84 replies to this topic

[FiremanDan]

So I have been fighting bad stars since... well since I started this hobby. But I had ordered the SV80 because it came highly recommended. It seems some folks get good stars with it but there has been an ongoing issue apparently with the SFFR80-25. Stellarvue apparently discontinued the product altogether. 

My latest bout of bad stars can be seen here: https://www.cloudyni...get-good-stars/

I got one session with good stars, I didn't change anything, and they are back to being horrible. 

 

 

My two best guesses now as to why I can't get decent stars consistently are: focuser sag and/or bad optics via the SFFR80-25. 

So, one of my main questions is, who uses the SV80 w/ another brand's 0.8 FR with good stars? Frankly, I just don't have the money for a feather touch or Moonlite. 

[bmhjr]

Could you have Stellarvue send you the standard f6 flattener to try.  That may tell you if it is the 0.8 FR or something else.  If it works well they may trade with you.  It is what I use and seems to work pretty good with my full frame DSLR.  I had to add a 1mm spacer to what should have been the correct spacing.  I havent messed with it anymore to try and get it "perfect"

[FiremanDan]

Ideally, I'd like to stay at 384mm f.4.8. I really like that FOV and the faster optics. 

[bobzeq25]

How much have you played with spacing?  Madratter had better results with the CCDT67 on his RC when he used a different spacing that one might have thought from the Astrophysics website.

Edited by bobzeq25, 27 June 2017 - 12:30 PM.

[terry59]

Hey Dan....curious here...why do you not use just a flattener?

 

Never mind

Edited by terry59, 27 June 2017 - 12:37 PM.

[mikefulb]

Back in the day I used the TV 0.8x (400-600mm) reducer with my LOMO TMB 80/480 and it was reasonable.

 

Really unfortunate SV has left people without a good option.

[FiremanDan]

How much have you played with spacing?  Madratter had better results with the CCDT67 on his RC when he used a different spacing that one might have thought from the Astrophysics website.

Madratter has or had an SV80 and was using a different brand's focal reducer. I'll message him, I am not sure he will get it. I know he's been sick. 

[terry59]

 

How much have you played with spacing?  Madratter had better results with the CCDT67 on his RC when he used a different spacing that one might have thought from the Astrophysics website.

Madratter has or had an SV80 and was using a different brand's focal reducer. I'll message him, I am not sure he will get it. I know he's been sick. 

 

Hey Dan....I think MR uses a TSFlat2

 

http://astrob.in/239107/0/

[FiremanDan]

Yeah, I saw that. I thought he was using a reducer at one point. I guess not. I think I'll end up having to just give up f4.8 and go f6. 

[terry59]

Will Vic take the acknowledged unmatched reducer back?

[FiremanDan]

Will Vic take the acknowledged unmatched reducer back?

They haven't replied to my last couple of emails asking for help with funky stars. 

I guess I should call. Initially, they said the ZWO camera probably had tilt issues, then said I probably should have gotten the feather touch focuser. So I suspect they'd want me to prove the focuser and camera are absolutely not the issues, which I can't prove for sure. There very well could be focuser issues. 
What is soo odd is I did get good stars that one time... 

[terry59]

I looked at one image you took with a DSLR and you seemed to have spacing issues then. Did you ever get a flat field with that?

[FiremanDan]

Eh.... It wasn't as obvious with the DSLR as it has been with the ASI1600. It was better than my C8 w/ the f6.3 but that is a really low bar. Others have pointed out that the DSLR tends to hide this kind of issue compared to a mono sensor with small pixels. 

[terry59]

Eh.... It wasn't as obvious with the DSLR as it has been with the ASI1600. It was better than my C8 w/ the f6.3 but that is a really low bar. Others have pointed out that the DSLR tends to hide this kind of issue compared to a mono sensor with small pixels. 

Ok...I was hoping a lighter weight on the focuser would make a difference

[RDBeck]

While I was looking at purchasing my SV80ST, I asked a question of Stellarvue about the precision required for spacing out the astrocamera sensor from the field flattener.  I got the impression from Vic's reply that even 0.5mm off may have negative effects.  

 

On the assumption that it's not the larger pixels in the DSLR giving you rounder stars, have you confirmed that your sensor distance is spot on?

 

I'm planning on getting the SFF6-25 when I do get a flattener.  I have much more to sort out before (being the newbie I am).

[FiremanDan]

Yeah. I have been working on spacing for literally over a year. Using manufactured supplied/recommended spacers and using digital calipers to verify. 

[gdd]

What happens if you rotate the asi1600 relative to the OTA? Does the problem follow the focuser sag or the camera orientation?

 

Gale

[FiremanDan]

The whole focuser rotates, everything threads right into it. 

However, any adjustments result in **** stars, especially in the corners.

[terry59]

The whole focuser rotates, everything threads right into it. 

However, any adjustments result in **** stars, especially in the corners.

