Askar FRA500 vs TS-Optics CF-APO 90mm vs [your suggestion]

[msacco]

Hello!

I'm looking for a 500mm~ focal length telescope, and I would love to hear some suggestion about 2 specific ones as well as general suggestions.
I'm looking for something not too big and not too small, so 90mm is a pretty good size for that.
I'm also looking for a rather fast scope, so around f/4-f/5.5~.
I'll mostly use an APS-C sensor, but also own a modified 6D which I might occasionally use, it's not mandatory but full frame support would be a nice to have feature.

I ended up looking at these 2 telescopes, the Askar FRA500 and the TS-Optics CF-APO 90mm.
Both seems to be rather new, and I couldn't find much results on any of them.
Here are some thoughts I have on each:
540mm@f/6 for the TS Optics is okayish, a bit slow, but it has both a 1x flattener and a 0.8x reducer/flattener so 432mm@f/4.8
According to specs the 1x is: "Fully illuminated and corrected image field: 45 mm full frame format."
The 0.8x: "The corrector is designed for cameras up to full frame format"
So according to specs this look like a pretty awesome scope, FPL55, TS Optics is known for its quality in general..
But according to the very few astrobin images I've find with this scope and the 0.8x reducer, it looks far from being flat with an APS-C sensor, this could be a user issue with incorrect backfocus as well, I don't know..

The FRA500 also looks pretty nice being a quintuplet scope, so natively at 500mm@f/5.6 it doesn't require a flattener or anything else, supports up to 55mm image circle which is far greater than a full frame sensor, as far as I could see the results with this scope seems really great, completely flat and no CA, it also has a 0.7x reducer, so there's also an option for 350mm@f/3.9, not my main goal, but it's a pretty nice option to have..
As far as I could see, with the 0.7x reducer there is the slightest CA and the frame is not completely flat with APS-C sensor, even though the description does indicate a full frame support, but same here, it might be due to incorrect backfocus or something similar, and regardless, my main objective with this scope is mostly the 500mm focal length, so the 0.7x is only a nice to have thing.
One more thing that really draws me towards the FRA500 is that at native 500mm is doesn't require any accurate backfocus, as long as you can reach focus, the field will be flat. That is an amazing feature in my opinion and will actually be very useful for me due to some gear that has 56mm backfocus instead of the common 55mm, and that makes quite a big difference.

So these are my thoughts about both of them, personally I lean a bit towards the FRA500, but I'm open for suggestions and opinions.
My budget is around the $2000~, could spend more but both of them seems quite sufficient for me so I don't see a reason to spend more here.

Anyone owns any of the scopes? What is your opinion?

Thanks

[Lee_P]

I own the Askar FRA400, so not the one you've shortlisted, but similar. It's very good. Review here.

[vlaiv]

https://www.teleskop-express.de/shop/product_info.php/info/p13758_TS-Optics-APO-Refractor-106-700-mm---FDC100-Triplet-Lens-from-Japan.html

with matching FF/FR:

https://www.teleskop-express.de/shop/product_info.php/info/p13759_TS-Optics-0-75x-REFRACTOR-Reducer-Corrector-for-TS-106-mm-f-6-6-Apo.html

Just an alternative - I don't have any info on how good that scope is - but it is 100mm+ and close to your target FL (525mm).

[msacco]

5 hours ago, Lee_P said:

I own the Askar FRA400, so not the one you've shortlisted, but similar. It's very good. Review here.

Lovely review, the pros and cons are pretty much what I knew, except for some things that were improved with the Askar FRA500.
The price tag of the FRA500 is much higher, but it also comes with a case, so I assume that's extra cost besides the actual size itself.

One thing I wonder, in the M31 image you shared, it seems like the blue channel is slightly more bloated than the other channels, did you notice that? Not sure if it's the result of editing or the optics itself.
With an OSC camera 55mm backfocus should be easy and precise with the supplied 16.5mm + 21mm adapters ZWO supply, but the lack of need to worry about that is really really nice.

