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Imaging with Dob OTAs/Overcoming the back-focus issue

DIY ATM collimation mirror making reflector
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#1 Mikhail Pavlichenko

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Posted 18 April 2019 - 06:40 AM

Hi all,

I bought a GSO 10" F/5 Dob in the month of January this year. I bought it with the intent of visual astronomy and have used it thus far for its intended purpose.

However, I want to mount it on an EQ mount for imaging purposes as well. When I used my T Ring and Adapter and attached my DSLR camera to the focuser of my telescope, I was unable to achieve focus. After asking around, everyone told me that there is lack of back focus - there isn't sufficient back focus for the focal plane to form on the sensor of my DSLR camera.

However, this issue can be overcome by pushing the primary mirror front (towards the secondary) so that the focal plane simply moves out further.

I would like to kmow how to push the primary mirror upwards. This is a GSO 10" f/5, and I'm sure many folks on this forum would have faced similar issues when they ventured into imaging with their originally-dob OTAs.

Is there a kit available on the market to solve this issue? Do we need to change the collimation knobs and buy longer ones so that the mirror sits higher? If so, what kind of kit to upgrade to?

Another issue with GSO dobs is that we cannot keep/leave them standing vertically. Because their collimation knobs jut/protrude further out from the end of the OTA, these telescopes always need to be stored horizontally, unlike SkyWatcher Dobsonias; which is frankly a big pain.
Can this issue also be solved if the mirror is pushed upwards?

Lastly, I would like to know how much of a push would suffice? Would 2" (50.8mm) be enough for me to start imaging with it? What is the ideal placement scenario here? I understand that there should be enough travel available in the focuser for accurate focus of stars (using AP softwares, before commencing imaging), which is directly proportional to the length of the focal plane available at the sensor of the camera.

Thank you all in advance for all valuable inputs. Also, do point out to me if I have missed accounting for any other factors here.

Regards,
Udayan
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#2 Jon Isaacs

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Posted 18 April 2019 - 07:39 AM

However, I want to mount it on an EQ mount for imaging purposes as well. When I used my T Ring and Adapter and attached my DSLR camera to the focuser of my telescope, I was unable to achieve focus. After asking around, everyone told me that there is lack of back focus - there isn't sufficient back focus for the focal plane to form on the sensor of my DSLR camera.

 

A few thoughts:

 

- If you are using a DLSR, that implies you are doing deep space;  How much experience do you have with Astrophotography?  The mount required for a 10 inch F/5 will have to be very sturdy and precise and very expensive.  The diameter, length, and weight of a 10 inch F/5 plus the precision required in tracking means the mount will be very costly. 

 

-Dob mirror cells are designed for alt-az use, Equatorial use can require a different mirror cell.

 

- The tube is relatively thin and under the weight of the DLSR, it will probably flex enough to cause image problems. 

 

 

I would like to kmow how to push the primary mirror upwards. This is a GSO 10" f/5, and I'm sure many folks on this forum would have faced similar issues when they ventured into imaging with their originally-dob OTAs.

Is there a kit available on the market to solve this issue? Do we need to change the collimation knobs and buy longer ones so that the mirror sits higher? If so, what kind of kit to upgrade to?

 

I think very few venture into EQ mounting an 10 inch Dob.  It's a can of worms. 

 

Realize, you will also have to increase the size of the secondary mirror if you move the mirror forward if you want to achieve a reasonably well illuminated field.  

 

I have a GSO 10 inch F/5 GSO.  I don't think it's practical to move the mirror forward with longer bolts. That can be done with a robust mirror cell but the GSO uses relatively small bolts and weak springs.  

 

A low profile focuser is probably your best bet.  It reduces the bending moment on the tube and does not require moving the mirror cell.  

 

Another issue with GSO dobs is that we cannot keep/leave them standing vertically. Because their collimation knobs jut/protrude further out from the end of the OTA, these telescopes always need to be stored horizontally, unlike SkyWatcher Dobsonias; which is frankly a big pain.
Can this issue also be solved if the mirror is pushed upwards?

 

I stand mine vertically when I am moving it, I store it in the mount. I removed the locking screws from the cell and replaced them with longer screws so the scope stands on the longer screws. 

 

- If you haven't spent much time with a larger Newtonian on a GEM, it's worth hooking up with someone who has one.  They are no fun.  

 

- Consider an equatorial platform:  Here is a 10 inch GSO on an EQ platform.  EQ platforms track for an hour or more before them must be reset,  Resetting takes maybe 10 seconds but then the object must acquired. 

 

6287012-10 inch Dob on EQ platform.jpg
 
This one is a fancy one, it is aluminum, it has dual axis drives and an autoguiding port.  It is not cheap, $2625 but it is almost certainly cheaper than an EQ mount sturdy enough for astrophotography with a 10 inch F/5.
 
Jon

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#3 Mikhail Pavlichenko

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Posted 18 April 2019 - 08:10 AM

Hello Jon,

I found your thoughts on the matter very insightful. Like you said, it is far too expensive to mount a heavy 10" f/5 OTA on a GEM for precision astrophotography. However, I am considering buying a GE Mount some time in the future and thought that I could use this OTA atop that.

