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First Use of Newt (F3.9) & Canon EOS 7D

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#1 Bestred1

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Posted 21 January 2020 - 09:28 PM

As topic says, I've done next to nothing with my Newt & Canon 7D so looking for some feedback on what I'm seeing here. Below is an 8-second sub taken at ISO 2500. My stacked photo output from DSS with just short of 5-minutes of subs using darks & flats looks about the same.

 

Take a look at the stars in the outer perimeter of this photo. Is this coma that can be expected as a normal view from a Newt?

 

I am using a coma corrector but it is one of the less expensive ones from High Point. Nothing else in the optical chain.

 

CN_4.jpg


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#2 Eric Horton

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Posted 21 January 2020 - 09:51 PM

I like it. Does appear collimation could be tweaked a bit more. What kind of newt is it?
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#3 klaussius

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Posted 21 January 2020 - 09:53 PM

Yes, that's coma.

 

Coma correctors need a specific distance to the sensor, and are designed for a particular F number. Assuming you got the right corrector for your Newt, make sure you get the spacing right. Otherwise it won't correct properly.

 

Check the specs on your corrector, it should mention the right spacing.


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#4 John Tucker

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Posted 22 January 2020 - 01:41 AM

Ditto the above. 

 

1. Set the spacing per the manufacturer's instructions

 

2. Collimate very carefully - I was never able to get it close enough on my F4 with a laser and Chesire.  Now I do a final collimation by taking an out of focus picture.  The fat side of the out of focus stars point to where the center of the primary is pointing, which should be in the middle of the FOV, not off center as in the pic below. To fix it, just use the collimation screws on the primary to move the "x" into the middle of the FOV.

 

3. Fine tune the spacing if needed

 

Capture.JPG


Edited by John Tucker, 22 January 2020 - 01:42 AM.

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#5 ks__observer

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Posted 22 January 2020 - 04:59 AM

To collimate i think a $5 collimation cap beats a laser.

Looks like a spacing issue above.

https://www.cloudyni...ocus/?p=8696478

Also a filter adds additional required BF distance equal to 1/3 filter thickness.

I was surprised recently at just how sensitive is the BF distance to getting good corner stars.


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#6 Bestred1

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Posted 22 January 2020 - 12:26 PM

I like it. Does appear collimation could be tweaked a bit more. What kind of newt is it?

Explorer Scientific N208/812 Newt (F3.9)

 

Yes, that's coma.

 

Coma correctors need a specific distance to the sensor, and are designed for a particular F number. Assuming you got the right corrector for your Newt, make sure you get the spacing right. Otherwise it won't correct properly.

 

Check the specs on your corrector, it should mention the right spacing.

Thanks......I haven't found any specs on the corrector but I bought it & the scope from High Point. The corrector was at their recommendation. When first received I tried the different spacers that came with the scope & landed where I'm at today as this combination was the only one where I could achieve focus. Assumption was that I had it correct.

 

Ditto the above. 

 

1. Set the spacing per the manufacturer's instructions

 

2. Collimate very carefully - I was never able to get it close enough on my F4 with a laser and Chesire.  Now I do a final collimation by taking an out of focus picture.  The fat side of the out of focus stars point to where the center of the primary is pointing, which should be in the middle of the FOV, not off center as in the pic below. To fix it, just use the collimation screws on the primary to move the "x" into the middle of the FOV.

 

3. Fine tune the spacing if needed

 

attachicon.gifCapture.JPG

See below...…...

 

To collimate i think a $5 collimation cap beats a laser.

Looks like a spacing issue above.

https://www.cloudyni...ocus/?p=8696478

Also a filter adds additional required BF distance equal to 1/3 filter thickness.

I was surprised recently at just how sensitive is the BF distance to getting good corner stars.

Thanks John & ks……………… I did have fits with collimation of this scope when first bought new. I was close to returning it as it had defective secondary vanes. They sent me replacements so I toughed it out. I do have a chelsire & a laser collimator & feel pretty confident in my set-up; however, I'll take another look at the star test as you've suggested at next opportunity.

 

One concern I still have is the offset of the secondary for this fast F3.9. Secondary is supposed to be 1/8" away from focuser & 1/8" closer to primary. I'm not confident that its there & have spent a bunch of time trying to tweak it. Any thoughts or experience with this? Could this be contributing to what I'm seeing?



