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Should I replace my telephoto lens by a small refractor?

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#1 systm

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Posted 20 February 2020 - 03:16 PM

Hello all wavey.gif

 

I am considering to make a small upgrade to my kit by adding a small refractor within the 1.5 kg range for the OTA. I am still a noob in AP, but I would like to get an high(ish)-quality instrument.

 

I am currently using a Pentax dfa* 300mm lens on a Pentax K1. While I am very happy with it, the lens is pretty sharp (Pentax is Pentax) and that K1 sensor is wonderful, I wonder if I would benefit into using a refractor. This will be used on a Star Adventurer (guided), hence the weight limit.

 

Now: I am seriously considering a Takahashi FS-60CB due to it's modularity so that I could add more modules when I get a better mount, and the fact that it fits my requirements of weight and cost, which is around 1500€ for OTA + flatteners and all the panoply of accessories that usually come with Taks.

 

My main question is regarding the quality of this Refractor in comparison with the Pentax lens. I've seen several reviews, some praise it, some others do seam disappointed with it, but as a starter telescope (it will be my first one), will I be pleased? Or would I come back to the lens after realising that I do have more resolving power with it?

 

I'm attaching an image of M35 taken with the lens  (I've messed up on polar alignment but managed to salvage a few frames) - will the Tak provide a better resolution? I am also not a fan, not at all, of the diffraction spikes produced by the lens, as it creates unnecessary noise on the image.

 

https://imgur.com/a/gBuRn3r

 

My K1 is a Full Frame. I am aware that the 0.72x reducer will produce an image circle that will encompass the sensor while turning the focal length to 256mm , but I would prefer to maintain a 300mm or higher magnification. I've heard reviews stating that they use this scope on a full frame just with the flattener attached and that it provides sharp starts until the corners - but as far as I understood, the available flatteners for this scope do not produce a suitable image circle for full frames.

 

I can shoot in Crop mode (APS-C size) so this might not be a problem, but Ideally I would like to use the full area of sensor, so are any other (non reducer) flattener that can provide a full image circle for FF?

 

To sum up, I really like the idea of having a modular instrument that can be upgraded over time (and of course that my snobbish side does love the idea of owning a Tak), but would I be disappointed?

 

Today is an achievement day! My first post on Cloudy Nights!  yay.gif

 

Cheers!

Eduardo


Edited by systm, 20 February 2020 - 03:17 PM.


#2 Jeff Struve

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Posted 20 February 2020 - 04:34 PM

I personally don't see that size is an upgrade, more of a different tool. I consider quality an upgrade.

 

With that in mind, compare the FoV you get with your camera lens to the FoV you get with the 60,mm scope you mention and maybe an 80mm f6 and a 127mm  f7.5 scope. You may find that one of them may complement your lens well and allow you to better frame a larger number of objects. Keep in mind that with any of the scopes you can add a reducer to fill in the missing FoV between the lens and scope.

 

I make these comparrisons visually by using the ocular functionality in Stellarium.

 

My 2 cents and welcome to CN!

 

Jeff


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#3 systm

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Posted 20 February 2020 - 06:48 PM

Hi Jeff, thanks for the reply.

 

Quality of optics and resolving power are indeed my main objective.

 

Still, and as I am still using a Star Adventurer, weight is a concern (official specs put weight limit at 6kg, but I am trying not to exceed half of that, including the camera, which weights 1 kg + guiding setup). So that's why I am, for the moment, thinking on getting a small, lightweight telescope that the SA could manage without too much trouble. Hence the option of getting the baby Tak.

 

Eventually when I get a better mount I could then add a few modules in, like the 1.7X Q extender, but still have a Grab-n-Go Telescope that I could easily put on my luggage when travelling, etc.

 

So considering larger telescopes might be out of the equation right now, but eventually I will get there when I get a mount that can handle them.

 

So my main concern which led to this post is that if the telescope I am considering would be an upgrade over the lens or if I might be disappointed with it.

