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6 inch F5 refractor vs 6 inch F4 reflector

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#1 GOLGO13

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Posted 01 June 2020 - 10:45 AM

I finally had a chance to actually test the 6 inch F5 refractor with night vision. I compared it to my 6 inch F4 reflector. My bottom line up front is the 6 inch F4 reflector seems to be better in general (especially on axis). 

 

The refractor itself is difficult to say on it's optics. It seems to be collimated just fine. But I don't think personally it's the best optics. Still, at low powers on the moon and such it's very sharp. It may just be that high powers brings out the CA. The 6 inch F4 as far as I can tell has excellent optics.

 

Comparing the views between the two on nebulas with night vision it's very close. Not sure there is a clear winner. On one had the 6 inch F4 without a coma corrector has a bit of issues off axis. But I can usually ignore those.

 

The big difference was on globular clusters and stars. The reflector did much better on M13. Just sharper all around. I suppose this could be expected because of the CA effect. I tried no filter and 610 and 695 long pass filters and it still wasn't great. So my opinion is it's not what I would use for star clusters. Because the scope is quite a bit heavier, I can't recommend it over the 6 inch F4 reflector.

 

Again, not ruling out the difference in optics could be a consideration here. 

 

The good news is I had my 103s with a binoviewer on the Moon and it was glorious with 24mm Brandons. A true clear night and I stayed up way too late.



#2 markb

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Posted 01 June 2020 - 11:05 AM

Try a Baader 495 Longpass. The super sharp cutoff does wonders for a fast achromat. Night and day different than the Wratten numbers, which have a much shallower curve. 

 

I would expect the difference to be even greater with a monochrome amplifier, especially if it is very blue sensitive.

 

I have 5 and 6 inch f5 Jaegers and the 495LP really upped my happiness level, and that was just visual use. The Wratten didn't come close.

 

And they are great with binoviewers.

 

Also, check the focuser is squared on to the objective. A laser should hit dead center in the lens. My Jaegers showed a significant improvement, and a f7 Carton 60mm had a night and day difference. I've read it doesn't much matter, but I found it really does.

 

My scopes have 3 screw focuser attachment. Not sure if you can make a change with a threaded on focuser (I have put a shim on one edge of the tube to get a small tilt from thread clearance on threaded on cells and focusers, if that helps), and I have no idea which you have.



#3 GOLGO13

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Posted 01 June 2020 - 11:07 AM

Try a Baader 495 Longpass. The super sharp cutoff does wonders for a fast achromat. Night and day different than the Wratten numbers, which have a much shallower curve. 

 

I would expect the difference to be even greater with a monochrome amplifier, especially if it is very blue sensitive.

 

I have 5 and 6 inch f5 Jaegers and the 495LP really upped my happiness level, and that was just visual use. The Wratten didn't come close.

 

And they are great with binoviewers.

 

Also, check the focuser is squared on to the objective. A laser should hit dead center in the lens. My Jaegers showed a significant improvement, and a f7 Carton 60mm had a night and day difference. I've read it doesn't much matter, but I found it really does.

 

My scopes have 3 screw focuser attachment. Not sure if you can make a change with a threaded on focuser (I have put a shim on one edge of the tube to get a small tilt from thread clearance on threaded on cells and focusers, if that helps), and I have no idea which you have.

I do have a Baader semi-apo filter I could try...is that similar to the 495?



#4 markb

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Posted 01 June 2020 - 02:46 PM

Yes, similar, but can be noticably different in effect since the cutoff curve is shallower. I have the semiAPO, M&S (the only visual filter that was satisfying with bad light pollution, less so since leds have hit the scene without municipalities allowing for three bad side effects), Fringe Killer and Contrast Booster. I did the Wratten filters too. The 495LP is so affordable Wrattens make no sense.

 

The 495LP exceeds them all, and is my go-to, except my f5 with some red error and less violet, for that one I use, IIRC, the Semi-APO.

 

The SemiAPO should give you a fair taste of the possible improvement, absolutely try it! The 495LP is a surprisingly large improvement so do the upgrade it the SemiAPO helps.

 

The 495LP is very affordable, particularly for a 2" bandpass, even more so for 1.25, I assume your sensor is sized so the 1.25 will cover. AgenaAstro is the US dealer. I usually buy direct but they have an Amazon store too.



#5 Wildetelescope

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Posted 01 June 2020 - 09:40 PM

I finally had a chance to actually test the 6 inch F5 refractor with night vision. I compared it to my 6 inch F4 reflector. My bottom line up front is the 6 inch F4 reflector seems to be better in general (especially on axis). 