#7 John Tucker

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Posted 22 January 2020 - 07:00 PM

Explorer Scientific N208/812 Newt (F3.9)

 

Thanks......I haven't found any specs on the corrector but I bought it & the scope from High Point. The corrector was at their recommendation. When first received I tried the different spacers that came with the scope & landed where I'm at today as this combination was the only one where I could achieve focus. Assumption was that I had it correct.

 

See below...…...

 

Thanks John & ks……………… I did have fits with collimation of this scope when first bought new. I was close to returning it as it had defective secondary vanes. They sent me replacements so I toughed it out. I do have a chelsire & a laser collimator & feel pretty confident in my set-up; however, I'll take another look at the star test as you've suggested at next opportunity.

 

One concern I still have is the offset of the secondary for this fast F3.9. Secondary is supposed to be 1/8" away from focuser & 1/8" closer to primary. I'm not confident that its there & have spent a bunch of time trying to tweak it. Any thoughts or experience with this? Could this be contributing to what I'm seeing?

 

I don't think the secondary is super critical to star shape.  I think that's mainly about maximizing your illuminated area on the camera sensor and getting any vignettting centered.  For now I think you can just make sure it points the laser beam into the middle of the primary, and come back to it later when you have sorted out the bigger issues. 

 

You're making much faster progress than I did.  For me the key was Chris Waters telling me I couldn't tell squat about the spacing issues or possible tilt in the system until I got the collimation dead on.  And by that, I mean better than I had succeeded in getting it with my laser and Chesire. The star collimation was the beginning of progress for me. It made it obvious I hadn't gotten it that close up to that point. It may have also been telling me that perfectly collimated with my Chesire in the focuser wasn't perfect collimation with my camera in the focuser.  I'm not smart enough to understand these things fully.

 

Once the collimation is dead on you can home in on the spacing as follows:  If stars are stretched radially (zooming out from the center of the FOV), you are too close.  If they swirl around the edge like a toilet being flushed, you are too far.  If you get one of those things in some corners and the other in other corners, you have a tilt issue.  That's usually solved by buying a tilt corrector and putting it between the corrector and the camera. 

 

I'm not a super expert or anything, but I don't thnk what you have here looks half bad except for the lower left corner, where I think you probably have a tilt issue (assuming that your collimation is good).  Gerd Neumann sells a very nice tilt corrector that allows you to adjust for tile in "real time".  Its pricey but the less expensive ones make you take the corrector off the camera each time you want to adjust.  Its pretty cumbersome.  If you buy a tllt corrector, make sure its optical path isn't longer than your current spacing, which looks pretty close.  I thnk the Gerd Neumann one is something like 17mm, and the shortest one I've seen is 9 mm  https://www.firstlig...t-adjuster.html

 

If you are tight for backfocus distance, Baader and Teleskop Service sell very short optical path (1 - 2mm vs the usual 10 to 11mm) "T rings" for Canon cameras.  One has a 48mm connection and the other is 42mm (complementary to the FLO tilt corrector)

 

Also, its easy to fret over star shape issues that turn out to be due to seeing.  As you go through the optimization process, stop once in a while and actually stack some subs.  In my case I find the stacked image generally looks better than the subs. 


Edited by John Tucker, 22 January 2020 - 07:41 PM.

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#8 Bestred1

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Posted 23 January 2020 - 05:53 PM

I don't think the secondary is super critical to star shape.  I think that's mainly about maximizing your illuminated area on the camera sensor and getting any vignettting centered.  For now I think you can just make sure it points the laser beam into the middle of the primary, and come back to it later when you have sorted out the bigger issues. 

 

You're making much faster progress than I did.  For me the key was Chris Waters telling me I couldn't tell squat about the spacing issues or possible tilt in the system until I got the collimation dead on.  And by that, I mean better than I had succeeded in getting it with my laser and Chesire. The star collimation was the beginning of progress for me. It made it obvious I hadn't gotten it that close up to that point. It may have also been telling me that perfectly collimated with my Chesire in the focuser wasn't perfect collimation with my camera in the focuser.  I'm not smart enough to understand these things fully.

 

Once the collimation is dead on you can home in on the spacing as follows:  If stars are stretched radially (zooming out from the center of the FOV), you are too close.  If they swirl around the edge like a toilet being flushed, you are too far.  If you get one of those things in some corners and the other in other corners, you have a tilt issue.  That's usually solved by buying a tilt corrector and putting it between the corrector and the camera. 