 

It is not cheap, so I am trying to avoid buyer's regret :)

 

Eduardo



#4 HydrogenAlpha

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Posted 20 February 2020 - 09:52 PM

I have the FS60CB, and it is in fact my second time owning it. I sold off my first one, but soon came to regret it because I realised how optically superb it is for imaging. 

 

That said, since you already have a 300mm lens, I'm not sure how much gain in resolution you can obtain. As far as I can tell, you've got pretty good stars for a camera lens. The FS60CB will not give you inferior performance for sure, but I am not sure how much gain you will find to justify spending that amount. You will, of course, fix the "issue" with the diffraction spikes. 

 

Also, it makes no sense for the reducer to fill the FF image circle where the flattener will not. There are several versions of the flattener, so make sure you get the latest one that turns the scope to f/6.2. 


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#5 Hesiod

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Posted 21 February 2020 - 10:30 AM

You could also evaluate the newer Vixen FL55ss which is extremely proficient and whose flattener and reducer are designed to be used with full frame sensors (can find several samples, especially on Japanese sites; I hope to update my review with full frame raws in a week or so).
Said so,I would be rather cautious about trading speed for slightly better star shapes, especially if shootibg with a startracker.
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#6 daquad

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Posted 21 February 2020 - 11:05 AM

That is a beautiful image of M45.  What causes those diffraction spikes; is it the iris diaphragm?  I can see why you would want a telescope for imaging.  Those spikes do detract from the quality of the image.


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#7 systm

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Posted 21 February 2020 - 11:18 AM

I have the FS60CB, and it is in fact my second time owning it. I sold off my first one, but soon came to regret it because I realised how optically superb it is for imaging. 

 

That said, since you already have a 300mm lens, I'm not sure how much gain in resolution you can obtain. As far as I can tell, you've got pretty good stars for a camera lens. The FS60CB will not give you inferior performance for sure, but I am not sure how much gain you will find to justify spending that amount. You will, of course, fix the "issue" with the diffraction spikes. 

 

Also, it makes no sense for the reducer to fill the FF image circle where the flattener will not. There are several versions of the flattener, so make sure you get the latest one that turns the scope to f/6.2. 

Thanks Ivan, your response, albeit indirectly, made me lean towards the telescope. I've see your wonderful site and a couple of images taken with the FS60CB and... that's some serious imaging quality. While I am aware that the glass is only part of the equation of those beautiful images, but nevertheless impressive!

 

But yeah, that's a conundrum. I believe that the lenses, at least for an interchangeable photographic lens standard are quite ok, while not extra fast (I usually shoot at f5). But those spikes... god help me.

 

In any case, the fact that you praise it and after seeing your work in imaging does give me confidence into plunging into it.



#8 systm

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Posted 21 February 2020 - 11:28 AM

You could also evaluate the newer Vixen FL55ss which is extremely proficient and whose flattener and reducer are designed to be used with full frame sensors (can find several samples, especially on Japanese sites; I hope to update my review with full frame raws in a week or so).
Said so,I would be rather cautious about trading speed for slightly better star shapes, especially if shootibg with a startracker.

I am in fact also considering the FL55ss... I've seen good reviews and it appears as an equivalent to the FS60CB, at the same price range and weight.

 

What makes me lean towards the Tak is that the FS60 has some expandability, being through attaching the 1.7X Q extender (which apparently improves image quality), or with the 76mm module. For the moment I wouldn't acquire any of these because of weight limits  of my Star Adventurer, but once I get a more capable mount, then I would have the possibility of expanding and have an all around package.

 

Another instrument that I got my eyes on was the WO Cat51, which everyone praises. But that would put me on the ~250mm range, and again, as the Vixen, no modularity.

 

I might be seeing things from a wrong perspective but the concept of having one scope that can be adapted to various situations/targets does entice me. I am aware that there are trade offs, most notably speed.

 

But yes. I need to see if the Vixen could be an option. Looks gorgeous as well.

 

 

Edit: I did seen your review of the FL55ss before here on CL, pretty good work! It is seriously making me consider it.


Edited by systm, 21 February 2020 - 12:16 PM.