I wasn't aware that the Dob mirror cells weren't designed to be used with GEM. I'll keep that in mind.

I did not know that it was okay to place the OTA in the vertical position, I was initially adviced against it. I used to always leave my SkyWatcher 8" Dob OTA vertical when temporarily not in use.

Yes, I knew about the flex (I had heard of it) but did not think it would be a problem with this 10".However, I am wrong.

I did not realise that the secondary mirror's size would need to increase. That is certainly something that would need to be done in order for this to work! That was silly of me.

Yes, I agree with you. A low profile focuser is my best bet. Maybe I'll travel down that route.

I do understand that imaging with large Newts makes no sense since just managing the structure and making it perfectly stable is next to impossible. SCTs and RCs are best for the job when one wants to image with larger apertures.

However, since mounts are already so expensive, I'm going to have to hold off for a long time until I procure an SCT. So it just got me thinking whether I could use my existing setup for imaging until the day I am able to buy a telescope for the purpose of Astrophotography.


The EQ Platform idea works best, like you said. I am definitely considering it (making one).

As for experience with using reflectors on EQ mounts, yes I have some prior experience with members of my club that have used that setup. They have had their fair share of issues with that, and a lot of them. Normally 8" newts are okay for AP (f/4-f/5). Its at the 10" aperture that the issues really start creeping up I guess.
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#4 Jon Isaacs

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Posted 18 April 2019 - 08:17 AM

As for experience with using reflectors on EQ mounts, yes I have some prior experience with members of my club that have used that setup. They have had their fair share of issues with that, and a lot of them. Normally 8" newts are okay for AP (f/4-f/5). Its at the 10" aperture that the issues really start creeping up I guess.

 

 

I agree.  that step up from an 8 inch F/5 to a 10 inch F/5 is a big one.  

 

I think you would be better off just buying a planetary camera, build an EQ platform and have some fun.  A planetary camera will almost certainly come to focus and won't need a robust mount for tracking.  

 

Jon


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#5 hamishbarker

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Posted 19 April 2019 - 05:37 AM


A few thoughts:

- If you are using a DLSR, that implies you are doing deep space; How much experience do you have with Astrophotography? The mount required for a 10 inch F/5 will have to be very sturdy and precise and very expensive. The diameter, length, and weight of a 10 inch F/5 plus the precision required in tracking means the mount will be very costly.

-Dob mirror cells are designed for alt-az use, Equatorial use can require a different mirror cell.

- The tube is relatively thin and under the weight of the DLSR, it will probably flex enough to cause image problems.

I think very few venture into EQ mounting an 10 inch Dob. It's a can of worms.

Realize, you will also have to increase the size of the secondary mirror if you move the mirror forward if you want to achieve a reasonably well illuminated field.

I have a GSO 10 inch F/5 GSO. I don't think it's practical to move the mirror forward with longer bolts. That can be done with a robust mirror cell but the GSO uses relatively small bolts and weak springs.

A low profile focuser is probably your best bet. It reduces the bending moment on the tube and does not require moving the mirror cell.

I stand mine vertically when I am moving it, I store it in the mount. I removed the locking screws from the cell and replaced them with longer screws so the scope stands on the longer screws.

- If you haven't spent much time with a larger Newtonian on a GEM, it's worth hooking up with someone who has one. They are no fun.

- Consider an equatorial platform: Here is a 10 inch GSO on an EQ platform. EQ platforms track for an hour or more before them must be reset, Resetting takes maybe 10 seconds but then the object must acquired.



This one is a fancy one, it is aluminum, it has dual axis drives and an autoguiding port. It is not cheap, $2625 but it is almost certainly cheaper than an EQ mount sturdy enough for astrophotography with a 10 inch F/5.

Jon


How does the Dec axis tracking work? Does the platform just move the south end up and down ( assuming it's used in the northern hemisphere. )

I was thinking of adding Dec tracking to mine but realized that the Dec axis on a gem is always at right angles to the ra axis and if I just raise and lower the south pivot ( north in my case as I am in no but trying to simplify for majority northerners here) that would only be the correct Dec movement for objects in the meridian.

#6 Jon Isaacs

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Posted 19 April 2019 - 06:53 AM

How does the Dec axis tracking work? Does the platform just move the south end up and down ( assuming it's used in the northern hemisphere. )

I was thinking of adding Dec tracking to mine but realized that the Dec axis on a gem is always at right angles to the ra axis and if I just raise and lower the south pivot ( north in my case as I am in no but trying to simplify for majority northerners here) that would only be the correct Dec movement for objects in the meridian.

 

Hamish:

 

That is how it works so if one wants to image, the object must be near the meridian to guide.  I am not an imager so I haven't used the autoguiding.  

 

Jon



#7 Stephen Kennedy

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Posted 20 April 2019 - 07:49 PM

To give you an idea of what you are looking at to mount your 10" OTA on a GEM for AP, this is what I use. It is a 210mm F/7.7 Mikage Newtonian reflector on a Pentax MS-5 GEM.

IMG_1168 (3).JPG

It is very stable and I get excellent tracking. Neither the OTA or GEM are being produced now and it is almost impossible to get either one of them now used but there are good mounts being made now that will carry your OTA. However, they will be very expensive.


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