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Globular clusters in a fast six inch achromat

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#1 gamma_ari

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Posted 30 May 2020 - 11:56 AM

I’m thinking about getting a large, fast achromat for deep sky, e.g. Skywatcher Startravel 150 mm f/5. I’m fully aware that this is not a planetary instrument. In the deep sky department, globular clusters also require large magnifications of up to 200x. Will they look good in a fast achro or will they be all mushy and blurry at 200x? And what about double stars in the range 6 to 10 mag, will they look sharp at high powers? Happy to hear your first-hand experiences.

 

I should add that I already own a C8. Advantages of a large achromat would be fast cool-down, stable collimation, obstruction-free optics and wider fields.

 

Best,

Viktor


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#2 jag767

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Posted 30 May 2020 - 12:32 PM

You have no chance of using that kind of magnification. Think more like 100x
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#3 daquad

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Posted 30 May 2020 - 12:48 PM

I have the Altair Starwave 6" f/5.9.  I've used it at 191X on M13 with great success.  The stars are not mushy, but in fact are pinpoints even at that magnification.  I can easily split Pi Aquirii (sep 1.4") and 52 Orionis (sep 1.1"). 

 

The scope even gives excellent views of Luna at 191X.  Lunar shadows are not perfectly black, but there is plenty of detail in the image.  I have detected 3 craterlets on the floor of Plato.  Even Jupiter puts on a good show and I have easily seen Ganymede transiting the planet.

 

The scope really shines at low power, wide field views.   I get 3 degrees at 28X with my UO 32mm MK 80 Koenig.  Would n APO do better?  Of course, but at 3-6 times the cost, depending on design and make.

 

Dom Q.

 

DSCN2493.JPG

 

 


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#4 astrohamp

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Posted 30 May 2020 - 12:53 PM

Why not consider a doublet (ED if funds permit) and gain some image quality.  I am a bit wanting for more from my 152 now that my aging eyes see things on a monitor rather than through an eyepiece.

FOV of camera is about the same as my 12mm Naglar T4 (~82x).  The 'dash board' image is from several nights ago and has only been (4k) screen saved then Gimp scaled.  There is an Astronomik UV-IR-Block L-3 filter in the optical path to effectively reduce considerable star bloat.

 I am still on the way steep learning curve for EAA, CMOS cameras, guiding, and off mount observing.  Although I have several decades of push-to DSC/planetarium use as a visual observer.  Some years with current achromat.

 

 

M10g21s128L3c5DisplayScaledc.jpg


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#5 markb

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Posted 30 May 2020 - 01:29 PM

The quality of the lens you get will be the determining factor and what you can do with it and how far you can push it.

 

High powers are definitely not a problem for my classic 6 and 5 inch Jaeger's f5 refractors, other than a small exit pupil and the limited eye relief that usually comes with short fl eyepieces. And CA of course.

 

Your biggest obstacle is likely to be chromatic aberration. The best solution I found is the Baader LP495 long pass filter. It gives some color, but the sharp cutoff wipes almost all of the Violet more effectively than any other filter I've used. The only exception is one of my Jaegers which has has some red that cleans up better with a Semi APO filter IIRC.

 

The CA is easiest to see slightly out-of-focus, one side has an obvious violet smearing. It also shows up very easily with daytime testing on distant tree leaves and branches and even power lines. It doesn't really bother me as such but it does definitely reduce contrast and sharpness.

 

Skywatchers can be very good, particularly in these days of computerized manufacturing. There is no getting away from the CA in achromatic designs, however.

 

If you can't set up a DPAC, research extra focal star tests (which I find difficult to interpret at times, in part due to a fair amount of unclear information on the web), but if inside and outside focus patterns are a match that's a good sign. And in good seeing, the final test is always on a star in focus.

 

Oddly enough, I've always found a daytime snap to focus test as pretty telling. The best Scopes do not require any hunting, and the point of focus is very definite and jumps out at you.

 

I'd pick up a good quality Ronchi tester. The Gerd Neumann looks like the best quality you can buy at the moment, but you may have to look around to get one. Just search the name with Ronchi or tester added. I believe his website in Germany has a reasonable amount of interpretation information IIRC. You can make one, but the last time I checked getting a decent ronchi screen was a little difficult.

 

Optcorp usually stocks them in the US. I did not check our host, astronomics, for what they sell along this line, and it is absolutely the first stop to make.

 

 A Ronchi is relatively easy to do a basic interpretation on a refractor since you are looking for straight lines of even width, top to bottom. The DPAC doubles the obviousness of any error, but a bare Ronchi still will give you a quick and reasonable test of the objective, enough to decide whether to send it back and get another one.

 

Swapping OTAs is no one's favorite thing to do, but with a reputable brand you should have a reasonably the chance of getting a decent lens if your testing shows one to be unacceptable.

