24 replies to this topic

[Grimstod]

If you do mono imaging with a filter wheel you do not need an expensive APO scope. Because you can focus for each color band and capture them separately. So if you have been debating with yourself the cost of all those filters you might be able to actually save money and get better images by using an Achromat or just a doublet rather then forking up all the extra cash for an expensive APO. 

 

 

 

 

[zakry3323]

I agree- "need" is a strong word sometimes. I've gotten some images that I've been pretty happy with from my ED doublet- an SW-ED100 with a reducer/flattener. However, I do still get some noticeable bloat on stars in the "B" and "Oiii" channels when I'm not using an OTA with better correction. Perhaps this could be mitigated with broadband filters that have a little sharper cutoff.  

[imtl]

This needs to be better explained. It is NOT just mono with Achromats. It is NB imaging with mono and Achromats. Broadband imaging will have much bigger effect with Achros compared to Apos. I believe another issue is that it is quite difficult (yet not impossible I guess) to find a dedicated or even a generic flattener that will match well with an Achro.

The last point regarding optical performance is that Achros (and some cheap apos) are badly corrected in the blue range. Which will affect OIII as well.

 

They usually come with mediocre to poor focusers so that is something to consider with heavy imaging trains.

 

But all in all yes definitely you can do mono+NB imaging with an Achro.

[jonnybravo0311]

A well-corrected doublet is a good choice. However, achromats typically aren't even if you're imaging monochrome. The poor optics have trouble even keeping a single color focused on the same plane. Usually blue suffers the most. You can try mitigating with tighter cutoffs in your filters. Chroma L and B cut at 420nm. Astronomik L-3 and B cut about the same point. Most other filter sets cut at 400nm or lower.

 

For your purposes with narrowband imaging, then sure, an achromat can work just fine since you're dealing with exceptionally tight bandpasses that even the poor optics of the scope can keep on the critical focus plane .

[Grimstod]

There was a thread about his last year but I cannot for the life of me find it.

Anyway with a mono camera you can do some pretty good imaging with an achromat. Just find a flattener/reducer that is a good fit and they make for some very good and sometimes fast imaging scopes.

 

Here are two combos that I found work.

Skywatcher ST120mm Achromat f5 version. Use the Skywatcher 80ED Evostar 0.85x focal reducer with it and its a perfect match.

 

Another combo that I found works is a Orion 100mm Achromat. No long in production. Use the same Evostar 80ED 0.85x focal reducer.

 

I use ASTAP for stacking so it can scale the image if there is a difference in size though that difference is usually less than 1mm in FL. I think just about every stacking solution out here can scale images during stacking except maybe DSS which is ancient anyways.

I made a video of my findings here in the first post. 

 

One thing I recommend if you want to try this is to get narrow band pass filters. I started with 7 and 8.5nm bandpass filters and have switched to 3.5 and 4nm band pass filters. They improved star size a good bid. Almost cutting their sizes in half.

And some photos. Because photos do not Lie.

Edited by Grimstod, 20 January 2022 - 11:19 AM.

[Grimstod]

There will be details on the capture of each image in their name. You know, how many subs. etc.

[Grimstod]

[Grimstod]

[imtl]

Thanks for posting the images. The chromatic aberrations are definitely there. Your processing is quite good!

Just shows that you can do a lot of stuff even with mediocre equipment. Each person will find their path and reach their goals in their way. This is not going to cut it with experienced imagers but I think if a beginner already has a scope like this then might as well try. The thing is that 70-80mm mass produced apos today are so relatively cheap then might as well go for much better quality.

 

Anyways, it's always good to have options. 

[Grimstod]

Thanks for posting the images. The chromatic aberrations are definitely there. Your processing is quite good!

Just shows that you can do a lot of stuff even with mediocre equipment. Each person will find their path and reach their goals in their way. This is not going to cut it with experienced imagers but I think if a beginner already has a scope like this then might as well try. The thing is that 70-80mm mass produced apos today are so relatively cheap then might as well go for much better quality.

 

Anyways, it's always good to have options. 

Although I am pretty happy with this scope and will probably keep it for a few years I do indeed want to move to an apo. Mainly for those really bright stars and perhaps for some more speed. Say f3.9 ASKAR   Though I think switching to a better camera that does not have the micro lensing issues would actually benefit me more right now. 

[imtl]

Although I am pretty happy with this scope and will probably keep it for a few years I do indeed want to move to an apo. Mainly for those really bright stars and perhaps for some more speed. Say f3.9 ASKAR   Though I think switching to a better camera that does not have the micro lensing issues would actually benefit me more right now. 

Since, like you wrote, they are so cheap, I would just keep it and get an apo. This can always be used as a visual instrument. Good luck with the new camera as well!

[terry59]

Something I didn't see mentioned is spot size. A doublet will have smaller stars than achro, triplet smaller than either

[Wildetelescope]

Something I didn't see mentioned is spot size. A doublet will have smaller stars than achro, triplet smaller than either

Is this true for narrow band imaging? 

