of back focus - barely enough for a DSLR + OAG.

A few positive notable exceptions are the AT65EDQ and (especially) the GSO RCs... but a common criticism in most reviews I've read is that they have too much back focus