Flight portable premium refractor (astrophotography)

[Staring]

Hello everyone,

 

I‘m looking for a perfect small refractor for astrophotography, and so far had not much luck with the ones I chose. The criteria are:

- Flight portable, so 48cm max. tube length

- FL 300-500mm

- no slower than f/6

- very good correction for CA

- premium build quality

- flat field with good stars up to APS-C with small pixels (IMX571) - dedicated reducer or reducer/flattener preferred

- Also usable visually

- available new

- Budget is high

My story to illustrate my expectations (no need to read except to exclude the 61EDPH and Tak FSQ-85):

I‘ve first tried the 61EDPH (v1). Its build quality is serviceable. As a doublet it suffers from rather bad CA and my sample also has some SA. The CA makes using it with a mono cam and dual-NB filters very difficult. 

Then I splurged for a Tak FSQ-85. The initial impression was very favorable: Built like a tank, nice to touch, comparably compact. That didn‘t hold long: The 0.73x reducer had terrible aberrations. With the flattener, stars were ok-ish, but still showed diffraction artifacts around bright stars. The focuser introduced image shift. I‘ve returned the scope.

I‘m hesitantly looking at the RedCat (but see many examples of non-flat field and do not like the helical focuser), the Vixen 55FL (which seems to have backspacing issues according to some posts), Borg (CA), the Askar telescopes (CA, aberrations) and the CFF 80mm (only f/6.9: tube too long).

Did I miss any other suitable telescopes?

[rl]

To be honest I'm not sure where you go from here. The FSQ85 would be the dream widefield astrograph for most people. I thought being a quad Petzval design the flattening was built-in? If you add an extra flattener the curvature will simply go the other way. I've seen some good reviews of the 5-element Askar and they're all checked in the UK before sale. 

[vlaiv]

https://www.teleskop-express.de/shop/product_info.php/info/p3881_TS-Optics-PHOTOLINE-80mm-f-6-FPL53-Triplet-APO---2-5--RAP-Focuser.html

+

https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html

[dmki]

That's a very tempting combination

[Clarkey]

What about the new StellaMira ED90 with reducer? Looks pretty good on paper.

Having said this, if you have rejected the FSQ85, the bar looks pretty high.

[Staring]

Thanks for helping.

Yes, I admit the bar is high. TS telescopes seem to be somewhat hit-and-miss, and it‘s difficult to know which is which. Maybe I can get them (or a reseller) to agree to a bench test and optimization. 

Concerning the FSQ-85: I also thought it‘ld be the dream astrograph, which is why I bought it. Its shortcomings are documented in many threads, though, if you look for it. For modern sensors with small pixels the field is not well-corrected enough and so it needs its dedicated flattener/corrector, giving up the advantage of the Petzval design. The flattener is now included by Tak (I think since 2019). But even with that I had these diffraction artefacts that look like vignetting, but on on-axis stars. You can also find that on many recent images with that telescope - I see many older ones that do not show it. The reducer really is only usable with pretty big pixels, even if you get the spots that Takahashi advertises. My sample must have some internal misalignment, though. The focuser can not compare to something from Moonlite or Starlight Instruments. The shifting could not be adjusted away. From what I researched about 50% of the Tak focusers have that problem, which is why many people accessorize theirs with 3rd party focusers. All in all the FSQ-85 feels like a dated design that can not hold up to the demands of modern cameras anymore.

[Staring]

I‘ve looked at the Askar scopes, too. The small ones show a lot of off-axis aberrations, even in the spot diagrams. The Askar 400 looks better,  but with the reducer it has significant CA. And I have not seen an image that has great corner stars on an IMX571 sensor with full resolution -  this probably means the scope can not do it.

[Icesheet]

If the FSQ 85 wasn’t up to scratch for you I’m not sure there’s anything on the market that will!
 

If you considered the Tak portable an Esprit 80ED may fit the bill. Although it sounds like you’re going to have to accept a compromise somewhere. 

Edited October 28, 2021 by Icesheet

[vlaiv]

@Staring

You seem to be hung up on small pixels thing.

