13 replies to this topic

[HMS_Endeavour]

I am starting to think about getting a longer scope to go along with my redcat51. I like the redcat but its quite wide and this summer I want to do as much nebula & galaxy imaging as I can.

 

So I have been browsing classified in my area and notice some scopes come with a reducer. Now I think I understand the application; they reduce the focal length but increase the focal ratio.

Why would I want this for DSO imaging with filters? (particularly NB) where my exposures are going to be quite long - i.e 120-300s am I not just losing focal length and not getting the benefit for shorter exposures? wouldn't I want to go for length over speed in this instance?

 

 

Secondly, whats a good option for DSO targets to compliment my Redcat? I am on a bit of a budget, so looking at used & something up to approx $1000 would be my spend limit

[unimatrix0]

Focal reducers reduce focal length and reduce F ratio.  

 

Here is how it works on paper: 

 

Let's say F6 scope with 580mm focal length 

 + introduce a 0.85X focal reduction   

 

580x0.85= 493mm. Your new focal length. 

 

 F6=  6x0.85 = F5.1  Your new focal ratio. 

[matt_astro_tx]

Franks got it right. As another example My 60mm F/6 scope has a 0.8x reducer which brings the focal length down from 360mm to 287mm and increases my speed from F/6 to F/4.8.

That may be the source of confusion is that we say a reducer increases speed or speeds up the optical system. This has the effect of reducing the exposure time required to image a target.

Why would you want this? Well, F/6 is kind of slow and would require more imaging time than F/4.8. Yes it increases the field of view effectively making your target appear smaller. So it’s a trade off.

[HMS_Endeavour]

yeah thanks, I meant reduce the f ratio number effectively increasing the aperture  - thinking from a photography point of view there.

 

but yes I get it. 

[scanner97]

Going back to something complementary to the RedCat.  I would stay with focal length and look at the targets you want to image.  (Telescopius is good for that.)

 

Someting in the 600-800mm range before reducer would likely be a good choice, again, depending on your targets.

 

There may not be many options that offer a flat field with full illum, at full frame and under your budget, but you can always crop.  (In which case maybe use an APS-C in Telescopius to check the FOV.)

Edited by scanner97, 19 May 2025 - 08:49 AM.

[imtl]

yeah thanks, I meant reduce the f ratio number effectively increasing the aperture - thinking from a photography point of view there.

but yes I get it.

Actually no. It does not effectively increase the aperture. You're still collecting the same number of photons with your scope. You are focusing them on a tighter area because your light cone is steeper.

[matt_astro_tx]

I think he meant to say increase the field of view.

[bobzeq25]

I am starting to think about getting a longer scope to go along with my redcat51. I like the redcat but its quite wide and this summer I want to do as much nebula & galaxy imaging as I can.
 
So I have been browsing classified in my area and notice some scopes come with a reducer. Now I think I understand the application; they reduce the focal length but increase the focal ratio.
Why would I want this for DSO imaging with filters? (particularly NB) where my exposures are going to be quite long - i.e 120-300s am I not just losing focal length and not getting the benefit for shorter exposures? wouldn't I want to go for length over speed in this instance?
 
 
Secondly, whats a good option for DSO targets to compliment my Redcat? I am on a bit of a budget, so looking at used & something up to approx $1000 would be my spend limit

You really need to understand some basic things to do this well.

The first is field curvature. With an ordinary doublet or triplet, the field is curved, and needs to be flattened. Your Redcat does this internally.

If you get a telescope that's a doublet or triplet, you need a field flattener. The overwhelming majority of these also reduce the focal length. So when we talk about a 0.8X reducer, what we're talking about is a field flattener that also reduces the focal length by 0.8X. 1.0x flatteners exist for some scopes, but they're rare. A number of scopes include the flattener internally. That's a significant convenience.

The reduction SHORTENS the exposure needed with a narrowband filter, another reason people use reducers(/flatteners).

Your Redcat is quite short at 250mm. Doubling that to 500mm would be good, you probably don't want to go too much longer yet. I'll give two examples, somewhat over your budget, new.

The first is a doublet with a matching flattener(/reducer). 0.8X. Matching the flattener is good, field curvature varies. The second has an internal flattener.

https://astronomics....num-f-7-doublet

https://astronomics....t-apo-refractor

The second. "Self-flattening design. No need for a separate field flattener"

https://agenaastro.c...-telescope.html

Getting a doublet or triplet that needs a flattener is cheaper. But flatteners need to be spaced properly, and it's not trivial.

Edited by bobzeq25, 19 May 2025 - 11:03 AM.

[sozen]

I got an Askar71F after having had a redcat.  Its slightly slow at its native focal length of 490mm and f/6.9 but have not found the slower optics to be that much of an issue. Its a petzval design similar to the Redcat so no need to worry about back focus.  There is an optional 0.75x reducer that brings the system down to  368mm f/5.2 which is similarly fast to the redcat with a bit more reach.  With the reducer you do need to ensure you have the correct 55mm backfocus.  Its reasonably priced and Ive really liked this scope after having used my redcat 51 for about a year

[Sacred Heart]

If it were me, I look not further than a 102 or maybe the 115.  800mm in focal length is getting up there, OAG range and closing in on limits for your CEM 40.  I like to have wiggle room for variables..like wind.

 

Personally I'd take the 102 over the 115, shorter, lighter.

 

Joe

[HMS_Endeavour]

Thanks. 

 

I think for Galaxy and small nebula, a 71 isnt going to get me where I want to be over the redcat. 

 

I'm using a guidescope on the CEM40 not OAG, so can assume going for something like an SCT is not viable

[JF1960]

I currently use a 533MC with my Redcat51 and can’t really justify an upgrade unless it allows me to pull in much smaller targets. So I wouldn’t bother unless you can get 800mm+ of focal length. But a scope that big means a complete upgrade including new mount. Spending $1K on a new scope will likely be not much of a change over what you have today. If you are using a DSLR to image, the focal length will improve your FOV. But in that case, I would use the $1K to buy a dedicated astrophotography camera instead.

Edited by JF1960, 24 May 2025 - 11:04 PM.

[Spaceman 56]

I think for Galaxy and small nebula, a 71 isnt going to get me where I want to be over the redcat. 

 

I'm using a guidescope on the CEM40 not OAG, so can assume going for something like an SCT is not viable

I agree. for smaller Nebulas and most galaxies a 71mm is not going to cut it.

 

I tried Galaxy imaging with my 106mm Takahashi which is 530mm and while the results were ok, it was not the right scope for the job.

for real Galaxy imaging you need both Aperture and Focal length.  But you will need to work within the limitations of your mount.

 

I bought a Stellarvue SVX-102T which is 714mm, as an intermediate scope, before I bought the big RC which is my Galaxy scope. its an in between type thing, being slightly too short for real Galaxy work, but long enough for a closer view of many nebulas.

 

perhaps a 200mm F4 newtonian might go on the CEM-40, which gets you to 800mm, but will require COMA correctors and collimation.

 

a 115mm Astrotech refractor might be worth considering as the focal lengths similar, but without all the newtonian mirror issues that people encounter.

 

https://astronomics....3RY7GnNPfNdnKx_

[Drothgeb]

For a $1000, I would be looking at used 4” SD triplets, similar to Spaceman’s 102. A 5” would be better and still work fine on your mount, but would be above your price range.