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Old 25-01-2008, 10:55 AM
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multiweb (Marc)
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In-Focus problems: options with Pentax *istD on 5" Newtonian

Got a Pentax *istD that I want to use on prime focus on a small 5" newtonian (FL=650mm). I measured the distance between primary and focus point after reflection on the secondary. With focuser completely in the tube I have 30mm to play with. By the time I stick an adapter + T-Ring + camera I don't think I can do that within my 30mm budget.

I'm new to this kind of set-up because I currently use a Nikon Coolpix 995, that allows me to zoon in and out with a William Optics 28mm lens adapter. That was very flexible to focus and zoom in/out.

Now with the pentax bare case there's no more lenses involved right?

What are my options to get as close as I can to the flexibility I had wit the Nikon? Is there some kind of set-up including extra intermediate lenses that would help me to achieve this? Thanks for your help.

PS: moving primary closer and re-drilling the tube is not an option. I don't think I can move it in that much. Maybe get a shorter focuser?
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Old 25-01-2008, 12:30 PM
Dennis
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Hi Marc

I have never used a Newtonian, but I do understand that usually, their design makes it impossible to rack the focuser in far enough to get to focus with a DSLR.

As you mentioned, most users get around this by fitting a low profile focuser, or shifting the primary mirror further up the tube to get more of the light cone outside of the tube at the eyepiece end.

In terms of additional optics in the light path, I have found that using a reducer/corrector makes the problem worse on my Vixen refractor and Tak Mewlon; that is, even MORE in-focus is required.

Cheers

Dennis
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Old 25-01-2008, 01:31 PM
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multiweb (Marc)
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Quote:
Originally Posted by Dennis View Post
In terms of additional optics in the light path, I have found that using a reducer/corrector makes the problem worse on my Vixen refractor and Tak Mewlon; that is, even MORE in-focus is required.
Hi Dennis, thanks for the feedback. By reducer/corrector do you mean eyepiece projection?
I googled this and wonder if it could be an alternative if I can't achieve prime focus.

http://cgi.ebay.co.uk/1-25-T-Mount-M...ayphotohosting

It looks a bit bulky but it sound like I could use existing 1.25" eyepieces by sliding them in the tube? Does this sound too good to be true or you reckon it's feasible?
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Old 25-01-2008, 03:31 PM
Dennis
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Hi Marc

The Vixen Reducer/Corrector (R/C) “reduces” the focal length (918mm) of my Vixen 4” F9 refractor to around 590mm making it an “faster” F5.75 system whereas the my Tak Reducer/Flattener (R/F) “reduces” the focal length (2160mm) of my Mewlon 180 to around 1728mm making it an F9.6 system.

This results in a wider field, correction of coma at the edges and shorter exposure times.

These are the opposite of eyepiece projection where you are effectively increasing the focal length, f-ratio, image scale and narrowing the field, typically the same behaviour as a Barlow lens.

I have read that some people who use bino viewers on their telescopes have to use a Barlow in order to get to focus; here again, the Barlow “extends” the focal plane further outside the optical tube.

So, a Barlow or eyepiece projection might work with your set up, but the major downsides are:
  • Image is much dimmer
  • Filed of view is much narrower
  • The increased image scale will severely stress the capabilities of the mount and it is unlikely that you would be able to take images of longer than 2 to 5 seconds duration before you get start trailing.
Also, be careful of “cheap” T-rings for fittings DSLR’s to telescope accessories. I had a problem where my focus kept changing during an imaging run and I eventually traced it to a sloppy Pentax K T-Ring purchased cheaply!

Cheers

Dennis
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Old 25-01-2008, 04:16 PM
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multiweb (Marc)
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Makes sense Dennis. I'll get the adapter and give it a go. My scope is not that flash so in the long run I'll be imaging with something else. At this stage I'm still learning and the curve is pretty steep. Getting all this gear to connect and work together is not straight forward for a noob like me. 99% of the work for me seems to get to the point where I'm in position to start shooting a decent picture. Right now I'll be happy to take a pic of anything as long as I sort out all my cables, lenses, adapters and others.

I got a lot of help on these forums, people like you and PM'd a few of the experts too so I'm hoping to post some shots soon when those clouds start clearing...
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