 

JMD

[terry59]

Is this true for narrow band imaging? 

 

JMD

My 80ED produced spot sizes decently larger than my SV70T in both NB and WB

[matt_astro_tx]

Something I didn't see mentioned is spot size. A doublet will have smaller stars than achro, triplet smaller than either

Huh.  I. Did. Not. Know. That.

[Grimstod]

Something I didn't see mentioned is spot size. A doublet will have smaller stars than achro, triplet smaller than either

Achronic scopes are doublets. 

[bobzeq25]

Mono with narrowband can work decently well with less complex lens designs.  But there are aberrations other than chromatic and cheaper scopes will have more of those.

 

A problem with broadband is that much of the value of mono is in the luminance, and chromatic aberrations will hurt stars in L.

 

Bottom line.  Inexpensive stuff can make decent images.  More expensive stuff works better.  Whether the difference is "worth it" is a personal decision.  But there is no magic cheap scope that works better than an expensive scope.

 

It's easy to come up with a creative idea about how to do this more cheaply.  They can work some.  But it's extremely hard to make it actually work better.

 

When I bought my Lunt solar scope (the ultimate in narrowband) they sold singlets and doublets.  Said the doublets just worked better.  I trusted their expertise.

Edited by bobzeq25, 20 January 2022 - 07:43 PM.

[freestar8n]

There are different types of chromatic aberrations and only longitudinal can be fixed by a focus change for each filter.  The blue halo effect is likely spherochromatism - which cannot be completely removed by focus.  So - combining multiple filters may have problems with different size stars.

 

So the idea may not work perfectly but it may work pretty well.

 

Frank

[Phishin_phool]

I have been fortunate I own to "apo" doublets William Optics Z71 SE  and William optics Z103. Both have very little CA (but it is there if you look very carefully using OSC) I also wouldn't call them cheap or inexpensive scopes.   WIth mono and narrowband however I am hard pressed to see any although a seasoned expert with a keener may disagree. MOst of the time though I am using my RASA to fit an imaging session in the alloted time f2 is so much faster than f/4.9 and f/5.9 evan after a .80 flattener/reducer.

 

Also I like the optics on the z103 a little better but the rack and pinion focuser has more backlash than the crayford focuser on the z71.

[unimatrix0]

You could also get a 6" or 8" Newtonian, that beats all the refractors in light gathering,  and no more issues with star sizes and channels out of focus.  Also, no dew heater required. 

More on the Svbony, it has a dual speed focuser, but whatever you do, don't try to adjust the grub screws. Just the bottom ones, not the ones on the side. 
They center the focuser axle, and 2 of them centers the bearings  and once its out of alignment it's a pain and lot's of time to get it back where it should be. 

 

Also, if you take it apart completely, make sure you take a day off from work to put it back together.  Otherwise nice lens, they are essentially the same as the Astrotech ED scopes rebranded. (FPL-51) 

Edited by unimatrix0, 20 January 2022 - 11:22 PM.

[unimatrix0]

Huh.  I. Did. Not. Know. That.

Well, when you see that the triplet APO owners don't even tolerate HFR 3.0, while anyone with doublet is happy with HFR 3.5.  It is still the focusing and bringing those colors into 1 point.  I mean, they better provide better stars, otherwise they would be a big waste of money. 

[Grimstod]

Well, when you see that the triplet APO owners don't even tolerate HFR 3.0, while anyone with doublet is happy with HFR 3.5.  It is still the focusing and bringing those colors into 1 point.  I mean, they better provide better stars, otherwise they would be a big waste of money. 

That is interesting. I am getting HFR in the low 2s if conditions are good though it mainly hovered in the low 3s. Any thing over 4 I toss. 

Perhaps some commenters earlier about star size should bear in mind that the 1600mm has considerable micro lensing issues. A different camera would probably yield stars even smaller. Especially bright ones. 

[imtl]

That is interesting. I am getting HFR in the low 2s if conditions are good though it mainly hovered in the low 3s. Any thing over 4 I toss. 

Perhaps some commenters earlier about star size should bear in mind that the 1600mm has considerable micro lensing issues. A different camera would probably yield stars even smaller. Especially bright ones. 

HFR in the low 2s of what? arcsec or pixels? That is actually quite high and bad if it's in arcsec.
Also, you should look at the FWHM to get a better idea of the size. Would you care to upload a single good frame and share? I would like to check it out. Fits file is great.

[Grimstod]

HFR in the low 2s of what? arcsec or pixels? That is actually quite high and bad if it's in arcsec.
Also, you should look at the FWHM to get a better idea of the size. Would you care to upload a single good frame and share? I would like to check it out. Fits file is great.

When I get the External out ill grab one and upload it. 

[imtl]

When I get the External out ill grab one and upload it. 

Thank you!