Yes, it is very nice to see modern CMOS sensors having 3.7µm or even smaller - down to 2.4µm pixel size, but that is just consequence of marketing - quest for higher megapixel cameras, and hurts more than it helps with Astrophotography.

If you want flight portable scope - it's going to be short and small aperture - like 60-90mm range.

With these scopes - best working resolutions are around 3-4"/px range. Maybe down to 2"/px - but that would be pushing it (depends on seeing and guiding). 80mm has airy disk of 3.21" in diameter - and that is diffraction limited optics on axis without influence of seeing or guiding.

With 400mm of FL - you need 6µm pixel size to get ~3"/px. So that is almost twice the size of small pixels (and sometimes even more - for very small pixels).

If you want to work with APS-C sized sensor - and you want your stars to be perfect - maybe look into doing things differently. Use scope at native focal length, get good flattener that will make good stars to the edge of the sensor and bin your data x2 to increase pixel size.

For example - 80mm F/6 will have 480mm of FL. With sensor that has 3.76µm pixel size (IMX571) and bin x2 you'll get very good resolution 3.23"/px, and I think that you'll be much happier with corner stars.

Remember - if you have 28mm diagonal sensor and you use x0.8 reducer for example - you want that scope to have pin point stars up to 35mm without reducer - just flattener. That is tall order for faster scopes with small aperture.

If you want to go higher resolution - at 2"/px or below - then you need to start thinking aperture - at least 5"-6" of it for those resolutions (2"-1.5" - and higher than that at least 8").

 

[Deadlake]

The Baader 95 mm caF2 is the only other scope I can think of...

https://www.baader-planetarium.com/de/teleskope/baader-planetarium/baader-apo-95560-caf2-travel-companion.html

[ollypenrice]

I agree with vlaiv. It's in the nature of widefield images to look fine down to 3.5"PP or so. These are 3.5"PP:

https://www.astrobin.com/383965/

https://www.astrobin.com/301531/?nc=&nce=

https://www.astrobin.com/327970/

It is a shame that the Tak FSQs have been plagued by quality control issues in recent years but they certainly have. A good FSQ85 is a great little scope, though you have to expect the 'inverse lighthouse beam' effect on bright stars. One advantage of buying second hand is that you can see images produced by the scope. All the refractors I use were bought second hand.

Would a TV85 go as carry on? It's a good imaging scope with the flattener, very strong indeed and, in the event of disaster, the front element can be re-collimated by an amateur at home. I used to carry on a Genesis for visual use.

Olly

 

[gorann]

I would seriously consider the Esprit 80 - never read anything bad about them. I have not tried one yet (but I will accidentaly get one tomorrow as part of a package deal together with an Esprit 150 and an EQ8 from a person that bought the stuff some years ago and esssentially never got around to use it). However, the Esprit 100 and 150 that I have had for several years are excellent scopes when used with their designated Esprit flatteners - no problems with star shapes even with an ASI6200 full-frame APS. Hopefully someone here have used the Esprit 80 and can tell us about it.

PS. I bought a TS 71 SDQ (quadruplet with FPL53 made by Brightstar with a supposedly flat field) two years ago and I am not impressed with the star shapes. It have since then lived a life as guide scope and it is apparently not sold anymore. @wimvb recently bougt a similar scope from TS and has a similar experience.

Edited October 28, 2021 by gorann

[Icesheet]

8 minutes ago, gorann said:

I would seriously consider the Esprit 80 - never read anything bad about them. I have not tried one yet (but I will accidentaly get one tomorrow as part of a package deal together with an Esprit 150 and an EQ8 from a person that bought the stuff some years ago and esssentially never got around to use it). However, the Esprit 100 and 150 that I have had for several years are excellent scopes when used with their designated Esprit flatteners - no problems with star shapes even with an ASI6200 full-frame APS. Hopefully someone here have used the Esprit 80 and can tell us about it.

A bit off topic but was that the one on FINN.no? Looked like a pretty good deal to be had. 

[gorann]

Just now, Icesheet said:

A bit off topic but was that the one on FINN.no? Looked like a pretty good deal to be had. 