 

this is one of the weaknesses of the used market, as many people dump their poor-performing Scopes without warning anyone. I've had two reversed element refractors, and two allegedly excellent performers that were merely very good. I would have been fine, but it was not what I was looking for at the time. A Ronchi test confirmed my visual impression.

 

It's almost impossible to get a single scope to do anything you want, but a very sharp short tube can get more done than many people might think. 

 

Add to that problems that come with a long tube 6 inch refractor, and a short tube looks better all the time. My f8 was a beast to transport, even within the house, that was a pain to get mounted stably due to the length and torque arm that comes with a heavy cell.


Edited by markb, 30 May 2020 - 01:31 PM.

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#6 RLK1

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Posted 30 May 2020 - 01:39 PM

I've got an Antares 6" F5.6 and I can go up around 200x power on globulars without issues although I do use a filter with planets. In fact, I'm not happy with it unless I can crank up the power...


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#7 stevew

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Posted 30 May 2020 - 02:14 PM

I've got an Antares 6" F5.6 and I can go up around 200x power on globulars without issues although I do use a filter with planets. In fact, I'm not happy with it unless I can crank up the power...

I'm pretty sure the Antares is F-6.5.

I have one, and it's a fun scope to use..



#8 RLK1

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Posted 30 May 2020 - 02:57 PM

I'm pretty sure the Antares is F-6.5.

I have one, and it's a fun scope to use..

Yup, my bad, it's F6.5 and, yup, it's a fun scope to use!


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#9 Echolight

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Posted 30 May 2020 - 03:44 PM

Globular clusters was one of the targets I had in mind with the C6R.

Hopefully some will be visible from the outer edge of the urban light dome.

I will find out tonight. Have a Baader zoom and an Ultima SV 2x barlow. So 300x max for now.


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#10 SeattleScott

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Posted 30 May 2020 - 05:25 PM

I found 100x the limit before CA started impacting the view with my 4” F6.5. DSO still looked good but side by side a 4”
Apo was a little sharper over 100x. With my 6” F8 I have found lunar details are still sharp at 150x but start breaking down at 240x. Again, not bad, but not as crisp. So I tend to stay with 150x max even on DSO. I have a 7mm now for 170x so I should try that on M13 this summer.

Of course this is a 6” F8 not a 6” F5. And yes someone chimed in about using 190x with a 6” F6 but that is a $1,000 refractor with better glass and figure than an entry level 6” F5. So I would tend to agree that 100x is about your max. When you start talking F5 achros, you are really talking about specialized low power sweeper scopes. Not that you can’t use 200x but you will start losing some sharpness.

Scott
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#11 russell23

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Posted 30 May 2020 - 08:15 PM

I’m thinking about getting a large, fast achromat for deep sky, e.g. Skywatcher Startravel 150 mm f/5. I’m fully aware that this is not a planetary instrument. In the deep sky department, globular clusters also require large magnifications of up to 200x. Will they look good in a fast achro or will they be all mushy and blurry at 200x? And what about double stars in the range 6 to 10 mag, will they look sharp at high powers? Happy to hear your first-hand experiences.

 

I should add that I already own a C8. Advantages of a large achromat would be fast cool-down, stable collimation, obstruction-free optics and wider fields.

 

Best,

Viktor

You've had a lot of good feedback already.  One thing I would add is that if you use the Baader 495 Longpass yellow filter it will allow you to get sharper views of globular star clusters and achieve higher magnifications. 



#12 SeattleScott

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Posted 30 May 2020 - 09:23 PM

Of course the downside is it turns everything yellow. Which bothers some more than others.

Scott

#13 The Ardent

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Posted 30 May 2020 - 09:34 PM

I would not use a 6” f/5 for magnifications over 50x.

Yes it possible but not recommended.
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#14 russell23

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Posted 30 May 2020 - 10:15 PM

Of course the downside is it turns everything yellow. Which bothers some more than others.

Scott

But when you are talking about globular clusters this is not going to be real easy to notice.  At the ages of these clusters the blue stars are long gone.

 

In general, when I was using the Baader 495 LP with the Vixen 140NA it really was hard to notice the yellow color in stars fainter than ~4th magnitude.   And the brighter stars took on a nice golden yellow color like Capella.

 

For the Moon and planets - sure the color is not the greatest.  But for deep sky it is not really noticed and the significant improvement in sharpness of stars with magnification certainly makes it worth it. 

 

Nobody ever complains about the blue/green shift that comes with a UHC filter.   The 495 LP is similar in the sense that like the UHC for nebula, the 495 LP allows you to see more details in globular star clusters with a large fast achromat than without.
 


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#15 Rudix

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Posted 30 May 2020 - 11:56 PM

I have recently acquired a Skywatcher Startravel 150/750 f/5. I have not really had the time to do any serious work with it but I did do some tests on it to see if it would work for my intended purpose of deep sky object photography. 

