some advice as to what im doing wrong

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D4n1el
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some advice as to what im doing wrong

#1

Post by D4n1el » Thu Jan 16, 2020 12:34 pm

Hi

I have an orion XT8 dob and a canon 750D dslr. Last night was the first time the skies have enabled me to test the camera out instead of using my smartphone.

I followed very strictly 3 videos with the same camera and scope as me and at first i was having a great deal of difficulty seeing anything on the cameras live view other then a black screen.
I waited for the moon to rise to test it on an easier target. I found i could use the barlow with the moon and it was a perfect crisp image i was seeing, problem was that i couldn't use the 25mm piece as it was blury and no sharpness at all no matter how much i focused it! I have the svbony adapter (1.25")

I prefer the 25mm as i get the whole moon in one shot, is there anything else i need to purchse to resolve this or am i simply doing something wrong?

Also my dob base mount seems a fair bit wobbly, any links to videos or advice to make this more sturdy?


many thanks
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#2

Post by yobbo89 » Thu Jan 16, 2020 2:51 pm

so you manage to use your camera and barlow , but adding a 25mm into the barlow and then the camera via eyepiece projection didn't work ?,if so i suspect it's a focusing problem. if you want more magnification i suggest you use the projection method with your phone , or you get a stronger barlow and use your dslr, you can get 1.5x ,2x , 3x, 5 x ectt.

here is a fov calculator
https://astronomy.tools/calculators/field_of_view/

i've added a camera similar to yours and a scope at the same focal length and introduced different barlows to give you an indication of fov
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#3

Post by D4n1el » Thu Jan 16, 2020 3:55 pm

Thank you for the response but i dont think ive advised what im after properly. I would like to use the 25mm lens on its own without the barlow but cant get focus with it. if i use the barlow on its own its fine but very close up
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#4

Post by JimMinCT » Thu Jan 16, 2020 4:08 pm

D4n1el wrote:
Thu Jan 16, 2020 3:55 pm
Thank you for the response but i dont think ive advised what im after properly. I would like to use the 25mm lens on its own without the barlow but cant get focus with it. if i use the barlow on its own its fine but very close up
That will probably not be possible unless you modify the optical train.
Either by pushing the main mirror up towards the secondary, or purchasing a low profile focuser that will allow you to get the camera's sensor close enough to achieve focus.
Most Reflector type telescopes are not suitable for AP without modifications. The one's marketed specifically for AP are usually called astrographs.
There are a number of YouTube videos out there on how to beef up your base. A search for dob base modifications should bring up some of them.
Good Luck!
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#5

Post by D4n1el » Thu Jan 16, 2020 4:19 pm

thank you! thats just the answer i was looking for! happy to purchase a separate focusser could you advise of one at all for my model?

this is the thing, every YT video ive watched with the same set ups all use the stock focusser and also just drop the camera in without a lens! i dont see how its not possible for me to replicate this as i have the exact same set up
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#6

Post by Voyageur » Thu Jan 16, 2020 4:51 pm

Daniel, can you post a link to a couple of those videos? There may be some subtle differences in the setup that you lack the experience to take note of.
Scopes: Vixen VMC200L, D=200mm, F=1950, f/9.75; Televue 2" Everbright diagonal. Coronado PST; AstroTech EDT 80mm, F=480, f/6.
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#7

Post by ARock » Thu Jan 16, 2020 5:18 pm

1. DSLR attached to focuser without any eyepiece is called prime focus.
2. DSLR + Eyepiece attached to focuser is called eyepiece projection.

These are two completely different techniques for Astro Photography.
Most people here do 1.

On a Xt8 for 1 with a DSLR, you have to use a barlow otherwise you will not reach focus. To reach focus without a barlow you need to change your optical train by pushing the mirror in, or changing the focuser (not easy), or sometimes getting a negative profile T ring/adapter for your DSLR which may or may not work. Or you could get a different camera.

In prime focus (case 1.) the image size is approximately equivalent to an eyepiece with with the same Focal Length as the sensor size, for your Canon 750D is about 27mm (diagonal length of your sensor). However using a Barlow makes it about 13.5mm causing the larger moon size. You can check the image you can get with your scope and setup at the FOV calculator site yobbo89 mentioned above.

Eyepiece projection (case 2.) has more flexibility in getting magnification I believe, though I don't know much about it.
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#8

Post by Voyageur » Thu Jan 16, 2020 6:01 pm

Here is a good video https://www.youtube.com/watch?v=7VAuW6GoTnM&t=392s that clearly and simply explains the problem of reaching focus with a DSLR on a Newtonian scope and presents four ways of addressing the problem.

One is prime focus (no eyepiece) using a Barlow, as you have done. It's simple and requires no hardware modifications. But it doubles the focal length of the scope and narrows the FOV, as you have discovered.

Another approach is to use eyepiece projection, as ARock says. That will require you to buy some additional adapters. I don't know a lot about it and don't see many people here using this method. It will be more heavy and cumbersome than prime focus due to the inclusion of an eyepiece along with the adapters.

