Using my Pentax DSLR with a Dobs 200p

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Using my Pentax DSLR with a Dobs 200p

Postby Sophiecentaur » Sat Apr 30, 2016 3:27 pm

I have bought a 200p and want to use my Pentax DSLR for some, albeit crude, photography of what I am seeing.
I got a T adaptor between the K camera mount and the 1.25" tube but I am having trouble focussing. I am ok when I choose the right combination of tubes and focus control for close objects (more than 20m and less than about 200m) but I can't rack the focus far enough in for more distant objects. That is with the adaptor ring hard up against the focus T (right term?).
This must be a pretty common situation and I would like a way round it. If I bought a Barlow (which I intend doing in any case, soon) would that solve the problem by putting the intermediate image further up the tube?
What's the accepted way of doing things?
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Re: Using my Pentax DSLR with a Dobs 200p

Postby Aratus » Sat Apr 30, 2016 4:51 pm

There is an issue with telescopes like this. What happens is that with the camera set up as you have it, it doesn't allow you focus close enough to the mirror.
Last edited by Aratus on Sat Apr 30, 2016 5:07 pm, edited 2 times in total.
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
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Re: Using my Pentax DSLR with a Dobs 200p

Postby Sophiecentaur » Sat Apr 30, 2016 5:04 pm

Thanks for the reply. I have extension tubes and they needed to be removed to focus the image from mid distance objects. The problem is that I can't get the camera any closer to the mirror, to focus on stars. Weird because the pictures of a Nikon on a Sky watcher Dob seem to show it working with the camera pulled a significant way out and the depth of a Nikon is pretty much the same as that of my Pentax.
I will need to go outside and try doing this again to confirm my problem is real. It's a shame that, despite being out in the country, our garden is surrounded by hedge and the Dob cannot see any distant objects without putting it up on a table. That is not convenient.
I am considering putting an elevated platform in the garden to allow for this and also to make using the scope easier.
I take your point about a Barlow.
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Re: Using my Pentax DSLR with a Dobs 200p

Postby Aratus » Sat Apr 30, 2016 5:09 pm

I had to change my previous post because upon reading it I realised your problem was the oposite to the one I was trying to solve! Sorry about that. (I've amended the post)

Somehow you need to bring the CCD nearer to the mirror. If shorter tubes don't work, then you may have to use something like a web cam instead of your DSLR. The only other thing I can think of is moving the mirror further up the tube ! That is rather drastic. Perhaps someone else has better solutions.
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
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Re: Using my Pentax DSLR with a Dobs 200p

Postby Gfamily2 » Sat Apr 30, 2016 5:14 pm

Yes, a Barlow should help.

The other things that can be done are :
get a lower profile focuser.
move the mirror forward - this can then mean your secondary is undersized, so you'll lose some effective aperture.

I think the best option would be to investigate the option of a lower profile focuser.
Scopes: Meade 8" SCT, Skywatcher 127mm Mak, Raffle winner of SW ST80
For imaging: Pentax K5, Asda webcam, Star Adventurer (new toy)
For companionship: Mid Cheshire Astronomical Group.
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Re: Using my Pentax DSLR with a Dobs 200p

Postby Aratus » Sat Apr 30, 2016 5:24 pm

Sophiecentaur - reading your posts again, you seem to be using your Dobsonian mostly to view terrestrial objects. A Dobsonian mount was not designed to do that, and being an astronomical telescope will of course render everything upside down. I think the focuser problem is only going to be the start of your problems if you are going to trying to use this for terrestrial work.
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
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Re: Using my Pentax DSLR with a Dobs 200p

Postby Gfamily2 » Sat Apr 30, 2016 6:10 pm

Aratus wrote:Sophiecentaur - reading your posts again, you seem to be using your Dobsonian mostly to view terrestrial objects. A Dobsonian mount was not designed to do that, and being an astronomical telescope will of course render everything upside down. I think the focuser problem is only going to be the start of your problems if you are going to trying to use this for terrestrial work.

I think he/she was saying that she/he could get a focus on nearby objects, which is fine; not that he/she wanted to use it for them.
Scopes: Meade 8" SCT, Skywatcher 127mm Mak, Raffle winner of SW ST80
For imaging: Pentax K5, Asda webcam, Star Adventurer (new toy)
For companionship: Mid Cheshire Astronomical Group.
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Re: Using my Pentax DSLR with a Dobs 200p

Postby Sophiecentaur » Sat Apr 30, 2016 11:56 pm

Gfamily2 wrote:
Aratus wrote:Sophiecentaur - reading your posts again, you seem to be using your Dobsonian mostly to view terrestrial objects. A Dobsonian mount was not designed to do that, and being an astronomical telescope will of course render everything upside down. I think the focuser problem is only going to be the start of your problems if you are going to trying to use this for terrestrial work.

I think he/she was saying that she/he could get a focus on nearby objects, which is fine; not that he/she wanted to use it for them.

Yes. Exactly. I have been using terrestrial objects to make life easier in the light whilst I am experimenting. Near-by (10m) is no problem at all but I have not been able to focus further than about 500m using the fittings I have. I have, however found that i can focus at a couple of km with just the focusser and the camera adaptor - hand held, I have a few mm to spare on the focus adjustment and that should allow sharp focussing on astronomical objects. What I really need is a 2" to 1.25" adaptor which will fit inside the focusser tube and not stand at all proud. I guess it's something I could fabricate, myself but it would need to be suitable for swapping easily between camera and eyepiece.
I was interested to find that the flare with terrestrial pictures is really shocking

Incidentally, my androgynous monicker goes back a while to when I had a Westerly Centaur sailing cruiser, named Sophie and I am an ageing bloke! I will fill in my profile just as soon as I get round to it. ;)
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Re: Using my Pentax DSLR with a Dobs 200p

Postby Gfamily2 » Sun May 01, 2016 8:52 am

It sounds like a low profile focuser is what you need then.
http://www.365astronomy.com/SkyWatcher- ... nians.html
Whether it's worth it with a Dobsonian is a matter for you, because that's a scope that is definitely a visual observers choice.

Alternatively, you could use Afocal photography where you stick the camera to the eyepiece. This often works better with a cheap digital or a phone camera. It'll allow you to capture a bit of what you see through the eyepiece at least, where tracking isn't needed.
Scopes: Meade 8" SCT, Skywatcher 127mm Mak, Raffle winner of SW ST80
For imaging: Pentax K5, Asda webcam, Star Adventurer (new toy)
For companionship: Mid Cheshire Astronomical Group.
(Not a moderator)
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Re: Using my Pentax DSLR with a Dobs 200p

Postby Aratus » Sun May 01, 2016 8:56 pm

Sophiecentaur wrote:Yes. Exactly. I have been using terrestrial objects to make life easier in the light whilst I am experimenting.

That's good news! 'Sophie Centaur' just means 'the wisdom of a centaur', and they are supposed to be very wise creatures! If you want a masculine version of the Greek you could use 'Sophos Centaur' . I hope you are able to solve your focusing problem.
I use an 11" Celestron SCT (CPC 1100) on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI1600MC and Canon 500D for imaging.
Aratus
 
Posts: 838
Joined: Tue Jan 12, 2016 2:17 pm
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