 

The refractor itself is difficult to say on it's optics. It seems to be collimated just fine. But I don't think personally it's the best optics. Still, at low powers on the moon and such it's very sharp. It may just be that high powers brings out the CA. The 6 inch F4 as far as I can tell has excellent optics.

 

Comparing the views between the two on nebulas with night vision it's very close. Not sure there is a clear winner. On one had the 6 inch F4 without a coma corrector has a bit of issues off axis. But I can usually ignore those.

 

The big difference was on globular clusters and stars. The reflector did much better on M13. Just sharper all around. I suppose this could be expected because of the CA effect. I tried no filter and 610 and 695 long pass filters and it still wasn't great. So my opinion is it's not what I would use for star clusters. Because the scope is quite a bit heavier, I can't recommend it over the 6 inch F4 reflector.

 

Again, not ruling out the difference in optics could be a consideration here. 

 

The good news is I had my 103s with a binoviewer on the Moon and it was glorious with 24mm Brandons. A true clear night and I stayed up way too late.

Out of curiosity, if the optics of a typical achromat is corrected primarily for the green and the very near red, It would seem to me that the farther down in the near IR you go, the worse things Will get, no?  I would question whether even a Tak or an AP would pull in everything out to 900 nm. I am curious as to what the Strehl number would be for any refractor beyond 700 nm.     I suspect that the BEST viewing of objects like Clusters is still in the visible spectrum, in a dark sight.   Just thinking out loud.

JMD



#6 Eddgie

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Posted 02 June 2020 - 10:54 AM

 

 

The big difference was on globular clusters and stars. The reflector did much better on M13. Just sharper all around. I suppose this could be expected because of the CA effect. I tried no filter and 610 and 695 long pass filters and it still wasn't great. So my opinion is it's not what I would use for star clusters. Because the scope is quite a bit heavier, I can't recommend it over the 6 inch F4 reflector.

 

Again, not ruling out the difference in optics could be a consideration here. 

 

The good news is I had my 103s with a binoviewer on the Moon and it was glorious with 24mm Brandons. A true clear night and I stayed up way too late.

This is not anything to do with the optical quality. This is just the affects of CA and spherochromatism.  These don't matter much for nebula, but for stars, they are huge energy robbers. 

 

An Apo by comparison will produce very fine and sharp stars even without a filter.  In fact, I am shocked at how close my 106mm f/6.5 FPL-triplet comes to my 6" reflector on stars.  Very close on limiting magnitude.

 

I  found it impossible to use larger or faster acrhomats for stellar work vs my 6" reflector.  Smaller Apos are hugely better on stars and can be run without filter in even suburban skies. 

 

In a 4" f/10 achromat, spherochromatism is the red line is far less of an issue than the defocus in red, and even here, because the human eye is not so sensitive to red, the fact that the red Airy Disk is twice the size of the green Airy Disk goes unnoticed.  

 

At 6" and f/5, though the spherical aberration in red is pretty severe.

 

From Telescope-Optics.net;

 

 

The primary spherochromatism wavefront error changes in proportion to the fourth power of the aperture, and in inverse proportion to the third power of the system focal length.

Now what that means is that while a 4" f/10 achromat is just barely diffraction limited in red, a 6" f/5 will be very poor in red.   A lot of the energy that the reflector is easily putting into the Airy Disk is being spread around into the rings on the achromat, lowering peak intensity of the Airy Disk and lowering contrast of the star against the background. 


Edited by Eddgie, 02 June 2020 - 11:10 AM.

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#7 GOLGO13

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Posted 02 June 2020 - 04:17 PM

Thanks Eddgie. I am pondering the best use for the scope. It is cool, but I am not sure it's a good fit for night vision. I will keep trying a bit.


For visual with my 34mm 68, the exit pupil is a bit much. So probably the best eyepiece combo I have is the ES 16mm 68 or 24mm pan. My 4 inch apo with the 34mm 68 is very good though. So I am unsure if the aperture is helpful in this case. The 10 inch dob is still my best NV scope.

#8 GOLGO13

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Posted 12 June 2020 - 11:59 PM

OK...finally had a really nice night to test out the 6 inch F5 refractor and compare it to the 6 inch F4 newt. SQM was 19.40 which is the best I've had since getting the tool that measures it. Tonight I was able to use a new strategy to improve the refractor view. I used my semi-apo filter for stars and globulars or anything I would use unfiltered. That actually did seem to work. Maybe cutting the light a bit but tightened up the stars a good amount. I would suggest trying something like that if you are using a fast achromat or even a slow one.

