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 »

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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Re: some advice as to what im doing wrong

#2

Post by yobbo89 »

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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D4n1el
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Re: some advice as to what im doing wrong

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Post by D4n1el »

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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Re: some advice as to what im doing wrong

#4

Post by JimMinCT »

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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Re: some advice as to what im doing wrong

#5

Post by D4n1el »

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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Re: some advice as to what im doing wrong

#6

Post by Voyageur »

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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Re: some advice as to what im doing wrong

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Post by ARock »

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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Re: some advice as to what im doing wrong

#8

Post by Voyageur »

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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Re: some advice as to what im doing wrong

#9

Post by Star Dad »

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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Re: some advice as to what im doing wrong

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Post by JimMinCT »

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!
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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D4n1el
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Re: some advice as to what im doing wrong

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Post by D4n1el »

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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Re: some advice as to what im doing wrong

#12

Post by JimMinCT »

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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Re: some advice as to what im doing wrong

#13

Post by D4n1el »

JimMinCT wrote:
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


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!
Thank you for your response and i completely agree! my OP and my last post were totally contradicting and didnt make sense at all! (i can only put this down to having our first child recently and im pretty tiered :))!

After reading/watching ive come to the realisation that the Dob is great for planetary viewing but not good at AP .

Could i buy something very cheap (svbony 20 or 48) and attach to my DSLR with a 25mm eyepiece lens and get good crisp shots of the moon?

If this is an option i may do this, then i have the small refractor for some light AP and the Dob for planetry viewing, which ever takes my interest more i could then proceed and upgrade for that particular thing.

thanks again for the help
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Re: some advice as to what im doing wrong

#14

Post by OzEclipse »

Eyepiece projection is a difficult technique and has problems with curved fields etc. I don't recommend this as a way forward.

As others have mentioned, if you move the mirror forward, the focus will be pushed out making prime focus available. However, the forward movement of the mirror and the weight of the camera will make the scope top heavy and out of balance so it will tend to fall down.

However, if you cut another hole for the focuser on the other side of the tube and a bit closer, you will also have to move the spider closer by the same amount and then turn the secondary mirror to suit. The prime focus will then be reachable by putting the camera directly on the focuser at prime focus allowing pictures of the moon. Deep sky photography requires a tracking mount and is out of reach of this scope. By moving the focus hole closer the scope will become bottom heavy and the extra weight of the camera body, if you are lucky will compensate for this and you'll end up with something that's almost in balance. Don't mount the focuser 180 deg opposite. The focuser is already at a 45 deg angle to the alt axis, you need to make the new hole about 90 degrees around the tube.
moving-the-focuser.jpg
I can't tell you the exact amount to move it in but you can work it out pretty easily.
Set the focuser travel as far in as possible, attach the T2 mounting ring. The focus needs to extend at least 45mm past the mounting plane of this ring.

Cut a piece of translucent material such as kitchen baking paper, greaseproof paper or tracing paper and put it over the focuser held with an elastic band. more narrow than the focuser diameter say 1" wide.

Point the scope at the Moon then bring it to focus.

If the moon comes to focus somewhere in the focuser adjustment range, then the new hole on the other side of the tube only needs to be about 46mm closer to the primary mirror- make it 50mm.

If it doesn't come to focus, come back here for advice. It can still be done, but it gets a bit more complicated to measure and calculate.

A piece of flexible black plastic can be fixed using the existing focuser mounting holes to cover up the old hole. A cheap way is to paint a panel cut out of the side of a 3L plastic milk or orange juice bottle. If you want to get fancy, get a metalworks to roll you a panel from Ally.

The spider mounting holes need to be re-drilled the same distance closer to the primary mirror.

You will need to buy and use a 50mm extension tube in the focuser to get eyepieces to focus.

However for the most part, this uses the equipment you already have, you only need buy the 50mm extension tube which costs less than $40.
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Re: some advice as to what im doing wrong

#15

Post by ARock »

Why not just use your DSLR and barlow? Most moon shots are partial shots.
Recent TSS APOD included.
app.php/article/1-17-2020-tss-astrophot ... of-the-day

Another way might be to use your Dob, but not your DSLR.
Get a different astro cam which will reach focus with the Dob. It will also be lighter and easier to balance. You should be able to do moon and planetary pictures with it. The moon will not be a full moon, but a part of it.
Here are some examples of what can be done, from the old AF forums (you might have to create an account there to see the pictures) from member @allisonribeiro who I believed has moved here as well. He was using a ZWO ASI224 with a Xt8
http://www.astronomyforum.net/astronomy ... o-far.html
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