12 replies to this topic

[Sreesha]

I want to do deepsky imaging.  What difference would it make to image using a APO ED Doublet vs APO ED Triplet?

 

Lets assume I use a flattener and I am able to do good correction on the Doublet.

Lets assume both Double and Triplet are in par when it comes to quality (same good quality in their own cadre).

 

Are there very good Doublet refractors that can be considered for astro imaging without having to go to Triplets?

 

Am looking for 900mm - 1200mm focal length.

 

My gear

Mount = AM5

Cameras = 2600mm

Filters = Optolong 3nm SHO and LRGB 

Edited by Sreesha, 13 April 2025 - 01:06 PM.

[Mike W]

SV503

[Rasfahan]

In general, at the same focal ratio, the doublet will have worse correction than the triplet. So, if you want a well-corrected doublet chose it with longer FL. What aperture/FL were you looking at? Off the top of my head, for 4“ I’ld consider a Takahashi FC-100 (DF/DC) with the 1.04x flattener as well-corrected.

 

Many of the Chinese mislabelled „Petzvals/Quadruplets“ are actually doublets with doublet correctors. Examples include the 61EDPHv1 (barely useable IMHO) and the Redcats 51, 61 and 71. They basically trade color correction for spherical aberration, as a rule.

[Mike W]

Check out "Narrow Band Channel" on you tube website he reviews the SV503 ED doublet for A/P mono with filter wheel. C/A is easiest ab. to fix in post.

Edited by Mike W, 13 April 2025 - 08:31 AM.

[Lookitup]

Best color corrected doublet I have ever looked through is my Altair 101 Planeta at F/11.3. FL is 1132, so a reducer would be needed for FL 900.

[Rasfahan]

Best color corrected doublet I have ever looked through is my Altair 101 Planeta at F/11.3. FL is 1132, so a reducer would be needed for FL 900.

I’m sure this scope is awesome for visual. I left it out because I don’t know a reducer that is specced to correct field curvature well enough for the OP‘s camera. At F/11 I feel it would be somewhat too slow with a flattener only.

[Rockarruda]

I have had nice results with my Astro-Tech AT102EDL doublet refractor.  I am not the only one either who have had some great images if you search on Astrobin on this model of doublet.  I know there quite a few others that are quite capable as well, but at least with mine, I have not felt the need to "upgrade" my scope.

 

I also have IMX571 based camera but OSC.  You can take a look at my Astrobin page in my signature and see what you think.

[Psion]

In general, it is hard to say which telescope is better corrected. You have to remember that every telescope is original, and they do not have the same optical parameters. You may find a doublet better than a triplet, depending on the glass and design used. Doublet has the advantage of faster tempering, and good correction of optical aberrations can be made up for with a well-calculated corrector. 
Shooting DSO is a very general concept, it depends on how big the objects (in arcsec) and with what camera. For example, a 100mm F6 telescope is not good for shooting small galaxies, and also the exposure length for Halpha nebulae will have to be significantly longer compared to a telescope like a 250mm F4.

[Sreesha]

In general, at the same focal ratio, the doublet will have worse correction than the triplet. So, if you want a well-corrected doublet chose it with longer FL. What aperture/FL were you looking at? Off the top of my head, for 4“ I’ld consider a Takahashi FC-100 (DF/DC) with the 1.04x flattener as well-corrected.

 

Many of the Chinese mislabelled „Petzvals/Quadruplets“ are actually doublets with doublet correctors. Examples include the 61EDPHv1 (barely useable IMHO) and the Redcats 51, 61 and 71. They basically trade color correction for spherical aberration, as a rule.

Thank you.  Am looking for 900mm - 1200mm focal length.

[Wildetelescope]

I want to do deepsky imaging.  What difference would it make to image using a APO ED Doublet vs APO ED Triplet?

 

Lets assume I use a flattener and I am able to do good correction on the Doublet.

Lets assume both Double and Triplet are in par when it comes to quality (same good quality in their own cadre).

 

Are there very good Doublet refractors that can be considered for astro imaging without having to go to Triplets?

 

Am looking for 900mm - 1200mm focal length.

 

My gear

Mount = AM5

Cameras = 2600mm

Filters = Optolong 3nm SHO and LRGB 

Album: TV102

9 images

0 comments

. Images taken with my TV102 F8.8 Doublet using an OSC and an UV/IR cut filter.  Decide for your self if you like what a good F9 ED Doublet can do.  I would expect similar performance from an SW 100 mm F9 ED.  If you are doing Narrow band, then I would expect things to be even better.  Nothing wrong with a good triplet either.   

 

JMD

[HydrogenAlpha]

You absolutely can. You can go to astrobin to compare images from doublets vs triplets, and to see sample images from specific models you're interested. 

 

Like others mentioned, triplets will have better correction. But whether that difference matters to you is the key here. 

 

In general, larger physical apertures and faster focal ratios will make the difference more obvious. For a small refractor (<80mm) that is f/6 or slower, it will probably be ok. 

[ELDavis]

My SV Access has been a joy for ap. It shows almost no perceivable ca. At $500 it was the least expensive of my telescopes, but is the most used.

[johrich]

So, I while back, I started this topic on cloudy nights, https://www.cloudyni...7-askar-107phq/  Comparing my Stellarvue SV102 Access, (now discontinued, but the same telescope as the Astrotech AT102EDL, a doublet with FCD100 glass) and the ZWO FF107 APO, (or ASKAR 107PHQ telescope).  I always felt, in general, a doublet with a high quality SD glass element like FPL-53 or FCD-100 would do as well as a similar APO triplet telescope with a lower quality ED glass like FPL-51, in terms of color correction, etc.  And I still believe that true, after using the AT72EDII doublet with an FPL-53 and the SV102 Access for several years.  However, time has passed, and those two scopes have been retired and replaced with a RedCat 51 Petzval with FPL-53 and the ZWO FF107 APO Petzval telescope.  Though I was quite happy with the images with my older scopes, I do believe the Petzvals produce slightly sharper images.  Color correction for all the scopes appears to me quite good, but not perfect.  In this hobby, there is always something better and more expensive than what you have now.

 

John