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High Point Scientific 2 inch Coma Corrector-Imaging

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#1 Astrohoven

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Posted 11 October 2018 - 05:48 AM

Hello!

 

I bought the HPS 2 inch coma corrector https://www.highpoin...r-photo-visual  and I thought it would at least come with it's own manual showing the correct spacing etc.. but apparently it doesn't.

 

I checked some forums, and they are comparing it to the GSO CC on which I found a huge thread and it requires 75mm of spacing. The problem is that I haven't had success with 75mm spacing and I don't have enough spacers to try different spacings.

 

So is there anyone using this specific CC with good results? and what's the spacing you're using? 

 

Edit: I'm using an 8 inch F5 newtonian.

 

Thanks!

Anthony


Edited by Astrohoven, 11 October 2018 - 06:25 AM.


#2 evan9162

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Posted 11 October 2018 - 06:03 AM

I think the correct spacing is going to vary, and may depend on the focal ratio of the telescope.

 

I've been playing with my TPO CC (same GSO made CC as your HPS), with an F/4 newt.  Last night, I found the best spacing was 90mm from the shoulder (with the 2" eyepiece holder removed) to the focal plane.  Keep in mind, with a DSLR, typically there's 55mm from the front edge of the T-adapter to the focal plane.   So I ended up with 35mm of spacing between the shoulder of the CC with the 2" holder removed, to the front edge of the T-adapter on my DSLR.



#3 Astrohoven

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Posted 11 October 2018 - 06:25 AM

I think the correct spacing is going to vary, and may depend on the focal ratio of the telescope.

 

I've been playing with my TPO CC (same GSO made CC as your HPS), with an F/4 newt.  Last night, I found the best spacing was 90mm from the shoulder (with the 2" eyepiece holder removed) to the focal plane.  Keep in mind, with a DSLR, typically there's 55mm from the front edge of the T-adapter to the focal plane.   So I ended up with 35mm of spacing between the shoulder of the CC with the 2" holder removed, to the front edge of the T-adapter on my DSLR.

Ok thanks but I would like someone with the exact same CC to give me the spacing they've been having good results in since the spacing seems to vary a lot between one CC and the other. I really can't currently afford to buy additional spacers or a variable spacer without making sure that it would work. My scope is F5 and I think the spacing between different focal ratios would differ by a slight amount so at least I'd get in the ballpark but I don't think it would differ by 20 mm or more. Would it?



#4 happylimpet

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Posted 11 October 2018 - 08:15 AM

Ok thanks but I would like someone with the exact same CC to give me the spacing they've been having good results in since the spacing seems to vary a lot between one CC and the other. I really can't currently afford to buy additional spacers or a variable spacer without making sure that it would work. My scope is F5 and I think the spacing between different focal ratios would differ by a slight amount so at least I'd get in the ballpark but I don't think it would differ by 20 mm or more. Would it?

I would expect the difference in spacing for different f/ratios would be of the order of 1-2mm. I have an ASA 2korr coma corrector, and I think its about that for me going from f4 to f6...but its been a while since i checked it.



#5 Astrohoven

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Posted 11 October 2018 - 08:26 AM

I would expect the difference in spacing for different f/ratios would be of the order of 1-2mm. I have an ASA 2korr coma corrector, and I think its about that for me going from f4 to f6...but its been a while since i checked it.

Ye that's what I would expect, not a difference of 20mm. So I just can't find any info on whether or not the HPS coma corrector should have a 75 mm or not.



#6 MitchAlsup

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Posted 11 October 2018 - 09:27 AM

I think the correct spacing is going to vary, and may depend on the focal ratio of the telescope.

The "criticality" of spacing does depend on the focal ratio.

The point of optimum correction does not.


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#7 Astrohoven

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Posted 01 November 2018 - 04:41 PM

Update:

 

Do not buy this Coma Corrector. Just save a bit more money and buy a better one. I've tried all possilbe spacings from 65 to 85 mm with no effect. Terrible chromatic aberration and coma is not fixed as it should have. here's a closeup on one of the stars in my image. This is terrible.

