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Denk II on an SCT and minimizing SA and aperture/contrast loss

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#1 miamichillin99

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Posted 22 May 2019 - 07:03 AM

Great thread ( https://www.cloudyni...-focal-reducer/ ) started by Russ on his WO binos which sparked this question in my mind. Eddie gave all the necessary tools to analyze the impact of SA and aperture and contrast loss when increasing the optical path. I have the following equipment:
1. Denk II = 115mm of backfocus
2. Powerswitch = ~15mm
3. 2” meade 5000 dielectric diagonal with VB = 133mm
4. Standard Celestron 1.25” diagonal with VB = 100mm
5. Denk multipurpose OCS.
6. Denk 2” and 1.25” nosepiece adapters
7. Celestron C8. 2000mm focal length.

I am trying to use what I have to mainly view planetary but sometimes DSOs as well. Also my budget is pretty tight so buying a T2 system to lower the optical path is not feasible.

I have read that putting the OCS on the nosepiece of the diagonal improves backfocus. Does this mean that it reduces the optical path and thus doesnt reduce aperture or contrast? Does using an OCS on the nosepiece also not increase SA?

My other option would be to use the standard 1.25” diagonal with the Denk 1.25” adapter. I would lose the ability to use the OCS but would gain 33mm in backfocus. (Meade diagonal - celestron diagonal).

Any advice and suggestions are greatly appreciated.


Edited by miamichillin99, 22 May 2019 - 07:41 AM.


#2 Eddgie

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Posted 22 May 2019 - 11:32 AM

For planetary use, there is no reason to buy a T2 diagonal .  If you have the OCS, just put it in front of the diagonal and this will fix all of your problems.   You will get full aperture in low power mode though the magnification will be 1.3x, but remember, you don't have to use it for wide field views) and the mirror spacing will be just about ideal when you use either straight through or high power. 

 

Now if you use your setup without the OCS, there is probably some aperture loss in low power mode (between an inch and an inch and a half most likely) and even in straight through (maybe .5 inches), but high power will be full aperture. Yes, there will be spherical aberration, but this is not all that critical on most very large, bright subjects.   Again, you can always just pop the OCS out for wider field views, but for higher power work, putting on the OCS fixes all of the problems of SA and aperture loss.


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#3 miamichillin99

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Posted 22 May 2019 - 01:18 PM

That is excellent news Ed. I will probably have to look for some longer focal length eyepieces to have some low power options. At the moment I mainly use a pair of 16mm Naglers. Ive read great things about the 24mm panoptics so may try those. Also read great things on the Denk 21mm but they are too close to my 16mm. I own a pair of 26mm celestron/vixen silvertops but the 50 degree FOV is limited and the lack of eyeguards makes it uncomfortable to use.

#4 Reid W

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Posted 22 May 2019 - 01:22 PM

I have the D2 & power x switch w/ the 2" & 1.25" ocs units.  

 

What I have found specifically with the 1.25" corrector in place is that all solar system views are better than those views w/o a corrector in place.

 

I have not spent too much time with the 2" multipurpose corrector in the nosepiece of a diagonal because it seems to have a slight thread mis-match with my TV Everbrite.  No problem when using with the Denk newtonian 2" corrector tubes, so it might be *my* diagonal.  If you install the corrector in the nosepiece of your diagonal, make sure you can remove it.... in a half turn, out a quarter turn until tight. 

 

A couple years ago when this started for me, I recall Eddgie talking about how SA increases as you lengthen the image train.  Maybe he or some other reference also noted that while the SCT has a wonderful focus range, the instrument's optimal focal point was an eyepiece in the 1.25" diagonal.  To that end, it was reasonable for me to apply the idea of using a corrector on an sct to *keep* the focus point as close to optimal as possible.   

 

*note- I was in a meeting and finished this note prior to updating- see:  Eddgie was right.


Edited by Reid W, 22 May 2019 - 01:23 PM.


#5 Eddgie

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Posted 22 May 2019 - 02:17 PM

For planetary viewing, even a pair of inexpensive Plossls will work incredibly well.  



#6 miamichillin99

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Posted 23 May 2019 - 04:33 PM

Thanks Ed. So what do you recommend to help with the following. When I use the 26mm plossl I find that they have a very narrow point where I get a good picture in each eye. I have to stay absolutely still and aim each eyepiece perfectly into each eye to not get any dark or dim areas in the FOV. Its incredibly difficult to merge the image on these eyepieces and furthermore to get as much light in the FOV as possible. Sorry for not knowing how to explain this in technical terms.

Reid. Thanks so much for chiming in and your advice. I have not had any problem screwing in and out the OCS into my Meade diagonal.

#7 Reid W

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Posted 24 May 2019 - 10:22 AM

The challenge for the longer eye relief eyepieces - for me- such as the 26mm- is that I have to "hover" at the binoviewer.   Now, with the ES 20mm 68°, I can rest on the eyecups, provides a spot for proper placement.




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