Yes, got an Esprit 150 with flattener, and Esprit 80 with flattener, an EQ8 with tripod, and a professionally modded Canon EOS 5D Mk III for the equivalent of 5400 GBP. I could not say no to that but I really just wanted the Esprit 150 for a dual rig. But, yes, cearly off the topic

[vlaiv]

1 minute ago, gorann said:

Yes, got an Esprit 150 with flattener, and Esprit 80 with flattener, an EQ8 with tripod, and a professionally modded Canon EOS 5D Mk III for the equivalent of 5400 GBP. I could not say no to that but I really just wanted the Esprit 150 for a dual rig. But, yes, cearly off the topic

While we are talking off topic - here is quick question (some what related to what you have written) - how do you find your CEM70 (and is it stock model without iThis and iThat)? How well does it guide?

(you mentioned that you purchased all of that together and I took a look at your equipment list and there it was - cem70 ).

[gorann]

10 minutes ago, vlaiv said:

While we are talking off topic - here is quick question (some what related to what you have written) - how do you find your CEM70 (and is it stock model without iThis and iThat)? How well does it guide?

(you mentioned that you purchased all of that together and I took a look at your equipment list and there it was - cem70 ).

Now we are way off the topic, so the OP can ignore this, but yes I bought an iOptron CEM70 about a year ago and used it with a RASA8 and guide scope (ST80). It did its job but I cannot really tell you how well it was guiding since the focuser of the ST80 is a bit flimsy to say the least, and I usually got the RMS around 1"/pix, but sometimes around 0.7"/pix, so seeing was probably also limiting. I thought the guiding was good enough for the RASA (FL 400mm) so I did not bother trying to fiddle with it. Since I decided to have a dual-RASA rig I needed to move it all to the Mesu 200 for weight reasons and I have now put an EdgeHD8 on the CEM70, with OAG. So if I get that up and running (and a clear sky) I will have a better knowledge about the guiding capabilities of that mount.

Edited October 28, 2021 by gorann

[Adam J]

On 27/10/2021 at 16:33, Staring said:

Hello everyone,

 

I‘m looking for a perfect small refractor for astrophotography, and so far had not much luck with the ones I chose. The criteria are:

- Flight portable, so 48cm max. tube length

- FL 300-500mm

- no slower than f/6

- very good correction for CA

- premium build quality

- flat field with good stars up to APS-C with small pixels (IMX571) - dedicated reducer or reducer/flattener preferred

- Also usable visually

- available new

- Budget is high

My story to illustrate my expectations (no need to read except to exclude the 61EDPH and Tak FSQ-85):

I‘ve first tried the 61EDPH (v1). Its build quality is serviceable. As a doublet it suffers from rather bad CA and my sample also has some SA. The CA makes using it with a mono cam and dual-NB filters very difficult. 

Then I splurged for a Tak FSQ-85. The initial impression was very favorable: Built like a tank, nice to touch, comparably compact. That didn‘t hold long: The 0.73x reducer had terrible aberrations. With the flattener, stars were ok-ish, but still showed diffraction artifacts around bright stars. The focuser introduced image shift. I‘ve returned the scope.

I‘m hesitantly looking at the RedCat (but see many examples of non-flat field and do not like the helical focuser), the Vixen 55FL (which seems to have backspacing issues according to some posts), Borg (CA), the Askar telescopes (CA, aberrations) and the CFF 80mm (only f/6.9: tube too long).

Did I miss any other suitable telescopes?

I think that you have to make some compromises if you want a truely compact travel imaging setup. 

My solution is the ASKAR FMA180 and yeah its not perfect in terms of CA / poly strel (not sure what other aberrations you mean)  but it can be corrected in processing with the use of a L3 UVIR filter too unless you really pixel peep. 

The reality is that with triplet refactors below F6 you will always have to trade between poly strel and CA and you will not get a perfect result. You need to go to a more optically complex design to get better performance below F6 and it gets harder as aperture gets bigger. 