4 hours ago, vlaiv said:

https://www.teleskop-express.de/shop/product_info.php/info/p13758_TS-Optics-APO-Refractor-106-700-mm---FDC100-Triplet-Lens-from-Japan.html

with matching FF/FR:

https://www.teleskop-express.de/shop/product_info.php/info/p13759_TS-Optics-0-75x-REFRACTOR-Reducer-Corrector-for-TS-106-mm-f-6-6-Apo.html

Just an alternative - I don't have any info on how good that scope is - but it is 100mm+ and close to your target FL (525mm).

Thanks, but I'm not looking for anything bigger here, I'm actually prefer something smaller and more compact, weight is not an issue, but the size and length does.
I already own an Esprit 120 so not looking for anything similar in size.

[Concordia000]

I am in the same spot. I own a Tele Vue NP-101 which is an exceptional scope, except that it doesn't fit into my carry on pelican case and makes transportation a mess. So I am selling it and hoping to get something smaller. Apparently the TS-CF APO is quite good.

Aside from the socpes you mentioned, in this range the lightest scope will be the Borg 90FL without a doubt, but you pay a price in price and CA for the extreme lightweight specs. 

Then there's the trifecta of premium 90mm triplets: the Astrophysics Stowaway, the CFF92 and the old TMB92, but people aren't going to sell those and you aren't going to buy those new...

[Lee_P]

11 hours ago, msacco said:

One thing I wonder, in the M31 image you shared, it seems like the blue channel is slightly more bloated than the other channels, did you notice that? Not sure if it's the result of editing or the optics itself.

Ah, can't say I noticed that. Could be the optics or my processing!

[iantaylor2uk]

I use a William Optics 81 GT IV f5.9 refractor with a 1x Hotech flattener which gives a focal length of 478 mm and has a good flat field with the APS-C sensor on my ZWO 071 MC Pro camera.

[msacco]

9 hours ago, Concordia000 said:

I am in the same spot. I own a Tele Vue NP-101 which is an exceptional scope, except that it doesn't fit into my carry on pelican case and makes transportation a mess. So I am selling it and hoping to get something smaller. Apparently the TS-CF APO is quite good.

Aside from the socpes you mentioned, in this range the lightest scope will be the Borg 90FL without a doubt, but you pay a price in price and CA for the extreme lightweight specs. 

Then there's the trifecta of premium 90mm triplets: the Astrophysics Stowaway, the CFF92 and the old TMB92, but people aren't going to sell those and you aren't going to buy those new...

I'm not really looking for a lightweight scope, weight is not a big factor for me really, only the length, so there's not much benefit for me in the Borg scopes.
I don't think I'll go for premium either, don't see the need for that for my needs.

If you're going to sell the NP-101 you're surely going to have enough for a smaller premium scope  

56 minutes ago, Lee_P said:

Ah, can't say I noticed that. Could be the optics or my processing!

Do you happen to have the raw stack perhaps?
Thanks

17 minutes ago, iantaylor2uk said:

I use a William Optics 81 GT IV f5.9 refractor with a 1x Hotech flattener which gives a focal length of 478 mm and has a good flat field with the APS-C sensor on my ZWO 071 MC Pro camera.

Can you share a raw image with the WO81 and the flattener? I was only aware of the WO 0.8x which isn't useful for me here.
Is it completely flat with APS-C? Is there any chance it could work with full frame as well?

Thanks.

[iantaylor2uk]

I think it's unlikely that it would work with a full frame, but it definitely seems flat with an APS-C sensor provided you tune the spacing correctly (you need 55 mm spacing from shoulder of flattener to camera sensor, so for my ZWO 071 MC Pro camera (which is 17.5 mm in) you need 37.5 mm of spacers, and I it comes with some spacers, so you don't need many extra. There is a photo of my set-up below - I'll take a close-up view when I'm next out with the scope. There is a good YouTube video of the flattener (which works for any refractor between f/5 and f/8) at: 

 

 

[Lee_P]

1 hour ago, msacco said:

Do you happen to have the raw stack perhaps?
Thanks

Here you go. Stacked using PixInsight, so I hope you have that to open it! https://drive.google.com/drive/folders/1b_HA8fBfA6t3GVAe1z6InCDYp5hyE_pp?usp=sharing

 

 

[msacco]

3 hours ago, iantaylor2uk said:

I think it's unlikely that it would work with a full frame, but it definitely seems flat with an APS-C sensor provided you tune the spacing correctly (you need 55 mm spacing from shoulder of flattener to camera sensor, so for my ZWO 071 MC Pro camera (which is 17.5 mm in) you need 37.5 mm of spacers, and I it comes with some spacers, so you don't need many extra. There is a photo of my set-up below - I'll take a close-up view when I'm next out with the scope. There is a good YouTube video of the flattener (which works for any refractor between f/5 and f/8) at: 

 

 

Thanks for the comment, do you happen to have a raw frame/stack using this filter with APS-C? I would love to take a look at that

2 hours ago, Lee_P said:

Here you go. Stacked using PixInsight, so I hope you have that to open it! https://drive.google.com/drive/folders/1b_HA8fBfA6t3GVAe1z6InCDYp5hyE_pp?usp=sharing

 

 

Sure do! Thanks a lot! The blues are just slightly slightly bloated, that's pretty fine though, you can also see that in the spot diagram, but the thing I'm not sure if is good enough for me is the 'outwards CA', not sure if there's a name for that, but I'm referring to the edges of the stars towards the 3/4 area of the image, like here:

It's a bit of a pixel peeping, it's not really bad and overall pretty good, but I need to think whether I'd like to avoid it or not.
Referring to this basically, around the 12-17mm I guess:

That was really useful, thank you!

[iantaylor2uk]

Here's a jpg of a single fits image I took of M42 on 15th December with the WO 81 GT IV + 1x Hotech flattener using an APS-C sized sensor. I uploaded the image to astrometry.net and it told me the FOV was 2.85 degrees x 1.89 degrees, and if I put the WO 81 GT IV + 1x flattener + ZWO 071 camera into the online field of view calculator (https://astronomy.tools/calculators/field_of_view/) it tells me it should be: 2.83 degrees x 1.88 degrees, so the flattener may be very slightly less than 1x but is certainly pretty close.  

It's possible from pixel peeking at the corners that I may need to increase the spacing on the flattener slightly - however you should see what it looks like if you don't use any kind of flattener at all!

 

[msacco]

6 minutes ago, iantaylor2uk said:

Here's a jpg of a single fits image I took of M42 on 15th December with the WO 81 GT IV + 1x Hotech flattener using an APS-C sized sensor. I uploaded the image to astrometry.net and it told me the FOV was 2.85 degrees x 1.89 degrees, and if I put the WO 81 GT IV + 1x flattener + ZWO 071 camera into the online field of view calculator (https://astronomy.tools/calculators/field_of_view/) it tells me it should be: 2.83 degrees x 1.88 degrees, so the flattener may be very slightly less than 1x but is certainly pretty close.  

It's possible from pixel peeking at the corners that I may need to increase the spacing on the flattener slightly - however you should see what it looks like if you don't use any kind of flattener at all!

 

It surely seems like you could perhaps use some more spacing, looks like the sensor is too close, hard to tell if that's the limit of the optics or lack of correct backfocus, guess that's just trial and error
From what I can see, I don't think it's the flat field I'm looking for, but quite hard to tell. Thank you for sharing!

[iantaylor2uk]

When I use the same flattener on my Takahashi 102 TSA f/8 refractor the field is noticeably flatter than for the WO 81 GT IV certainly.

[msacco]

23 minutes ago, iantaylor2uk said:

When I use the same flattener on my Takahashi 102 TSA f/8 refractor the field is noticeably flatter than for the WO 81 GT IV certainly.

In that case that would indicate the optics of the WO 81 GT itself I assume?

[iantaylor2uk]

Well possibly. The WO 81 GT IV is f5.9 whereas the Tak is f/8, so it may be just a case of optimizing the flattener spacing for each scope. The longer focal length of the Tak will help too