 

I'm not a super expert or anything, but I don't thnk what you have here looks half bad except for the lower left corner, where I think you probably have a tilt issue (assuming that your collimation is good).  Gerd Neumann sells a very nice tilt corrector that allows you to adjust for tile in "real time".  Its pricey but the less expensive ones make you take the corrector off the camera each time you want to adjust.  Its pretty cumbersome.  If you buy a tllt corrector, make sure its optical path isn't longer than your current spacing, which looks pretty close.  I thnk the Gerd Neumann one is something like 17mm, and the shortest one I've seen is 9 mm  https://www.firstlig...t-adjuster.html

 

If you are tight for backfocus distance, Baader and Teleskop Service sell very short optical path (1 - 2mm vs the usual 10 to 11mm) "T rings" for Canon cameras.  One has a 48mm connection and the other is 42mm (complementary to the FLO tilt corrector)

 

Also, its easy to fret over star shape issues that turn out to be due to seeing.  As you go through the optimization process, stop once in a while and actually stack some subs.  In my case I find the stacked image generally looks better than the subs. 

Thank you for sharing your experience John. This helps a lot. Its been cloudy & raining the past few days but is looking like things might clear nicely tomorrow night so I can do some star testing. I'll post my results as follow-up. 


Edited by Bestred1, 23 January 2020 - 05:56 PM.


#9 AhBok

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Posted 24 January 2020 - 07:51 AM

I have this same OTA and had similar problems at first. I use the Explore Scientific coma corrector which does a very good job taming coma--once I got the spacing correct. I also see you have the too common flaring around your brightest star. This is caused by one of your mirror clips intruding into the light path. Many of us masked off the outer 3-4mm of our primaries to prevent that flaring. I cut a mask out of a black PVC sheet I bought on Amazon and glued 3 small neodymium magnets to it 120 degrees apart. Then I laid is on top of the mirror so that the magnets adhered to the mirror clip screws and just covered the clips. This gives me clean round stars. I know of may other who have done similar fixes.

 

The only issue with collimating my N208CF was getting the secondary placed directly under the focuser. It is best to use a sight tube for this. I am not sure why, but it seems that many with this OTA had them delivered with the secondary way back toward the primary. Once you get it centered, you should never have to touch that again unless you disassemble the secondary for some reason.

 

I use a 2" Farpoint Laser and a 2" Farpoint Cheshire for collimation. I keep my rig setup semi-permanently so collimation stays dead on for many weeks at a time. I check it weekly, but rarely have to touch collimation more than every other month or so, and then it is just a minor tweak on the primary. My secondary only requires tweaking after many months.

 

Once I got my spacing, collimation and mirror clip reflections out of the way, I found this to be a very capable OTA.

 

Best wishes.


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#10 Bestred1

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Posted 25 January 2020 - 09:35 PM

I don't think the secondary is super critical to star shape.  I think that's mainly about maximizing your illuminated area on the camera sensor and getting any vignettting centered.  For now I think you can just make sure it points the laser beam into the middle of the primary, and come back to it later when you have sorted out the bigger issues. 

 

You're making much faster progress than I did.  For me the key was Chris Waters telling me I couldn't tell squat about the spacing issues or possible tilt in the system until I got the collimation dead on.  And by that, I mean better than I had succeeded in getting it with my laser and Chesire. The star collimation was the beginning of progress for me. It made it obvious I hadn't gotten it that close up to that point. It may have also been telling me that perfectly collimated with my Chesire in the focuser wasn't perfect collimation with my camera in the focuser.  I'm not smart enough to understand these things fully.

 

Once the collimation is dead on you can home in on the spacing as follows:  If stars are stretched radially (zooming out from the center of the FOV), you are too close.  If they swirl around the edge like a toilet being flushed, you are too far.  If you get one of those things in some corners and the other in other corners, you have a tilt issue.  That's usually solved by buying a tilt corrector and putting it between the corrector and the camera. 

