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Skywatcher 130p and prime focus

PostPosted: Thu Nov 21, 2019 11:40 pm
by BrendanC
Hi,

After much research and head-scratching, I'm starting to realise that basically, a Skywatcher 130P , DSLR camera and prime focus don't get along well together.

There are mods I could do (scary), new OTAs I could consider (pricey), new lenses I could buy (ditto, plus I want to use my scope, not buy new lenses).

So, I'm considering selling the DSLR, and trying a dedicated astro camera instead. I already have the Bresser Mikrokular which is a joy to use but pretty limited. I'm after better planetary and moon shots, and the possibility of at least starting to delve deeper into the brighter DSOs, nebulae and galaxies and suchlike. I'm very aware that I'm using a small scope and don't have huge expectations, but I'd like to take my imaging a step further.

However, I don't want the same palaver I've had with the DSLR re achieving prime focus. I want something like the Bresser - pop it in the eyepiece holder, off you go. Looks to me like the best candidates are those with 224 and 290 sensors, from Bresser, Altair and ZWO.

Which brings me to my question: if I go for one of these, will I be able to achieve prime focus ie remove the Barlow, in goes the camera, get focus? If it's just not possible with a 130P then I'll accept it, maybe have a little cry to myself in the corner, and then just crack on with what I've got ie good observing, sort of ok imaging.

I'd just hate to take delivery, get all excited, remove the Barlow, pop it in, then realise that I just cannot get prime because it's not possible with my scope.

I'm not after specific recommendations for cameras, but I really would like owners of Skywatcher 130P scopes (not PDS, because that works, apparently, darn it) to tell me their experiences.

Thanks, Brendan

Re: Skywatcher 130p and prime focus

PostPosted: Fri Nov 22, 2019 7:00 pm
by rwillits
I don't have a Skywatcher 130p, but I do have a 6" Newt. Which I think is very similar. A DSLR with an adapter should drop into your telescope and focus. What DSLR do you have ? I still use my old Canon 450D from time to time. You can get some beautiful photos with a DSLR. What mount do you have ?

Re: Skywatcher 130p and prime focus

PostPosted: Sat Nov 23, 2019 3:52 am
by gassman
Hi Brendan,
depends on how much backfocus you have in your camera, and the prime focus of the scope. possible you might not be able to get the sensor close enough to the secondary, i had this issue with first scope, a 12" dob.
a mirrorless dslr or astrocam will have the sensor closer to the front this will help.
or move the mirror up to alter the prime focus into range.
btw if you have not already, try check out the field of view you will get with a particular camera and scope setup. stellarium can do it, and https://astronomy.tools/calculators/field_of_view/ is quite convenient. both those sensors are quite small compared to a DSLR for sure, might find the moon and larger DSO's are too big for it.

Re: Skywatcher 130p and prime focus

PostPosted: Sat Nov 23, 2019 7:17 am
by AstroChris
Yea that's a common problem. Skywatcher sells a variant of their scopes called "PDS" like my skywatcher explorer 150pds. They have a shorter tube and a bigger secondary on order to move the focus point out of the scope. Then you need extension tubes for eyepieces but cameras can move into focus rather easy.
I think they overdid it slightly. I wanted to attach my ZWO ASI120MC-S colour (a small planetary webcam). With only the 2"-T2 adapter I slewer all the way out and was still out of focus. I then used the 2"-1.25" -> 1.25" t2 adapters to get the few more millimeters. Whatever. I find the whole topic rather annoying. But it gets worse if you are playing with different stuff like field flattened and stuff i believe.

If you don't have a modified scope then
A) you can use a dedicated astro cam. Maybe that works given my cam needed to be shifted all the way out.
B) a 2" (high quality) low magnification Barlow (1.25 ?) can move the focus out...

Greetings
Chris

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Re: Skywatcher 130p and prime focus

PostPosted: Sat Nov 23, 2019 7:33 am
by AstroChris
Image

Is that the scope? Question: can't you loosen the side screws and then move the secondary slightly to the primary (like you would to fold it, but just a tiny bit) and then lock again? That still shift the focus point out. Because the secondary is flat we don't have to worry about that distance too much. It might "cut" some of the light coming back from the primary because we move it but that s shouldn't be too bad.

Try it and report on the results. I would be interested!!

Greetings
Chris


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Re: Skywatcher 130p and prime focus

PostPosted: Sat Nov 23, 2019 3:14 pm
by BrendanC
Great responses all! Thank you!

@rwillits - this is the full scope I have: https://www.tringastro.co.uk/sky-watche ... -528-p.asp. I can get focus with a Barlow, but not prime focus, which is what I want to at least try to capture the brighter DSOs. I'm under no illusion about what I'll get, I'm not expecting amazing things, I just want to make a small step in that direction.

@gassman - thank for the fov calculator, I also use Stellarium's feature to do something similar. The mod to move the primary up is something I've considered but I'm really reluctant to try it, not least in case I screw it up. Also it compromises the viewing because you see less from the primary, and it seems you also then need to have longer eyepiece holders to view in focus.

@AstroChris - "I find the whole topic rather annoying" - my sentiments exactly! I've been very seriously considering the PTS OTA, so it's interesting - and annoying, as you say - to find that it doesn't achieve prime focus readily with a dedicated astrocam! The low-power Barlow approach is another idea I've also considered, but the lowest I can find is 1.5x and that still slows the scope compared to prime focus.

For me, given my reluctance to mod my scope, it seems to boil down to changing the OTA, or changing the camera. Given what AstroChris says about the PDS having its own problems, I'm tending towards changing the camera. I have to say, the DSLR has been very frustrating and fiddlesome with the need to get adapters, T-rings and the like. The idea of popping in a dedicated camera, using pretty much exactly the same set-up I already have with the Bresser Mikrokular, is very appealing.

Anyway, really appreciate the feedback. Much food for thought!

Cheers, Brendan

Re: Skywatcher 130p and prime focus

PostPosted: Sat Nov 23, 2019 6:28 pm
by paraman
I'm not a reflector user ordinarly, I do have an RC which has a huge back focus so it never has a problem regardless of what camera or even eyepiece is used. That said the skywatcher PDS will reach focus with an astro camera without resorting to hacking bits off, Its normal practice to use a barlow with a planetary cam in order to get a longer focal length, narrower field of view and a bigger image scale. Planets are bright objects and the resultant increase in focal length and lowered light grasp is not an issue, pretty much the exact opposite of what is required for DSO imaging.
There are thousands of images out there taken with 130 and 150 PDs telescopes and astro cameras.

Re: Skywatcher 130p and prime focus

PostPosted: Sat Nov 23, 2019 8:36 pm
by bengourben
Hi Brendan.
I use a Skywatcher 200p with my canon.
I have mine connected directly to the 2 inch part of the focuser.
I've never had issues with achieving focus.. I can't remember if I've tried it with the 1.25 adapter.
Maybe this is the way to go? Directly to the 2 inch part?


Jason

Re: Skywatcher 130p and prime focus

PostPosted: Sat Nov 23, 2019 8:43 pm
by bengourben

Re: Skywatcher 130p and prime focus

PostPosted: Sat Nov 23, 2019 11:03 pm
by BrendanC
Thank you, and it's a great idea - but one I've already tried. The Skywatcher adapter allows for very close connection to the focuser, but even so, I'm about 1cm away from prime focus. I don't know about your model, but certainly on the 130P it's a well known problem, and one that I'm trying to get around. Somehow.