 

My personal opinion between the two scopes on nebulas (both with the 6nm HA filter) is it's pretty darn close. Close enough that I think the 6 inch F4 newt is probably worth using instead since it's much lighter and can be used on my Vixen Porta mount. The 6 inch F5 actually stressed my porta mount out so much I had to repair it on both axis's. I re-greased and fabricated a washer and it seems fine again. 

 

Anyways I looked at the Lagoon, Trifed, Swan, and Eagle and it truly was close. I will admit I didn't check the collimation in the 6 inch F4. I think the 6 inch F4 has better optics in general. But the views between the two were very close. Maybe a bit of a different view, but similar. My personal guess is the optics being better in the 6 inch F4 makes up for the central obstruction. 

 

However, one negative about the 6 inch F4 newt is when using it for afocal. It ends up being a gigantic stack because it really is meant for something that needs some back focus. Does it work, yes, but it's a bit awkward. I use it with Afocal to get the most reduction possible. I put a .7 reducer on the 2 inch to 1.25 inch adapter. Then use my 40mm plossl in afocal. This allows me to get good views of the North American Nebula and everything else in that region. Of course, I could always use my guide scope to get a wider view. 

 

So I guess I'm personally a bit disappointed in the 6 inch F5. Not that it doesn't work well for night vision (with some strategies). Just that it doesn't work better (or enough better) than the 6 inch F4 newt. 

 

If you guys don't have some sort of SCT, I highly suggest picking up a cheap C8 or something. The combination of a C8 and the 6 inch F4 newt would be pretty good. Actually, my 10 inch dob does the best, but the 6 inch F4 newt is more convenient. But the C8 is really good on globulars and small planetary nebulas. I'm sure a C6 would work well also.



#9 The Ardent

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Posted 13 June 2020 - 12:35 AM

Having used both a 6" f/4 newt and 6" f/5 refractor for NV, my result was the newt was near-perfect portable widefield scope for NV. The refactor was only usable for h-a, just like those solar viewing singlet refractors. 

 

For visual, The 6" refractor is an excellent Widefield low-power DSO seeing machine. The pic may help illustrate why it's not for NV. The CA here is a combination of visual and near IR wavelengths exaggerated by the camera's sensitivity. 

 

For visual use the CA is imperceptible under 40x 

With NV it's worse than the pic, as the IR is spread out more.

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#10 bobhen

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Posted 13 June 2020 - 07:53 AM

I use a 102 F5 achromatic refractor with NV and the CA is not much of an issue because I only use it at 20-power or less and I only use it with filters. It can be reduced to F3.5 as well.

 

The NV, very low power, wide views are pretty great.

 

There are fast, photographic-oriented Newtonians and of course fast apos but they are a lot more costly and would they add much if “only used” for very low power, wide field NV observing?  I would think that the distortion introduced by the intensifier tube and the strong filters used for NV would impact or negate the high optical quality of the apo, especially when used at very low power. Of course if I already had an AP traveler I would use it and be very happy.

 

I did add a C-8 and it is really great for the small stuff.

 

When used together, the C8 and fast refractor make a nice NV combination.

 

Bob


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#11 GOLGO13

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Posted 13 June 2020 - 11:11 AM

I know this is an EAA thread, but I know you guys have some experience with refractors. I decided to look at the lens to see if there was anything amiss. Looking visually at it with the dew shield off I could see some marks which are meant for lining up the two lenses. They were not in line by about 1/8th an inch. So just a little off but certainly off. I carefully took off the retaining ring, a spacer and a rubber ring. I then moved the lens carefully to line up with the marks. Put everything back together and took it outside. Everything looked nice and sharp for a during the day test. I don't think I'll have another good night for awhile to star test it.

 

So, my question is, would the lens being slightly off of it's marks make a difference on optics and the views?

 

The dude that shipped this to me put just about no padding in it and I could tell it had quite a rough ride over here. 

 

I suppose I shouldn't be surprised by poor shipping, but it's always crazy to get a 16 pound giant refractor basically placed in a box with very little packing. I was impressed the glass didn't break.

 

The star test for this scope is a bit strange, but I don't have much experience star testing achromats. Especially really fast ones. One side of focus looks pretty nice. The other side has the outer half of the image dark and the inner half bright. Everything is circular as far as I can tell. When looking at the Moon all looked OK. 

 

After adjustment I used it on a far away building with a 25mm Ortho (looked very nice with a little to be expected CA). with a 12mm ortho it still looked sharp with some obvious CA but still not too bad. With a 2x barlow and the 12mm still basically sharp and more CA. So it seems like it's pretty good. I think the next star test will be interesting to see if anything has changed by this slight adjustment of the lens.