I have a friend with a WO GT71 that delivers good results, perhaps combined with a 1.0x corrector as opposed to a reducer this will give you the performance you are looking for?

https://www.firstlightoptics.com/william-optics/william-optics-gt-71-ii-2019-checked-tuned.html

In the end no budget if going to defy the laws of physics, you cant have it all.  The only other thing that comes to mind is a TV-76, FLO have an ex demo avaliable? Combine with a 0.8x reducer. 

https://www.firstlightoptics.com/offers/offer_tele-vue-76mm-f6-3-apo-refractor_187502.html

Although i would still go with the GT-71 if it was me. 

Adam 

 

Edited October 28, 2021 by Adam J

[Ken82]

I’ve got to say my fsq 85 is a really good scope! Yes it struggles now with my full frame asi 6200 but if I was using an aps-c type sensor it would be my first choice. 
 

Incidentally I wasn’t happy with an fsq 106 I bought a while back and returned it (due to the focuser). And I was concerned buying another one. But thankfully my concerns have not been justified. So I now have two very good fsq’s 😀

[Whirlwind]

On 27/10/2021 at 16:33, Staring said:

Did I miss any other suitable telescopes?

What about some of the Borg telescopes, they are collapsible so should be easily transportable as well being modular to meet different needs?

[Concordia000]

16 hours ago, Whirlwind said:

What about some of the Borg telescopes, they are collapsible so should be easily transportable as well being modular to meet different needs?

I have one. Or soon will be "had", because I am selling it to cover fund my purchase of a NP-101.

Those scopes are the most transportable scopes of their aperture and it is impossible to contest that. The Borg 90FL weighs 1.8kg without rings, 1.5kg without rings and with a helical focuser. The 107FL is 2.7kg without rings. No other scopes come close to that. Even the biggest 107FL is only about 40cm long fully collapsed; its telescoping design makes other scopes look ancient and unwieldy. To add insult to injury, you can even break the scope apart and put all of that in a backpack and reassemble it later. 

They are fantastic for visual observation. My 90FL is sharp and incredibly bright, and I was really impressed by the view of M31 through it. Not as good as a Takahashi, but the CA is generally invisible unless you are looking at very bright edges and looking for it. The field curvature is noticeable but not acceaptable. 

But literally all of them will have some degree of blue bloat for AP, even with flatteners and reducers. The bigger 90FL and 107FL basically have to use the extremely expensive 7770 reducer (it costs 2 grand on its own) if you want to photograph at F/4, or so I was told. With the multi-flattener the CA is more under control, but it still won't compare with a good triplet. The smaller 72FL might be the only one of the family that has a somewhat inexpensive reducer, since it was specifically designed to match the cheaper, 600 quid, 7872 reducer... 

Are they worth it? I want to say yes, since I loved my 90FL and I am actually hesitant to let it go. I mainly used it visually, but many people on Astrobin had success with them. But I also have to acknowledge the fact that the scope + flattener + adapters, even when bought second hand, will approach that of a used NP-101, which blows the 90FL out of water. But that is the price you pay for the portability: the NP-101 is twice as long and twice as heavy as the 90FL and it is definitely not a travel scope.

[Concordia000]

Also I still think the FSQ-85 is OP's best bet. It's not like the scope is slow at its native FL. Maybe OP had a bad sample of the FSQ-85, maybe it was miscollimation or tilt, or maybe it was backfocus issues.

I had tilt and/or collimation issues with my RedCat 51, admittedly I use a FF DSLR; I have no idea how it translates to APS-C performance. Field curvature is not an issue imo. However it is 250mm FL and cannot be reduced any further; it is also not in your range of FL. The larger RC71, hopefully coming soon to the UK , will be inside your FL range, but those two scopes practically cannot be used visually. You will need to remove the tilt adjuster to install the special diagonal, which to be frank is not good, and you risk introducing tilt into your system because you will might mess with the tilt adjuster plate when you remove/reinstall it.

You might want to consider the Vixen, though I have no idea how that performs, and it is a very small scope and will not be a satisfying visual scope. A very good triplet like the CFF is probably your safest bet however, and you have a lot of choices. You can even go for the bigger CFF 92 f/6 if weight isn't a big issue.