I wish a 2.5" Moonlite was doable for you Dan. Perhaps crowdfunding? I'll contribute

[FiremanDan]

I have heard the Moonlite actually has some play in it. But it's still a consideration. I guess I should start saving up now. 

[dkeller_nc]

Dan - These guys sell a ton of different reducers/flatteners.  It should not matter which brand, so long as it's made for the correct focal length.  

 

However, in looking at your images, I'm going to go with focuser sag.  That's not a definite, however.  If you want to test it, it's pretty easy.  Simply rotate the whole scope in the rings so that your camera is 180 degrees from the starting point.  Rather obviously, you need to take a series of test frames, then rotate the scope and image train 180 degrees, and then take the same set of exposures, all done on the same night, and preferably, on the same target.

 

If this is focuser sag, the stars will change shape on the exact opposite side of the frame.  If it's the FF/FR, the orientation of wonky stars will stay the same.

[Jon Rista]

Dan, looking at your old examples, I think you have a combination of some sag (not a lot, it doesn't look as bad as I had with the ASI071) and somewhat incorrect spacing. The thing that I think is making this hard to diagnose is those two factors are combining to give you very inconsistent stars throughout the field, making it difficult to really figure out the direction of the tilt, and the tilt completely throws off the star angles in the corners, making it difficult to figure out how off your spacing might be. 

 

If SV really does require a 0.5mm spacing or less to get good results with the reducer, then while you might need another spacer or two, I think what you also want is a set of adjustment rings. Adjustment rings come in packages of a dozens or so, with a couple rings at each thickness. The thicknesses range from 0.3mm to 1.5mm or so. You can add a ring between a couple of spacers or a spacer and the focuser and/or camera, to finely tune your spacing. 

 

In order to really resolve the spacing issue, I think you will want to tackle the tilt issue first. It's not a lot of tilt, but it seems to be just enough to produce a very confusing issue. I couldn't really say more about where the tilt was coming from, not without handling the scope myself. I find that feeling for wobble or movement, with very light pressure at locations you think might have tilt, is the best way to identify potential problem points in your imaging train. If you think the focuser might be a source, that's where I would start. The simple fact of the matter here is...to fix your tilt, you'll need to play with the scope...and that means you'll have to deal with potentially worse stars until you get the tilt sorted. Also keep in mind...there could be more than one source of tilt. You might have a little sag in the focuser, you might have some tilt at the focuser/scope interface, you might even have some slight sensor tilt. It could even be a filter that isn't entirely seated properly in it's retainer or in your filter wheel...a darn dog hair caught between the edge of the filter and the rim in the filter wheel could actually be enough to introduce some slight tilt. 

[rflinn68]

Sorry to hear you're having problems Dan. Being that it sometimes has good stars and other times it doesn't, it makes me wonder if its a focus issue. If you're sure focus is dead on, then the other thing that comes to mind is where are you pointing the scope when you have good stars/bad stars. Focuser sag with the scope pointing low in the sky could be the issue. Try pointing the scope straight up to zenith and see how the stars look.

 

If you ever decide to go to f/6 with the scope, I can vouch for the TSFlat2. I use it with my 107/700 and 10RC with great results. Since I have a new camera for the RC, I'm planning to buy another TSFlat2 for that setup. Or maybe a TSFlat2.5 for the 107 in case I ever get a larger sensor down the road. It's by far the best flattener I have ever used. I haven't really gotten that great of results with any reducer/flattener, but the Stellarvue 70T with reducer seems to work well. I've even seen a guy use that reducer on an RC with good results.

 

If you're set on using a reducer with the scope and can't get this figured out, I think I'd work with SV to try to get their newest FR/FF for the SV80 if you determine it's not another problem. Best of luck with it! 

[FiremanDan]

I think you guys are right, it's probably a mix of imperfect spacing and focuser sag. 

I wonder if the temperature changes could cause enough change to affect the overall spacing? 
I think tonight if things go right, I'll image my target across the sky, and use blink to see if the orientation of the distortion changes. That should clue me into the sag issue, then I can do a rotation test at a fixed area of the sky to see how that changes. 

This is so frustrating, I thought I finally had this issue fixed back in April. https://www.cloudyni...tars/?p=7847501

 

I guess I do need to order one of those spacer packs, my camera orientation and such are whacky, my OAG prism actually sits in the corner of the main camera sensor. So that's not ideal.  

[Madratter]

Dan did message me. I only use the TS-Flat2.

 

It is quite difficult to make a really good reducer/flattener for optics that end up being this fast. And even if you have a reducer/flattener that will do the job exactly, you will run into two other issues:

 

1) Spacing becomes really critical

2) Any tilt causes big problems because your critical focus zone becomes so small

 

The bottom line is that as Dan is finding out, this can be a real real bear. It is a bear I chose not to tackle, especially with the backspacing requirements my rig has.

 

There is a reason people pay so much for Tak's and even then you still need to get all tilt out of the system.