Third approach is the low-profile focuser already mentioned. This is expensive and requires some hardware modifications and the expertise to do the work.

Finally, the primary mirror can be moved up the tube to move the focal plane of the scope outward. Again, requires mods and a lot of know-how, and the scope will not function for visual use without extension tubes, etc. More of permanent setup for AP.
Scopes: Vixen VMC200L, D=200mm, F=1950, f/9.75; Televue 2" Everbright diagonal. Coronado PST; AstroTech EDT 80mm, F=480, f/6.
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#9

Post by Star Dad » Thu Jan 16, 2020 9:03 pm

I replaced my focuser (on an Orion f4.9 1000mm) with a low profile one. I am able to achieve prime focus with my Canon (requires 55mm back focus). But your scope is 1200mm, so I'm not sure you'd be able to achieve focus. I recommend you make darn sure whatever focuser you get can, in fact, achieve focus with your Canon.
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#10

Post by JimMinCT » Thu Jan 16, 2020 9:45 pm

Star Dad wrote:
Thu Jan 16, 2020 9:03 pm
I replaced my focuser (on an Orion f4.9 1000mm) with a low profile one. I am able to achieve prime focus with my Canon (requires 55mm back focus). But your scope is 1200mm, so I'm not sure you'd be able to achieve focus. I recommend you make darn sure whatever focuser you get can, in fact, achieve focus with your Canon.
It might end up being both a low profile focuser, and moving the mirror up in the tube.
At this point, we need to see the videos showing it was done and how before any disassembly takes place. :)
To the OP...
Could you post links to those YT vids, please? It will help a lot in getting you set up.
Thanks!
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#11

Post by D4n1el » Fri Jan 17, 2020 8:13 am

JimMinCT wrote:
Thu Jan 16, 2020 9:45 pm
Star Dad wrote:
Thu Jan 16, 2020 9:03 pm
I replaced my focuser (on an Orion f4.9 1000mm) with a low profile one. I am able to achieve prime focus with my Canon (requires 55mm back focus). But your scope is 1200mm, so I'm not sure you'd be able to achieve focus. I recommend you make darn sure whatever focuser you get can, in fact, achieve focus with your Canon.
It might end up being both a low profile focuser, and moving the mirror up in the tube.
At this point, we need to see the videos showing it was done and how before any disassembly takes place. :)
To the OP...
Could you post links to those YT vids, please? It will help a lot in getting you set up.
Thanks!

Apologies for late reply I didnt get any notifications.

https://youtu.be/8607YGFG12Q

This is a similar setup to my dob

I want to view planets but also want to capture nebula with my dslr so what scope would achieve this at a relative price point?
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#12

Post by JimMinCT » Fri Jan 17, 2020 12:40 pm

D4n1el wrote:
Fri Jan 17, 2020 8:13 am
JimMinCT wrote:
Thu Jan 16, 2020 9:45 pm
Star Dad wrote:
Thu Jan 16, 2020 9:03 pm
I replaced my focuser (on an Orion f4.9 1000mm) with a low profile one. I am able to achieve prime focus with my Canon (requires 55mm back focus). But your scope is 1200mm, so I'm not sure you'd be able to achieve focus. I recommend you make darn sure whatever focuser you get can, in fact, achieve focus with your Canon.
It might end up being both a low profile focuser, and moving the mirror up in the tube.
At this point, we need to see the videos showing it was done and how before any disassembly takes place. :)
To the OP...
Could you post links to those YT vids, please? It will help a lot in getting you set up.
Thanks!

Apologies for late reply I didnt get any notifications.

https://youtu.be/8607YGFG12Q

This is a similar setup to my dob

I want to view planets but also want to capture nebula with my dslr so what scope would achieve this at a relative price point?
Please do not take this the wrong way, but your first post states you followed 3 videos using identical equipment as yours, but your link is for a different camera and scope.
There are so many variables involved with optical spacing that you really need to compare apples to apples.
I'm still going to wager that a movement of the main mirror, and/or a focuser change will be required to get to focus.

On the other topic in question... Please explain what you mean by "relative price" as compared to what, the Dob?
You will be looking for a small refractor and a mount to put it on, or a tracking mount for your camera and a telephoto lens.
Both options will be far better, far easier, and far more rewarding than trying to struggle with the dob.
My 2¢ adjusted for inflation and the expansion of the universe.
Good Luck!
OTA's: Kson 1026-C, 4" Carbon Fiber ED Refractor, Home-built, 6.1", f/2.? APO refractor... (In Progress) 8" Meade LX2 SCT Mounts: CGX Imaging: ZWO ASI 1600MM, Canon 550D (T2i) Software: PixInsight, APT, PHD2, SharpCap, SGP, Stellarium, Registax, Stuff: Astro-Tech 0.8x FR/FF, Hotech SCA FF, ZWO 7nm 31mm LRGB-SHO filter set, ZWO 8 position EFWObsy: "Maybe Spaceship" Observatory
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