#9 systm

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Posted 21 February 2020 - 12:05 PM

That is a beautiful image of M45.  What causes those diffraction spikes; is it the iris diaphragm?  I can see why you would want a telescope for imaging.  Those spikes do detract from the quality of the image.

Thanks!!! (although I think that I still have a long long path to go) but yes - 18 spikes - 2 for each diaphragm blade smile.gif

 

M45 is particularly troublesome due to the extra bright stars but it occurs in most of the images I've took to more or less extent.

 

Apparently there is a way to go around it through the use of step down rings - basically rings that are put on the front of the lens until the light that comes in to the lens has a smaller circumference than the aperture. Sounds sketchy, so I will not even try it. smile.gif

 

Cheers


Edited by systm, 21 February 2020 - 12:06 PM.


#10 Hesiod

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Posted 21 February 2020 - 01:01 PM

I went through a similar dilemma last spring.
At first was leaning toward the fs60, since already was using a Tak for AP.
However was not totally sold by the focuser and the adapter system looked a bit clumsy (as that of my fc100 is).
On the other hand the fl55 had a brilliant design and was able to dig a few Japanese reviews which, even through the barbaric google translator, sounded very positive.
I therefore opted to "risk" (am not a Vixen "fanboy") and chose the Vixen over the Takahashi (a brand I place higher on the scale).
And was a stroke of luck, as the tiny refractor is just as good than my Taks and have a better focuser, better reducer and more elegant adapters system (at least, as long as there is a camera angle adjuster for your dslr...).
The lack of a "q module" is nowaday, when are available excellent and comfortable eyepieces under 5mm, of little concern (even for the purpose of traveling sibce these eyepieces are smaller than the q module and more convenient to swap); the possibility to upgrade it with the fc76 lens assembly is nice but, again, not something I would like to do often; furthermore, I would prefer the fc76df* (the photographic version: its focuser, which is the same employed on fc100df and dl, is better than that of the fs60 but not as good as that of the fl55).
Said so, both are excellent choices, as could be the Borg55


*I seem to remember that can not use the fs60 flattener or reducer with the fc76

#11 systm

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Posted 21 February 2020 - 02:05 PM

Ahhh dilemmas, dilemmas.

 

Borg 55 also looks stunning, but it is a bit north of what I was hoping - I still need to save for a good mount later on :)

 

Not that either Tak and the Vixen plus the adapters are cheap, but still around the limit that I set (1.5k euro). On the Tak, there's also a microfocuser compatible with the FS60 but then I would need to add another 300€ to the bill.

 

I am without urgency, so I have the time to carefully think about it. In any case, as you say, I believe that I would love either of them.



#12 Hesiod

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Posted 21 February 2020 - 03:06 PM

I did not upgraded the fc100df nor the fl55 as use a focusing mask and both stock focusers are up to the task.
In truth, had been interested in a merely visual telescope, now would be the owner of a Takahashi FOA60😁

#13 systm

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Posted 21 February 2020 - 07:03 PM

Yeah, details aside, the fact that the FS60 was primarily introduced as an imaging scope versus the FL55, which was recently made (and for sure thought from the ground up with AP in mind) does make me wonder.

 

That's actually the point that divides me between the two.



#14 Jon Isaacs

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Posted 21 February 2020 - 10:02 PM

My two cents:

 

For astro-photography, the most important thing is the mount, not the telescope.  It doesn't make sense to buy a relatively expensive scope for an entry level mount.  Your first serious investment should be in a quality mount that is capable of handling a decent sized scope.  If you buy the 60mm Takahashi, your spending a good deal of money on a scope of limited capability. 

 

Your 300mm camera lens is a reasonable match for your current mount. Learn what you can, when it's time to spend some money, buy a real mount.

 

Jon


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#15 systm

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Posted 22 February 2020 - 08:33 AM

Hi Jon, thanks for the advice, which does put a nice layer of reality on top of this discussion.

 

I am aware of the mount > glass > camera hierarchy, but the camera lens at this point does give me results that I am not very fond of, most notably the diffraction spikes. On the other end, the mount/tracker, while pedestrian, can still track reasonably well on the ~300mm range if guided. I definitively don't want to sound pretentious when stating this, please don't take me wrong.