#12 markb

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Posted 28 June 2020 - 02:54 PM

Having the alignment marks off slightly should not make a difference, or, at most, a very small difference. It's my understanding that the marks were more important in earlier days as a way of tuning out wedge and astigmatism, but that is second or third-hand information and should be treated as such.

 

it probably did not happen during shipping, but may have resulted from someone messing around with the lenses or cleaning them in the past.

 

in my experience, people who ship refractors another optics incorrectly are generally not the most reliable people to have dealt with, and may have abused or been careless with those scopes while in their possession. Harsh, but there it is. And I've been fortunate enough to be able to research and correct some of these messes that came into my possession. Almost always undisclosed.

 

Which raises a possibility, in the event there is an actual problem.

 

You sound unhappy, so I'll take a flyer with one suggestion, since it's happened to me twice now.

 

I've had two refractors with a reversed element, and one with a completely reversed cell. My memories on the specific if shoes are faint, but I seem to recall an increase in spherical aberration in possibly chromatism. Possibly field curvature as well, but that may have been the reversed cell, not the reversed single element.

 

If the edge marks were not arrows, but simple lines as is most common, an element, likely the front, could have been flipped. The rear is usually a convex concave Flint, I assume your lens is a contact or air spaced fraunhofer achromat.

 

I researched optical prescriptions back then to resolve what I thought was a flipped element. My recollection, and only a recollection at this time, is that fraunhofer achromats have identical or almost identical curves on the two innermost surfaces. Assembled, with thin spacers or in contact, Newton's rings should be visible, more so with monochromatic light such as a green filter over a fluorescent bulb or a diffused laser from a collimator. I used a green laser pointer. But the 'lenses kind of stuck together by suction' test works just as well for identical curves.

 

I first estimated the curve by measuring the height of the edge when the lens was balanced face down on a pec pec on a table IIRC. The Baker achromat had a very large spacer, but again, the curves were nearly identical so I simply put them in contact with one another and the different curvatures of the two sides of the front element were immediately obvious. 

 

The correct, matching, curves matched so closely the lack of air space made them almost want to stay together.

 

But, of course, a warning.

 

It may be nothing wrong with your lens, or nothing wrong, at least, with its assembly.

 

I've hit enough of them, that I wonder how many SCTs with misrotated alignments, and refractors with flipped elements, are out there, almost certainly the result of cleaning that probably wasn't necessary, and was done without care to maintain the original alignments and orientations of the elements. I guess I'll have to mention overtightened retainers damaging flints on older lenses, or simply causing astigmatism by mechanical stress.

 

I make sure I mark all possible points of alignment, as well as sky and eyepiece side, if I'm forced to clean something. but usually, I grit my teeth and tell myself 'don't even think about cleaning it'.



#13 GOLGO13

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Posted 02 July 2020 - 04:13 PM

Wow...we have our own forum now!!

 

So I posted over in the ATM forum and found out I had pretty bad under-correction.

 

I've been working on the spacing and I think I have it pretty close now. Just have to star test it tonight. The good news is I should be able to adjust it. Not sure why the spacing was so far off...maybe someone else messed with it before, but the tabs that were on there looked original.

 

I've learned quite a bit however through the experience. Hoping to not have to do this re-spacing ever again though!

 

And of course I'm glad it's a cheaper scope and not one of my nice APOs.



#14 GOLGO13

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Posted 02 July 2020 - 04:22 PM

Based on this development on the lens spacing on the 6 inch F5, I'd say the above analysis is a bit null and void. Maybe I can do another one once I get the 6 inch F5 as perfect as I can get it.

 

Though I would say a 6 inch F4 imaging newt would be an easier going solution. Easier to mount, but you do have to mess with collimation. My 6 inch F4 seems to hold collimation quite well though despite the super fast focal ratio.



#15 GOLGO13

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Posted 02 July 2020 - 10:08 PM

Good news is I fixed the 6 inch F5 scope. I actually went a bit too far on the spacing this time, but after taking off two pieces of the spacer tape it's good to go. The Ronchi eyepiece I purchased awhile back (and Suiter's book at the same time) finally has paid off. It let me know I was under-corrected and now the lines are strait after the adjustment. The star test looks pretty good. I don't think this scope has as good of optics as my other scopes, but it was putting up a really nice Moon image now even with an 8mm (with some CA of course). But my Vixen scopes are really good (seriously) so I think this is probably just fine for what it is.

 

Once the fair weather clouds clear out I'll try some NV, though with the Moon so bright it may not be the best night for judging things.

 

This should allow using a bit more power with NV as the images with a barlow were soft last time.




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