Tests on stars, bright and faint does not show noticeable CA (61mp Full frame sensor and 32.5MP crop sensor). Some moon shots does show a marginal amount (especially on over exposed shots but why over expose anyway?) but I treat it like I would treat CA on any of my camera lenses.I take the RAW (or TIFF) file into Adobe Light Room, the fix there takes a few seconds with a slider so for me it is of no concern! 

 

I am really looking forward to capturing some great images using the scope under my Bortle 2 sky at home!


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#16 Jon Isaacs

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Posted 31 May 2020 - 12:12 AM

I’m thinking about getting a large, fast achromat for deep sky, e.g. Skywatcher Startravel 150 mm f/5. I’m fully aware that this is not a planetary instrument. In the deep sky department, globular clusters also require large magnifications of up to 200x. Will they look good in a fast achro or will they be all mushy and blurry at 200x? And what about double stars in the range 6 to 10 mag, will they look sharp at high powers? Happy to hear your first-hand experiences.

 

I should add that I already own a C8. Advantages of a large achromat would be fast cool-down, stable collimation, obstruction-free optics and wider fields.

 

Best,

Viktor

 

For resolving globulars, a wide field is not necessary, the collimation and obstruction are not important issues, I think the CA is a small issue, the 6 inch achro and 8 inch SCT will be very similar.

 

Are you looking for better views of globulars than the C-8 provides?

 

Jon


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#17 Diomedes

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Posted 31 May 2020 - 12:24 AM

I would not use a 6” f/5 for magnifications over 50x.

Yes it possible but not recommended.

Why not?



#18 Jon Isaacs

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Posted 31 May 2020 - 01:28 AM

Why not?

 

Chromatic aberration would be my guess.

 

The chromatic blur of a 150 mm F/5 achromat is about 11 times the diameter of the Airy disk, the resolved star. 

 

Jon



#19 RLK1

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Posted 31 May 2020 - 02:04 AM

Yup, my bad, it's F6.5 and, yup, it's a fun scope to use!

Actually, I think I had it right the first time, I just took a quick measurement of mine from objective to diagonal and it's about 34".  



#20 The Ardent

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Posted 31 May 2020 - 02:25 AM

Because I have a 6" F/5 refractor. I use its strengths, not its weaknesses. 

 

 

Why not?



#21 jag767

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Posted 31 May 2020 - 04:45 AM

I would not use a 6” f/5 for magnifications over 50x.

Yes it possible but not recommended.


🤔 that's a tad extreme. My f5.9 (which allegedly has CA in the ballpark of a standard f8) looks quite good at 100x. I had it out this morning and because they were there swung in for a view of Jupiter and Saturn. Sure there's CA, but theres also a ton of detail. I would say the view of Jupiter up to 100x is far more detailed than in my 4"f11ED. After that it slowly swings in favor of the 4" (particularly Jupiter), but I'm not going to change instruments for one or two targets. Same is accurate for pretty much any other target. Personally, I don't really bother with anything over a 1mm exit pupil on a manual mount, so for the most part the 6" f5.9 suits my viewing habits quite fine. And fwiw, filtering can make a huge difference, potentially allowing for higher magnifications.
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#22 gamma_ari

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Posted 31 May 2020 - 12:21 PM

For resolving globulars, a wide field is not necessary, the collimation and obstruction are not important issues, I think the CA is a small issue, the 6 inch achro and 8 inch SCT will be very similar.

 

Are you looking for better views of globulars than the C-8 provides?

 

Jon

My C8 never reaches thermal equilibrium, so stars are never pinpoints. I’m not sure by how much contrast suffers in other deep sky objects such as galaxies. Would be interesting to compare with an achro.



#23 JuergenB

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Posted 31 May 2020 - 12:51 PM

In the case of galaxies or nebulae, the sky quality, i.e. transparency and  background brightness, will have more influence on contrast than the type of telescope, therefore, the C8 should be quite fine for the task. For globulars, it should be on par with a 6" achromat or even a tad better.

 

I agree with some comments here that an ED doublet will show significantly better performance at higher magnifications compared to a fast achromat. The APM 152 mm ED comes into my mind here. Not too many of them are still available because production has stopped meanwhile.

 

Juergen 


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#24 GOLGO13

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Posted 31 May 2020 - 12:59 PM

C8's are quite good for Globulars. I'm not sure yet on the 6 inch F5 as I have not had a very good night yet. However, I don't expect it to do too well at high powers.

 

I'd choose something a bit different.

 

One thing the C8 does very well is Globulars with night vision! Really good because of the 2032 focal length.


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#25 Jon Isaacs

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Posted 02 June 2020 - 03:51 AM

C8's are quite good for Globulars. I'm not sure yet on the 6 inch F5 as I have not had a very good night yet. However, I don't expect it to do too well at high powers.

 

I'd choose something a bit different.

 

One thing the C8 does very well is Globulars with night vision! Really good because of the 2032 focal length.

 

That seems odd to me. The night vision folks always seem to going for fast scopes.

 

Jon


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