Attached Thumbnails

  • 1234.jpg


#8 Xeroid

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Posted 01 November 2018 - 09:14 PM

Possibly defective device? Contact Highpoint Support!



#9 Astrohoven

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Posted 03 November 2018 - 08:13 PM

Possibly defective device? Contact Highpoint Support!

I did contact them just to double check what spacing I should be using and they actually seriously did not know. They started by saying it should be 55mm then once I told them to check the manufacturer's "manual" tolf me 75mm should work. Once I started telling them the CC might be defective, they didn't get back to my emails. I'm not buying anything from HP anymore.



#10 evan9162

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Posted 03 November 2018 - 09:22 PM

Which camera?

 

Here is what corner stars look like with my TPO (another rebranded GSO) coma corrector on a 6" F/4 newt, Canon T4i DSLR (APS-C, 4.3um pixels), 84mm spacing, stack of 21 300s exposures:

 

cornerstars.jpg

 

There is a little bit of chromatic error, but maybe just a couple pixels....


Edited by evan9162, 03 November 2018 - 09:24 PM.


#11 Astrohoven

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Posted 04 November 2018 - 05:23 AM

I'm using a Canon 500D mod. Your CA is much less than mine.

Could you tell me how do you deal with that in processing? Do you split the RGB channels and try to realign or do you leave it as is? What's frustrating is that everyone I know using the GSO is using a completely different spacing. Some are using 70, others 75 and 85. I don't think a lensing system can have that much tolerance even if the manufacturer said so. I think the best thing for me to do is try and sell the CC for someone who needs it for visual use and buy a new better one. I just need to make 100% sure that  the issues I'm facing are due to the CC not something else.



#12 evan9162

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Posted 04 November 2018 - 09:35 AM

If you're asking did I do anything in my image to counter the CA, then the answer is no, I didn't touch anything.

 

If you're asking how one would go about addressing it, that's a tough question.  If there is none in the center, and it gets progressively worse towards the corner, you may have to re-size the R and B channels slightly (like 0.1% or less) and re-align the channel.  

 

If you post one of your images, I could take a swing at seeing if one could re-align the channels.

 

I don't know if GSO has a huge range of manufacturing tolerances on this CC, or if (like I indicated earlier) the spacing is depending on the scope's F-ratio.  A coma corrector flattens the field.  A refractor field flattener needs different spacing depending on the focal length of the scope in use - so to me, the same concept would hold true for a coma corrector.

 

I optimized my spacing by using a Bahtinov mask, focusing on a centered star, then moving that star a corner, and looking at the Bahtniov spike pattern of the star in the corner.  A properly spaced coma corrector will show an focused star in the corners using a Bahtinov mask.  I adjusted the spacing until the star looked good in all four corners.  That was a compromise, since the sensor in my DSLR is tilted, and the best spacing for one corner wasn't the best for the rest.



#13 evan9162

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Posted 04 November 2018 - 10:10 AM

Does this CA appear the same in all 4 corners?  

Also, what kind of modification was done on your camera?  Full-spectrum, or was an IR-block filter installed?  



#14 Astrohoven

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Posted 04 November 2018 - 06:48 PM

Does this CA appear the same in all 4 corners?  

Also, what kind of modification was done on your camera?  Full-spectrum, or was an IR-block filter installed?  

Yes I think the CA is the same across the whole image (attached). I did not install a block filter but I am using the Astronomik CLS-CCD filter which takes care of that. My issue has nothing to do with the camera since I'm having the same problem with another unmodded DSLR.

Attached Thumbnails

  • CC Test.jpg


#15 evan9162

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Posted 04 November 2018 - 06:57 PM

Sounds like the CC is faulty - I would possibly expect some CA in the corners, but not in the center of the FOV.



#16 Astrohoven

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Posted 04 November 2018 - 07:00 PM

Sounds like the CC is faulty - I would possibly expect some CA in the corners, but not in the center of the FOV.

I guess that's the case and HighPoint doesn't even care. I guess I'm done with them.




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