 

I am aware that the Star Adventurer is the weak link on my setup, even when considering the diffraction spikes that my (otherwise fine piece of) glass produces.

 

Astrophotography is a marathon. I believe that for the foreseeable future I will always have the need of a portable mount on my setup and corresponding portable glass (I do travel a bit both professionally and on leisure) so the SA will have a place on my equipment portfolio for the medium/long-ish term.

 

So I thought that acquiring a good quality compact, portable telescope that can be with me on the long term (since I will always want a portable one), while perhaps counter productive and unreasonable if we look at the tracker I currently have, might be an option I wouldn't regret? Again, I can get reasonable tracking results on the focal length that I am using right now, hence the option of a telescope of similar power.

 

It is still a conundrum, and I will need to think this carefully.

 

Thanks again for the valuable advise - reality is always appreciated. salute.gif



#16 Passerine

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Posted 22 February 2020 - 12:12 PM

If you're really committed to sticking with your Star Adventurer mount, and it seems you're really focused on imaging, not visual, the Williams Optics RedCat 51 might be an option.

 

Dave



#17 JJexp

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Posted 07 March 2020 - 12:46 AM

I don’t know where you’ve ended up by now, but I have had bad luck running a small refractor on my star adventurer, even with the balance weights. 
 

I was having great success with my 24-70 and my 70-200, so I thought I might give my WO Z73 a shot, which is a 430mm focal length. Given that the scope and camera came in at around 7 pounds, which is definitely under the star adventurer advertised load of 11 pounds, I thought I’d be in good shape. I was wrong. Even perfectly balanced with the counterweight I was unable to exceed 30 second subs and then they also showed significant trailing when imaging at higher declinations, whereas I was easily able to obtain 180 seconds or more with the camera lenses. I just don’t think the mechanism is as robust as they claim to handle the weight. 
 

That said, buy the scope but also buy an entry level mount to throw it on, and leave the star adventurer for the camera lenses. Can’t ever have enough gear. 



#18 systm

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Posted 09 March 2020 - 08:12 PM

I don’t know where you’ve ended up by now, but I have had bad luck running a small refractor on my star adventurer, even with the balance weights. 
 

I was having great success with my 24-70 and my 70-200, so I thought I might give my WO Z73 a shot, which is a 430mm focal length. Given that the scope and camera came in at around 7 pounds, which is definitely under the star adventurer advertised load of 11 pounds, I thought I’d be in good shape. I was wrong. Even perfectly balanced with the counterweight I was unable to exceed 30 second subs and then they also showed significant trailing when imaging at higher declinations, whereas I was easily able to obtain 180 seconds or more with the camera lenses. I just don’t think the mechanism is as robust as they claim to handle the weight. 
 

That said, buy the scope but also buy an entry level mount to throw it on, and leave the star adventurer for the camera lenses. Can’t ever have enough gear. 

I am still waiting to decide. This is more of a careful consideration than an impulse smile.gif

 

The fact is that I'm currently using a 300mm lens that weights ~1kg (2.2lbs) and when looking for a refractor I did set the weight limit at around the same mark. Now with a telescope I know I need to consider a panoply of adaptors/accessories that do contribute to weight, but starting at around the same weight for the OTA sounded to me like a good beginning for this consideration.

 

With the 300mm I do get satisfactory results (~2kg: lens + camera) - I do use guiding (maybe that's why you aren't having good results with the WO Z73?, it wasn't clear in your post if you were guiding or not) - and I was wondering that if I could get a bit further regarding weight I could just get a bit more optical quality. To be honest, what annoys me the most is the refraction spikes on the camera lens, so before committing to a new telescope I might try to mitigate that.

 

In the end I might go with saving for a better mount, if I manage to correct the issues that I perceive with my lens. I am fully aware that the mount is everything, but... I just wanted to get that bit more with my current setup.


Edited by systm, 09 March 2020 - 08:13 PM.



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