 

I'm not a super expert or anything, but I don't thnk what you have here looks half bad except for the lower left corner, where I think you probably have a tilt issue (assuming that your collimation is good).  Gerd Neumann sells a very nice tilt corrector that allows you to adjust for tile in "real time".  Its pricey but the less expensive ones make you take the corrector off the camera each time you want to adjust.  Its pretty cumbersome.  If you buy a tllt corrector, make sure its optical path isn't longer than your current spacing, which looks pretty close.  I thnk the Gerd Neumann one is something like 17mm, and the shortest one I've seen is 9 mm  https://www.firstlig...t-adjuster.html

 

If you are tight for backfocus distance, Baader and Teleskop Service sell very short optical path (1 - 2mm vs the usual 10 to 11mm) "T rings" for Canon cameras.  One has a 48mm connection and the other is 42mm (complementary to the FLO tilt corrector)

 

Also, its easy to fret over star shape issues that turn out to be due to seeing.  As you go through the optimization process, stop once in a while and actually stack some subs.  In my case I find the stacked image generally looks better than the subs. 

I spent about an hour tweaking the primary last night via star testing & really didn't get any noticeable improvement. It actually looked round/concentric at the start but I tried to get perimeter looking better. When I did, the center of the FOV didn't look good. In the end I probably put it back close to where it was originally. Took some  pics & they looked about the same if not a little worse. Didn't have time to recheck it using my laser collimator today but maybe tomorrow. 



#11 Bestred1

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Posted 25 January 2020 - 09:52 PM

I have this same OTA and had similar problems at first. I use the Explore Scientific coma corrector which does a very good job taming coma--once I got the spacing correct. I also see you have the too common flaring around your brightest star. This is caused by one of your mirror clips intruding into the light path. Many of us masked off the outer 3-4mm of our primaries to prevent that flaring. I cut a mask out of a black PVC sheet I bought on Amazon and glued 3 small neodymium magnets to it 120 degrees apart. Then I laid is on top of the mirror so that the magnets adhered to the mirror clip screws and just covered the clips. This gives me clean round stars. I know of may other who have done similar fixes.

 

The only issue with collimating my N208CF was getting the secondary placed directly under the focuser. It is best to use a sight tube for this. I am not sure why, but it seems that many with this OTA had them delivered with the secondary way back toward the primary. Once you get it centered, you should never have to touch that again unless you disassemble the secondary for some reason.

 

I use a 2" Farpoint Laser and a 2" Farpoint Cheshire for collimation. I keep my rig setup semi-permanently so collimation stays dead on for many weeks at a time. I check it weekly, but rarely have to touch collimation more than every other month or so, and then it is just a minor tweak on the primary. My secondary only requires tweaking after many months.

 

Once I got my spacing, collimation and mirror clip reflections out of the way, I found this to be a very capable OTA.

 

Best wishes.

 

Thanks for the feedback Randy. In looking at the photos you have on astrobin using this scope I'm impressed & at the same time encouraged as it hasn't been much fun getting to this point. My adventure on initial set-up is in another post as follows.

 

http://www.cloudynig...with-new-scope/

 

I was wondering about the flaring & can notice it somewhat during a star test. Looks like the adventure with this scope will continue as I'll be headed down the road to also make a mask. Before that I'm going to recheck collimation with what I have now & play around with spacing a bit to see what effect it has on what I'm currently seeing.

 

Thanks again!



#12 AhBok

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Posted 25 January 2020 - 10:02 PM

Yes! That is the attitude that will help you get the most from this OTA. It took me about 6 months of minor tinkering to get what I consider to be the most out of this particular scope. It is definitely not perfect, but the Explore Scientific version of the 8” F4 reflector is definitely better than the less expensive ones (I previously had a cheapie 6F4 that was pretty awful). I’ve been a newt lover for decades and really like the N208CF.

Best wishes with yours!
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#13 Bestred1

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Posted 17 March 2020 - 10:27 AM

Yes! That is the attitude that will help you get the most from this OTA. It took me about 6 months of minor tinkering to get what I consider to be the most out of this particular scope. It is definitely not perfect, but the Explore Scientific version of the 8” F4 reflector is definitely better than the less expensive ones (I previously had a cheapie 6F4 that was pretty awful). I’ve been a newt lover for decades and really like the N208CF.

Best wishes with yours!

Following up on this topic...………….

 

It turns out that I have the infamous GSO comma corrector. Didn't know this until poking at it some more. No instructions came with it nor anything on the seller's webpage to indicate that it had been rebranded; however, after reaching out they directed me to the topic on CN.  Based upon what I read on several threads I added a 20mm spacer.

 

The seeing wasn't so great when I took the photos that contributed to this stack but the edges look a lot better to me. Comments please?

 

Autosave-cn.jpg


Edited by Bestred1, 17 March 2020 - 10:33 AM.

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